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How Long Does a Mission to Every Planet Actually Take _ Science for Sleep · Boring Science

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  Speaker 1: Mercury is the closest planet to the Sun and one
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  Speaker 1: of the most difficult planets to actually reach. That should
• 0:06
  Speaker 1: bother you. It took one spacecraft nearly seven years to
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  Speaker 1: get there. Another one launched in 2018 and will not
• 0:14
  Speaker 1: complete its arrival until late 2026. Meanwhile, missions to Jupiter,
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  Speaker 1: more than five times further from the Sun than Earth,
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  Speaker 1: have made the journey in 18 months. The rules of
• 0:26
  Speaker 1: space travel are not the rules you would guess. Venus
• 0:30
  Speaker 1: sits closer to Earth than any other planet. At its
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  Speaker 1: nearest approach, it is roughly 25 million miles away. And
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  Speaker 1: yet the engineers who designed Cassini, a mission to Saturn,
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  Speaker 1: used Venus as a waypoint twice on the way out.
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  Speaker 1: A spacecraft bound for a planet nearly 900 million miles
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  Speaker 1: away stopped at the closest planet first, swung around it,
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  Speaker 1: and kept going. That is not a detour. That is
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  Speaker 1: the only way it could be done. A spacecraft launched
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  Speaker 1: toward Neptune today would still be in transit when children
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  Speaker 1: born this year are finishing graduate school. 12 years is
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  Speaker 1: the fastest any mission has made that crossing. There is
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  Speaker 1: no shortcut. There is no faster road. The solar system
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  Speaker 1: is not arranged conveniently, and the physics of getting across
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  Speaker 1: it do not care what you would prefer. Here is
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  Speaker 1: the question at the center of all of this. If
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  Speaker 1: you tried to send missions to every planet, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune,
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  Speaker 1: all of them, one after another. How long would that
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  Speaker 1: actually take? Not in the abstract. Not in a diagram
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  Speaker 1: where everything is labeled and the distances look manageable. In reality,
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  Speaker 1: with real spacecraft, real orbital mechanics, and real constraints on
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  Speaker 1: when you can launch and how long it takes to arrive,
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  Speaker 1: the answer is not comfortable. Mercury alone requires years of
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  Speaker 1: careful approach just to slow down enough to enter orbit.
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  Speaker 1: Mars requires waiting for an alignment that comes around roughly
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  Speaker 1: every 26 months. Saturn took Cassini nearly seven years. Uranus
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  Speaker 1: has not been visited since 1986, and a mission launched
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  Speaker 1: today would not arrive until the late 2030s at the earliest.
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  Speaker 1: Neptune is even further. the numbers stack up in a
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  Speaker 1: way that stops feeling like astronomy and starts feeling like
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  Speaker 1: something else entirely. To send missions to all eight planets,
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  Speaker 1: one after another, and account for the actual physics of
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  Speaker 1: getting there, you are talking about timelines that would outlast
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  Speaker 1: entire careers, entire generations, in some cases entire human lifetimes.
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  Speaker 1: The first thing to understand about getting anywhere in the
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  Speaker 1: solar system is that space travel has almost nothing to
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  Speaker 1: do with distance. This is not intuitive. When most people
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  Speaker 1: picture a mission to Mars, they imagine something like a
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  Speaker 1: long road trip. Point the spacecraft at the red dot
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  Speaker 1: in the sky, fire the engines, and drive until you arrive.
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  Speaker 1: The problem is that Mars is not sitting still. It
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  Speaker 1: is moving. Earth is moving. The Sun is pulling everything
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  Speaker 1: in curves. By the time a spacecraft launched toward Mars
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  Speaker 1: today would cross the gap between the two planets, Mars
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  Speaker 1: will have travelled hundreds of millions of miles from where
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  Speaker 1: it was at launch. If you aimed at where Mars is,
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  Speaker 1: you would arrive at empty space. Think of it like
• 3:40
  Speaker 1: throwing a football. A good quarterback does not throw to
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  Speaker 1: where the receiver is standing. He throws to where the
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  Speaker 1: receiver is going to be. The receiver runs, the ball
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  Speaker 1: travels in an arc. And if the calculation is right,
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  Speaker 1: they meet at the same point at the same moment.
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  Speaker 1: Interplanetary navigation works on the same principle, except the receiver
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  Speaker 1: is a planet moving at tens of thousands of miles
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  Speaker 1: per hour. The ball is a spacecraft following a curved
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  Speaker 1: path through a gravitational field, and there is no second
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  Speaker 1: throw if the timing is wrong. The other thing to
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  Speaker 1: understand is that a spacecraft is not driving through space.
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  Speaker 1: It is falling through it. From the moment a rocket
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  Speaker 1: cuts its engines after launch, the spacecraft is essentially a projectile.
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  Speaker 1: It is not pushing against anything. It has no road
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  Speaker 1: beneath it. What it has is momentum from the launch
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  Speaker 1: and the gravitational pull of every massive object in the
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  Speaker 1: solar system, principally the Sun, constantly bending its path. Mission
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  Speaker 1: planners do not steer a spacecraft the way you steer
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  Speaker 1: a car. They calculate a trajectory, a curved path through
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  Speaker 1: the gravitational field that will intersect with where the target
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  Speaker 1: planet will be at a specific moment in the future.
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  Speaker 1: Then they launch at precisely the right time on precisely
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  Speaker 1: the right heading and let gravity do the rest. This
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  Speaker 1: is where Walter Hohmann comes in. In 1925, a German
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  Speaker 1: engineer named Walter Hohmann worked out the mathematics of the
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  Speaker 1: most fuel-efficient path between two planetary orbits. His solution was
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  Speaker 1: elegant and counterintuitive. The most efficient trajectory is not a
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  Speaker 1: straight line. It is a half ellipse. A spacecraft leaves
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  Speaker 1: Earth moving in the same direction Earth is already moving
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  Speaker 1: around the Sun, which adds Earth's orbital speed to the
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  Speaker 1: spacecraft's own speed essentially for free. That combined velocity carries
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  Speaker 1: the spacecraft out on a wide curving arc that stretches
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  Speaker 1: across space until the far end of the arc intersects
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  Speaker 1: the orbit of the target planet. At that moment, if
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  Speaker 1: the timing is right, the planet arrives at that same location.
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  Speaker 1: Spacecraft and planet meet. This path is called a Hohmann
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  Speaker 1: Transfer Orbit. It uses the minimum amount of fuel of
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  Speaker 1: any trajectory between two orbits, which is why nearly every
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  Speaker 1: interplanetary mission in history has followed some version of it.
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  Speaker 1: Fuel is the most constraining resource in spaceflight. Every kilogram
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  Speaker 1: of fuel you carry requires more fuel to lift it
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  Speaker 1: off the ground, which requires more fuel to lift that fuel,
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  Speaker 1: and so on in a compounding cycle that mission designers
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  Speaker 1: spend careers trying to minimize. The Hohmann transfer solves this
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  Speaker 1: problem by working with orbital mechanics rather than fighting them.
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  Speaker 1: But here is the part that breaks people's expectations. The
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  Speaker 1: Hohmann transfer is efficient, not fast. The arc a spacecraft
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  Speaker 1: follows to Mars takes roughly 260 days, the arc to
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  Speaker 1: Jupiter takes years, and the arc to Mercury, the planet
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  Speaker 1: closest to the Sun, is not necessarily short just because
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  Speaker 1: Mercury is nearby. How long the arc takes depends not
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  Speaker 1: only on the distance between the two planets, but on
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  Speaker 1: the difference in their orbital speeds, on the geometry at
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  Speaker 1: the moment of launch, and on what the spacecraft needs
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  Speaker 1: to accomplish when it arrives. A flyby requires one calculation.
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  Speaker 1: Entering orbit requires a completely different one. Orbital speed is
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  Speaker 1: the key variable that most people overlook. Every planet moves
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  Speaker 1: around the Sun at a speed set by its distance
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  Speaker 1: from it. Close planets move fast. Distant planets move slowly.
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  Speaker 1: Earth moves at about 67,000 miles per hour. Mars moves
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  Speaker 1: at about 54,000. Jupiter crawls along at about 29,000. When
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  Speaker 1: a spacecraft leaves Earth for Mars, it starts with Earth's
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  Speaker 1: orbital speed and needs to bleed off some of that
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  Speaker 1: speed to fall inward toward the Sun's gravity. Except for Mars,
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  Speaker 1: which is outward, it needs to gain speed. For Mercury,
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  Speaker 1: which is inward and moving much faster than Earth, the
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  Speaker 1: problem is reversed and severe. the spacecraft has to shed
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  Speaker 1: an enormous amount of orbital energy to match Mercury's velocity.
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  Speaker 1: That process takes time and fuel in quantities that define
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  Speaker 1: the entire mission structure. Here is the other layer. The
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  Speaker 1: planets are not aligned in a way that makes every
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  Speaker 1: departure equivalent. The angle between Earth and the target planet
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  Speaker 1: at the moment of launch determines whether the Hohmann arc
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  Speaker 1: will intersect the target's orbit at the right moment. too
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  Speaker 1: early or too late and they miss each other. This
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  Speaker 1: means there are launch windows, specific periods when the geometry
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  Speaker 1: lines up correctly, that may only open once every year
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  Speaker 1: or two or sometimes longer. Miss the window and you
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  Speaker 1: wait for the next one. The window for Mars, for instance,
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  Speaker 1: comes around roughly every 26 months. The window that allowed
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  Speaker 1: Voyager 2 to visit all four outer planets in sequence
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  Speaker 1: came around once every 175 years. This is the system
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  Speaker 1: that every mission is built inside. The distance between Earth
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  Speaker 1: and another planet is almost beside the point. What matters
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  Speaker 1: is where both planets will be at the moment of arrival,
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  Speaker 1: how fast they are moving relative to each other, how
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  Speaker 1: much energy is available to change the spacecraft's velocity, and
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  Speaker 1: whether the geometry at the moment of launch allows the
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  Speaker 1: arc to work at all. These factors together, not simple distance,
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  Speaker 1: determine whether a mission takes seven months or seven years.
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  Speaker 1: Every mission in history to every planet has been a
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  Speaker 1: negotiation with those same orbital mechanics, and the planets have
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  Speaker 1: never once agreed to make it easy. Mariner 10 launched
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  Speaker 1: in November of 1973, and it reached Mercury after a
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  Speaker 1: journey of 147 days. That number sounds almost reasonable, less
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  Speaker 1: than five months to the innermost planet. If you told
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  Speaker 1: someone that at a dinner table they would probably nod
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  Speaker 1: and move on this was because mercury is the closest
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  Speaker 1: planet to the Sun that means from Earth it sits
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  Speaker 1: sometimes as near as 77 million miles away at closest
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  Speaker 1: approach closer than Mars closer than Jupiter closer than anywhere
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  Speaker 1: else in the outer solar system. Five months to a
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  Speaker 1: destination that near does not sound unreasonable. Then you learn
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  Speaker 1: what Messenger required. Messenger launched in August of 2004. It
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  Speaker 1: did not reach Mercury until March of 2011. That is
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  Speaker 1: six years and seven months of travel. Not because engineers
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  Speaker 1: made poor decisions or chose a slow route. That was
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  Speaker 1: the fastest viable path. Mercury sits closer to us than
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  Speaker 1: almost anything beyond Venus. It is one of the hardest
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  Speaker 1: destinations in the entire solar system to actually stop at.
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  Speaker 1: The reason has nothing to do with distance in any
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  Speaker 1: straightforward sense. It has everything to do with speed, with
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  Speaker 1: what the Sun's gravity does to anything that falls toward it.
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  Speaker 1: Here is the problem in its simplest form. When you
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  Speaker 1: fire a spacecraft toward the Sun, gravity accelerates it. The
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  Speaker 1: deeper into the Sun's gravitational well you fall, the faster
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  Speaker 1: you go. Mercury sits very deep in that well. Its
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  Speaker 1: orbital speed is roughly 107,000 miles per hour, nearly twice
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  Speaker 1: what Earth manages as it circles the Sun. A spacecraft
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  Speaker 1: arriving at Mercury after falling sunward from Earth is moving fast,
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  Speaker 1: far too fast for Mercury's gravity to hold. Mercury is
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  Speaker 1: a small planet. Its surface gravity is about 38% of Earth's.
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  Speaker 1: At typical interplanetary arrival speeds, a spacecraft simply flies past.
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  Speaker 1: The planet cannot capture it. To enter orbit, the spacecraft
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  Speaker 1: has to arrive moving slowly enough that Mercury's weak gravity
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  Speaker 1: can take hold. Slowing down while falling toward the Sun
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  Speaker 1: is not something that happens naturally or cheaply. The brute
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  Speaker 1: force solution would be to carry enough rocket fuel to
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  Speaker 1: fire massive engines in reverse upon arrival, killing velocity through
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  Speaker 1: sheer thrust. Engineers calculated what that would require for Mercury.
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  Speaker 1: The fuel mass needed would be so enormous that no
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  Speaker 1: practically buildable spacecraft could carry it to Mercury in the
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  Speaker 1: first place. The rocket equation punishes heavy spacecraft. The fuel
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  Speaker 1: required to slow down at Mercury adds weight that requires
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  Speaker 1: more fuel to launch. That adds more weight, which spirals
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  Speaker 1: into a vehicle too large to exist. So engineers solve
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  Speaker 1: the problem a different way. They take time instead of fuel.
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  Speaker 1: A lot of time. Gravity assists work in either direction.
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  Speaker 1: When a spacecraft passes a planet ahead of the planet's
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  Speaker 1: orbital motion, it gains speed from the planet's momentum. When
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  Speaker 1: it passes behind the planet's motion, relative to the Sun,
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  Speaker 1: it loses speed. That lost velocity goes into the planet
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  Speaker 1: in a technical sense. But the planet is so massive
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  Speaker 1: that the effect on it is completely immeasurable. For the spacecraft,
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  Speaker 1: the effect is significant. A carefully designed gravity assist flyby
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  Speaker 1: can bleed thousands of miles per hour from a spacecraft's
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  Speaker 1: solar orbital velocity without burning any propellant at all. The
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  Speaker 1: cost is trajectory complexity and time, sometimes months of additional
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  Speaker 1: cruise just to set up a single flyby at the
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  Speaker 1: right geometry. For Mercury, engineers need to line up many
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  Speaker 1: of these maneuvers in sequence, each one ratcheting the spacecraft's
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  Speaker 1: velocity slightly downward until arrival speed falls within the range
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  Speaker 1: Mercury's gravity can manage. MESSENGER's actual path through the solar
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  Speaker 1: system was one of the most intricate trajectories ever flown.
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  Speaker 1: After launching in August 2004, the spacecraft flew back past
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  Speaker 1: Earth in August 2005 in a gravity assist that reshaped
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  Speaker 1: its orbit around the Sun. It then moved toward Venus,
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  Speaker 1: swinging past in October 2006 and again in June 2007.
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  Speaker 1: Two Venus flybys, each one pulling the trajectory slightly inward,
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  Speaker 1: slightly slower relative to Mercury's orbital frame. Then began the
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  Speaker 1: Mercury flybys themselves. January 2008, October 2008, September 2009. Three
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  Speaker 1: separate passes of the actual destination, each one skimming close
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  Speaker 1: to the surface, each one shedding more orbital energy, each
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  Speaker 1: one making the spacecraft's path around the Sun more compatible
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  Speaker 1: with the orbit it needed to achieve. In March 2011,
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  Speaker 1: after six and a half years, one Earth flyby, two
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  Speaker 1: Venus flybys, three Mercury flybys, Messenger fired its engines and
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  Speaker 1: let Mercury's gravity pull it into orbit. The science mission
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  Speaker 1: had not yet begun. The travel was only just finished.
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  Speaker 1: Mariner 10 had none of those constraints. It was not
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  Speaker 1: trying to stop. It executed a Venus flyby in February
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  Speaker 1: 1974 and then swung past Mercury for the first time
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  Speaker 1: in March of that year, photographing roughly 45% of the
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  Speaker 1: planet's surface over a few hours of close approach. Because
• 15:39
  Speaker 1: it was in a solar orbit that kept returning it
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  Speaker 1: to Mercury's neighborhood, it flew past twice more, September 1974
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  Speaker 1: and March 1975, collecting additional images each time. What it
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  Speaker 1: never did was slow down. The spacecraft passed through Mercury's
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  Speaker 1: space at whatever speed it happened to be carrying. Mercury's
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  Speaker 1: gravity had no opportunity to hold it. A flyby mission
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  Speaker 1: has the physics working in its favour. There is no
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  Speaker 1: orbital insertion requirement. No need to shed thousands of miles
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  Speaker 1: per hour of velocity through carefully staged gravity breaking that
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  Speaker 1: takes years to execute. You arrive, you pass, you observe
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  Speaker 1: for hours, and then you continue into space. The science
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  Speaker 1: is genuinely limited. Mariner 10 saw less than half the
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  Speaker 1: planet across all three passes, at distances that left enormous
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  Speaker 1: questions unanswered. But 147 days is fast. It is fast
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  Speaker 1: because the mission demanded nothing from Mercury except that it
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  Speaker 1: be present. This divide between a flyby and an orbiter
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  Speaker 1: is the fundamental split in planetary mission design. Mercury makes
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  Speaker 1: the difference more dramatic than almost anywhere else in the
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  Speaker 1: solar system. A flyby to Mercury requires a relatively modest trajectory.
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  Speaker 1: An orbiter requires a looping, doubling back, multi-year journey that
• 17:07
  Speaker 1: uses three or four planets as stepping stones before the
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  Speaker 1: spacecraft is finally moving slowly enough to be claimed by
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  Speaker 1: Mercury's gravity. The years spent in transit are not wasted.
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  Speaker 1: Engineers take measurements throughout the cruise phase. The gravity assist
• 17:23
  Speaker 1: flybys of Venus produce valuable science of their own. But
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  Speaker 1: the core reason for the detour is mechanical and unavoidable.
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  Speaker 1: MESSENGER spent four years in Mercury orbit after it arrived,
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  Speaker 1: mapping the planet completely, discovering that water ice exists in
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  Speaker 1: permanently shadowed craters near the poles. confirming that Mercury has
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  Speaker 1: a liquid iron core despite its small size, measuring a
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  Speaker 1: magnetic field that turns out to be substantially offset from
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  Speaker 1: the planet's center. None of that was possible from Mariner
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  Speaker 1: 10's three rapid passes. The years of travel purchased years
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  Speaker 1: of answers that could not have come any other way.
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  Speaker 1: Scale comparisons help make this concrete. If you shrunk the
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  Speaker 1: solar system to the size of a football field, Neptune
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  Speaker 1: would sit near one end zone and the Sun at
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  Speaker 1: the other. Earth would land somewhere around the 10-yard line.
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  Speaker 1: Mercury would be barely four yards from the Sun, practically
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  Speaker 1: standing on the goal line. On paper, on that diagram,
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  Speaker 1: it is within almost effortless reach. But getting there, stopping there,
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  Speaker 1: entering orbit around a planet sitting four yards from the goal,
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  Speaker 1: that demands a path that crosses half the field, loops back,
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  Speaker 1: swings wide, crosses again, before finally arriving at a destination
• 18:43
  Speaker 1: that looks from a distance like it should have taken
• 18:45
  Speaker 1: 20 minutes. Orbital mechanics does not care about apparent proximity.
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  Speaker 1: It cares about velocities and energy, and the geometry of
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  Speaker 1: intersecting paths. For Mercury, every one of those factors works
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  Speaker 1: against the simplest route. BepiColombo is a joint mission of
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  Speaker 1: the European Space Agency and JAXA. the Japanese Space Agency.
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  Speaker 1: It carries two separate science orbiters stacked together during the
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  Speaker 1: cruise phase, equipped with instruments significantly more capable than anything
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  Speaker 1: Messenger carried. It launched in October 2018 on a trajectory
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  Speaker 1: that includes one Earth flyby, two Venus flybys, six Mercury
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  Speaker 1: flybys before final orbital insertion. Those six Mercury encounters are
• 19:33
  Speaker 1: spread across years each one reducing the spacecraft's velocity incrementally,
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  Speaker 1: edging it closer to the speed where Mercury can finally
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  Speaker 1: hold on. The mission's arrival timeline was pushed repeatedly as
• 19:47
  Speaker 1: trajectory planners worked through the constraints. The final orbital insertion
• 19:52
  Speaker 1: for both spacecraft is now scheduled for late 2026. BepiColombo,
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  Speaker 1: launched in 2018, will not complete its arrival at Mercury
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  Speaker 1: until late 2026. A journey of more than eight years
• 20:07
  Speaker 1: door-to-door to the planet that sits closest of all to
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  Speaker 1: our own sun. Venus sits closer to Earth than any
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  Speaker 1: other planet, just over 25 million miles away at its
• 20:18
  Speaker 1: nearest approach, and on paper it should be the simplest
• 20:21
  Speaker 1: destination in the solar system. The numbers make it look easy.
• 20:26
  Speaker 1: Mars at closest approach is roughly 35 million miles away.
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  Speaker 1: Mercury at its nearest is about 77 million miles. Venus
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  Speaker 1: beats both of them. For the earliest missions launched toward it,
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  Speaker 1: the travel time reflected that closeness in a way that
• 20:42
  Speaker 1: almost nothing else in planetary exploration does. Mariner 2, launched
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  Speaker 1: in August of 1962, reached Venus in 109 days. The
• 20:54
  Speaker 1: Soviet Venera probes average somewhere between 100 and 140 days
• 20:59
  Speaker 1: in transit. These are short journeys by the standards of
• 21:02
  Speaker 1: interplanetary space. Four months to cross tens of millions of
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  Speaker 1: miles and arrive at another world. Compared to what Mercury demands,
• 21:12
  Speaker 1: or what Saturn requires, Venus looks almost neighbourly. But that
• 21:17
  Speaker 1: framing misses something that turned out to matter enormously. Venus
• 21:22
  Speaker 1: is not just a destination, it is the most useful
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  Speaker 1: gravitational tool in the solar system. The history of what
• 21:29
  Speaker 1: humans have done with Venus has less to do with
• 21:32
  Speaker 1: understanding Venus itself, and more to do with using it
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  Speaker 1: to reach everything else. To understand why, you have to
• 21:40
  Speaker 1: understand how a gravity assist actually works, because the mechanics
• 21:44
  Speaker 1: of it are not obvious. A spacecraft approaching a planet
• 21:48
  Speaker 1: is falling into that planet's gravitational field. As it falls closer,
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  Speaker 1: it speeds up. As it swings around and climbs away,
• 21:57
  Speaker 1: it slows back down. If nothing else were involved, The
• 22:01
  Speaker 1: spacecraft would enter and exit with essentially the same speed
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  Speaker 1: relative to the planet, and the whole encounter would appear
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  Speaker 1: to accomplish nothing. But the planet is not sitting still.
• 22:12
  Speaker 1: It is moving through space at its own orbital velocity,
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  Speaker 1: and the direction of the spacecraft's approach changes everything. If
• 22:20
  Speaker 1: the spacecraft approaches from a specific geometry, the planet's motion
• 22:25
  Speaker 1: adds to the spacecraft's motion. When the spacecraft exits the encounter,
• 22:30
  Speaker 1: It is moving faster than when it entered, not relative
• 22:34
  Speaker 1: to the planet, but relative to the Sun. The planet
• 22:37
  Speaker 1: loses an infinitesimal fraction of its own orbital momentum. The
• 22:42
  Speaker 1: spacecraft gains a meaningful fraction of speed. Nothing burns. No
• 22:47
  Speaker 1: fuel is spent. The laws of physics do the work.
• 22:51
  Speaker 1: For missions heading deep into the outer solar system, this
• 22:54
  Speaker 1: matters more than almost any other single factor in trajectory design.
• 22:58
  Speaker 1: Cassini used Venus twice. The spacecraft launched in October of
• 23:03
  Speaker 1: 1997 and reached Venus for its first flyby in April
• 23:07
  Speaker 1: of 1998, less than six months after leaving Earth. It
• 23:12
  Speaker 1: came back around for a second Venus flyby in June
• 23:15
  Speaker 1: of 1999, then Jupiter in December of 2000, and finally
• 23:20
  Speaker 1: Saturn in July of 2004. Each of those encounters had
• 23:25
  Speaker 1: to be executed with extraordinary precision. because the geometry required
• 23:30
  Speaker 1: for one flyby to set up the next was calculated
• 23:33
  Speaker 1: across years. Cassini did not go to Venus because scientists
• 23:38
  Speaker 1: wanted data about Venus. It went to Venus because Venus
• 23:42
  Speaker 1: was the only practical way to build up enough energy
• 23:45
  Speaker 1: to reach Saturn with the rockets that existed in 1997.
• 23:50
  Speaker 1: Galileo did something similar on its way to Jupiter. Launched
• 23:54
  Speaker 1: in 1989, It swung past Venus in February of 1990,
• 23:59
  Speaker 1: used Earth twice more in 1990 and 1992, and finally
• 24:04
  Speaker 1: arrived at Jupiter in December of 1995. Six years of travel,
• 24:10
  Speaker 1: much of it spent looping through the inner solar system,
• 24:13
  Speaker 1: picking up speed before climbing outward. The Parker Solar Probe
• 24:18
  Speaker 1: takes this logic and runs it in the opposite direction,
• 24:21
  Speaker 1: which is where things become genuinely strange. Parker launched in
• 24:25
  Speaker 1: August of 2018, and its mission was to get as
• 24:29
  Speaker 1: close to the Sun as any spacecraft had ever been.
• 24:32
  Speaker 1: To do that, it needed to slow down relative to
• 24:35
  Speaker 1: the Sun, to fall inward rather than orbit outward. Counter-intuitively,
• 24:40
  Speaker 1: you cannot simply point a spacecraft at the Sun and
• 24:43
  Speaker 1: expect it to fall in. Earth is moving sideways at
• 24:47
  Speaker 1: about 67,000 miles per hour in its orbit, and anything
• 24:51
  Speaker 1: launched from Earth inherits that velocity. To get closer to
• 24:55
  Speaker 1: the Sun, you have to shed that sideways speed. The
• 24:58
  Speaker 1: way Parker's designers chose to do it was to use Venus,
• 25:02
  Speaker 1: not to speed up, to slow down. Each Venus flyby
• 25:07
  Speaker 1: adjusted Parker's orbit, gradually reducing its perihelion, the closest point
• 25:12
  Speaker 1: of its path to the Sun, by a little more
• 25:15
  Speaker 1: each time. The mission required seven Venus flybys over the
• 25:20
  Speaker 1: course of its primary mission, each one precisely timed. each
• 25:24
  Speaker 1: one stealing a small portion of Parker's orbital momentum and
• 25:28
  Speaker 1: depositing it into Venus's gravitational account. Parker reached its closest
• 25:34
  Speaker 1: solar approach of 3.8 million miles from the Sun's surface
• 25:39
  Speaker 1: in December of 2024, moving at 430,000 miles per hour.
• 25:46
  Speaker 1: Venus made that possible, not through speed, but through subtraction.
• 25:52
  Speaker 1: There is something worth pausing on here. Gravity assists are
• 25:56
  Speaker 1: usually described as accelerating spacecraft, and most of the time
• 26:00
  Speaker 1: they do. But the direction of the effect depends entirely
• 26:04
  Speaker 1: on the geometry. A spacecraft approaching in front of a
• 26:08
  Speaker 1: planet in the direction of its orbit gets pulled forward
• 26:11
  Speaker 1: and gains speed. A spacecraft approaching from behind gets pulled
• 26:15
  Speaker 1: backward and loses it. Parker needed the braking version. Most
• 26:19
  Speaker 1: outer planet missions need the boosting version. The planet does
• 26:23
  Speaker 1: not care which way the spacecraft approaches. It simply exerts gravity.
• 26:28
  Speaker 1: The engineers decide what that gravity accomplishes. The Venera program,
• 26:33
  Speaker 1: by contrast, went to Venus simply to study Venus. The
• 26:37
  Speaker 1: Soviet missions dropped probes into the atmosphere, survived the crushing
• 26:42
  Speaker 1: pressure and searing heat for brief periods. The surface temperature
• 26:46
  Speaker 1: on Venus is around 900 degrees Fahrenheit and returned data.
• 26:51
  Speaker 1: Mariner 2 confirmed that the planet had no detectable magnetic
• 26:55
  Speaker 1: field and surface conditions hostile to anything resembling life. These
• 27:00
  Speaker 1: were fast missions with direct trajectories and straightforward objectives. They
• 27:05
  Speaker 1: arrived quickly, accomplished what they could, and ended. They used
• 27:10
  Speaker 1: Venus the way you would expect, as a place to go,
• 27:13
  Speaker 1: not a lever to pull. Modern mission design has moved
• 27:17
  Speaker 1: far beyond that framing. Venus now functions less like a
• 27:21
  Speaker 1: destination and more like infrastructure. It sits at an orbital
• 27:26
  Speaker 1: distance from the Sun that makes it extraordinarily useful for
• 27:30
  Speaker 1: adjusting trajectories heading both inward and outward. The geometry works.
• 27:35
  Speaker 1: The planet is massive enough to produce meaningful speed changes.
• 27:40
  Speaker 1: It is close enough to Earth that spacecraft can reach
• 27:43
  Speaker 1: it quickly before beginning the longer work of using its gravity.
• 27:47
  Speaker 1: Every time a spacecraft swings around Venus, trading a fraction
• 27:51
  Speaker 1: of the planet's orbital momentum for a change in trajectory,
• 27:55
  Speaker 1: it is borrowing from a bank that has been remarkably
• 27:58
  Speaker 1: generous to human space exploration. Mars is the most visited
• 28:03
  Speaker 1: planet in the solar system, which says less about how
• 28:06
  Speaker 1: easy it is to reach and more about how relentlessly
• 28:09
  Speaker 1: humanity has tried. More than 50 missions have been aimed
• 28:13
  Speaker 1: at Mars since the early 1960s. Rovers have driven across
• 28:18
  Speaker 1: its surface. Landers have measured its wind. Orbiters have mapped
• 28:22
  Speaker 1: its craters and canyons down to the width of a desk.
• 28:26
  Speaker 1: By any measure, Mars is the most thoroughly pursued destination
• 28:30
  Speaker 1: in the solar system beyond Earth. And yet, even with
• 28:34
  Speaker 1: all of that accumulated effort, getting a spacecraft there remains
• 28:38
  Speaker 1: a serious commitment of time. Not weeks, months, closer to
• 28:43
  Speaker 1: a year, Depending on when you leave and where the
• 28:46
  Speaker 1: planets happen to be sitting, the average distance from Earth
• 28:50
  Speaker 1: to Mars is roughly 140 million miles. But that number
• 28:54
  Speaker 1: is almost meaningless on its own, because the distance between
• 28:58
  Speaker 1: Earth and Mars is never fixed. Both planets are moving.
• 29:02
  Speaker 1: Earth completes an orbit in 365 days. Mars takes 687 days.
• 29:04
  Speaker 1: Their relative positions are constantly shifting. the gap between them
• 29:13
  Speaker 1: swings dramatically as a result. At closest approach, when the
• 29:17
  Speaker 1: two planets line up on the same side of the Sun,
• 29:20
  Speaker 1: Mars is only about 35 million miles away. At their furthest,
• 29:25
  Speaker 1: when they sit on opposite sides of the Sun, the
• 29:28
  Speaker 1: distance stretches out to nearly 250 million miles. That is
• 29:33
  Speaker 1: a 7 to 1 swing in raw distance depending purely
• 29:37
  Speaker 1: on timing. A standard Hohmann transfer from Earth to Mars
• 29:41
  Speaker 1: takes roughly 259 days, eight and a half months. The
• 29:46
  Speaker 1: spacecraft does not travel in a straight line across that gap.
• 29:50
  Speaker 1: It arcs along a curved path following the geometry that
• 29:54
  Speaker 1: was introduced in the previous section. aiming at where Mars
• 29:57
  Speaker 1: is going to be, rather than where it is when
• 30:00
  Speaker 1: the mission launches. The result is a transit that consumes
• 30:04
  Speaker 1: most of a year before the spacecraft ever reaches the planet.
• 30:08
  Speaker 1: By the time Perseverance hit the Martian atmosphere on February 18th, 2021,
• 30:11
  Speaker 1: it had been travelling since July 30th, 2020. That is
• 30:12
  Speaker 1: a journey of just over 200 days. more than six
• 30:21
  Speaker 1: and a half months of coasting through empty space before
• 30:24
  Speaker 1: the first contact with another world. Perseverance was not unusual
• 30:29
  Speaker 1: in that regard. Mars Odyssey, launched in April of 2001,
• 30:33
  Speaker 1: took about 200 days to reach orbit. Viking 1 and
• 30:35
  Speaker 1: Viking 2, launched in the summer of 1975, each took
• 30:38
  Speaker 1: roughly 11 months to complete their journeys. The travel times
• 30:48
  Speaker 1: vary based on the specific launch window and the trajectory chosen,
• 30:52
  Speaker 1: but the range is fairly consistent. Call it 6 months
• 30:56
  Speaker 1: on the short end, 11 or 12 on the long end,
• 30:59
  Speaker 1: with the majority of missions landing somewhere around 7 to
• 31:03
  Speaker 1: 9 months in transit. There is no shortcut available within
• 31:07
  Speaker 1: current propulsion technology. The physics of the Hohmann transfer defines
• 31:11
  Speaker 1: the floor, and the floor is measured in months. What
• 31:15
  Speaker 1: determines exactly how long any given mission takes is largely
• 31:19
  Speaker 1: the question of when it leaves. Earth and Mars reach
• 31:23
  Speaker 1: their most favourable alignment for a home and transfer roughly
• 31:26
  Speaker 1: once every 26 months. This is called the synodic period,
• 31:32
  Speaker 1: the time it takes for the two planets to return
• 31:34
  Speaker 1: to the same relative positions after one has lapped the other.
• 31:38
  Speaker 1: During these windows, the geometry lines up in a way
• 31:42
  Speaker 1: that makes the trip as short and fuel efficient as
• 31:45
  Speaker 1: it is ever going to be. Outside those windows, the
• 31:49
  Speaker 1: trajectory gets worse. The spacecraft has to travel farther, burn
• 31:53
  Speaker 1: more fuel, or both. Mission planners are not choosing between
• 31:58
  Speaker 1: a fast trip and a slow one. They are choosing
• 32:01
  Speaker 1: between launching during the window or not launching at all.
• 32:05
  Speaker 1: This creates an unusual rhythm to Mars exploration. Missions cluster
• 32:10
  Speaker 1: around those 26-month windows. The 2020 window, which opened in midsummer,
• 32:16
  Speaker 1: saw three separate missions launch. Perseverance from the United States,
• 32:22
  Speaker 1: Hope from the United Arab Emirates, and Tianwen-1 from China.
• 32:27
  Speaker 1: All three left within weeks of each other, because the
• 32:30
  Speaker 1: window was open and all three mission teams understood what
• 32:34
  Speaker 1: closing it meant. Miss the window, and the next opportunity
• 32:38
  Speaker 1: is more than two years away. The science waits. The
• 32:42
  Speaker 1: funding continues to flow. The engineers sit and watch the calendar.
• 32:47
  Speaker 1: These extended delays between launch windows have shaped the entire
• 32:52
  Speaker 1: cadence of Mars exploration, creating periods of intense activity, followed
• 32:57
  Speaker 1: by years of relative quietude. A mission delayed beyond its
• 33:02
  Speaker 1: window might not launch for another two years, meaning that
• 33:05
  Speaker 1: careful planning and rigorous testing must be completed on a
• 33:09
  Speaker 1: rigid schedule. or the entire effort is deferred indefinitely. Now
• 33:14
  Speaker 1: consider what this means for a crewed mission. A human
• 33:18
  Speaker 1: crew flying to Mars on a standard trajectory would spend
• 33:21
  Speaker 1: roughly eight months in transit. That is eight months of
• 33:25
  Speaker 1: exposure to cosmic radiation, eight months of microgravity effects on
• 33:30
  Speaker 1: bone density and muscle mass, eight months of confinement in
• 33:34
  Speaker 1: a spacecraft likely not much larger than a few rooms
• 33:37
  Speaker 1: of an average house, They would arrive at Mars with
• 33:40
  Speaker 1: bodies that have already taken a significant toll from the journey.
• 33:44
  Speaker 1: Then comes the harder part. The crew cannot simply turn
• 33:48
  Speaker 1: around and fly home. Earth and Mars will not be
• 33:52
  Speaker 1: aligned for a return trajectory for well over a year
• 33:55
  Speaker 1: after arrival. The planets have to travel far enough along
• 33:58
  Speaker 1: their respective orbits to reach the next favorable configuration. That
• 34:03
  Speaker 1: wait is typically around 14 to 16 months on the
• 34:06
  Speaker 1: Martian surface. Not a brief visit. Not a week of
• 34:11
  Speaker 1: planting flags and collecting samples. More than a year of
• 34:15
  Speaker 1: living on another planet, in a pressurised habitat, dependent entirely
• 34:19
  Speaker 1: on systems brought from Earth, before the window opens for
• 34:23
  Speaker 1: the return flight. Then another eight months to get home.
• 34:27
  Speaker 1: The entire round trip runs to roughly three years. Three
• 34:31
  Speaker 1: years is the number that tends to land differently than
• 34:34
  Speaker 1: the others. Not for a robotic spacecraft. which does not
• 34:38
  Speaker 1: age and does not get sick and does not need
• 34:41
  Speaker 1: to call its family. Three years for people. Three years
• 34:46
  Speaker 1: away from Earth on a mission to a planet that,
• 34:49
  Speaker 1: for most of history, was nothing more than a reddish
• 34:52
  Speaker 1: point in the night sky. Mars is described as humanity's
• 34:57
  Speaker 1: nearest realistic target for crude exploration. And that framing is accurate.
• 35:03
  Speaker 1: It is the nearest one. But nearest does not mean close.
• 35:07
  Speaker 1: The engineering problem of getting humans to Mars is not
• 35:10
  Speaker 1: primarily a vehicle problem or a life support problem or
• 35:14
  Speaker 1: a landing problem, though it is all of those things too.
• 35:18
  Speaker 1: At its foundation, the problem is time. The distances are
• 35:22
  Speaker 1: so large that the transit itself becomes a mission, a
• 35:25
  Speaker 1: months-long ordeal that has to be survived before any exploration
• 35:29
  Speaker 1: can begin. Viking 1 took 11 months to reach a
• 35:33
  Speaker 1: planet with no crew aboard, no need for food or
• 35:36
  Speaker 1: water or breathable air, no biological clock counting the days.
• 35:42
  Speaker 1: It was a machine, and 11 months was simply how
• 35:45
  Speaker 1: long the trajectory required. A crewed mission would likely aim
• 35:49
  Speaker 1: for a faster approach to reduce radiation exposure, but the
• 35:53
  Speaker 1: physics of the home and transfer imposes hard constraints. Trading
• 35:57
  Speaker 1: efficiency for speed means burning more fuel, which means a
• 36:01
  Speaker 1: heavier spacecraft, which means more fuel to lift that spacecraft
• 36:05
  Speaker 1: out of Earth's gravity. The trade-offs compound quickly. The 26-month
• 36:11
  Speaker 1: window is not a bureaucratic inconvenience. It is a gravitational fact.
• 36:16
  Speaker 1: Earth and Mars are in motion, and they do not
• 36:19
  Speaker 1: pause to accommodate schedules or budgets or political timelines. The
• 36:24
  Speaker 1: window opens, the window closes, and the next one arrives
• 36:28
  Speaker 1: on its own schedule regardless of whether humanity is ready.
• 36:32
  Speaker 1: Every Mars mission ever launched, from Mariner 4 in 1964
• 36:36
  Speaker 1: to Perseverance in 2020, has had to negotiate with that constraint.
• 36:41
  Speaker 1: All of them planned around it. None of them changed it.
• 36:45
  Speaker 1: And if you miss the window, the planets simply will
• 36:48
  Speaker 1: not cooperate. You wait two years and two months for
• 36:51
  Speaker 1: the geometry to come back around, and then you try again.
• 36:55
  Speaker 1: Past the asteroid belt, the solar system stops feeling like
• 36:59
  Speaker 1: a neighborhood. and starts feeling like something else entirely. The
• 37:03
  Speaker 1: asteroid belt itself is not a dramatic physical barrier. Spacecraft
• 37:08
  Speaker 1: pass through it without incident. There are no walls, no
• 37:12
  Speaker 1: visible edges, no moment where an alarm sounds and the
• 37:15
  Speaker 1: scenery changes. But the numbers on the other side of
• 37:19
  Speaker 1: it are different in kind, not just in degree. Mercury
• 37:23
  Speaker 1: sits roughly 77 million miles from Earth at its closest. Mars,
• 37:28
  Speaker 1: at its nearest, is about 35 million miles away. Jupiter,
• 37:33
  Speaker 1: at its closest approach, is nearly 365 million miles from Earth.
• 37:39
  Speaker 1: That is not a longer version of the same journey.
• 37:42
  Speaker 1: That is a different category of problem. Saturn is roughly
• 37:46
  Speaker 1: 750 million miles away at closest approach. Uranus sits at
• 37:51
  Speaker 1: around 1.6 billion miles. Neptune at nearly 2.7 billion. Each
• 37:58
  Speaker 1: step outward is not a continuation of the pattern from
• 38:01
  Speaker 1: the inner solar system. It is an escalation. There is
• 38:05
  Speaker 1: a way to feel this that raw numbers do not
• 38:08
  Speaker 1: quite capture. If you shrink the solar system down so
• 38:11
  Speaker 1: that Earth and the Sun are one inch apart, Mars
• 38:14
  Speaker 1: is about one and a half inches from the Sun.
• 38:17
  Speaker 1: Jupiter is more than five inches. Saturn is closer to ten.
• 38:22
  Speaker 1: Neptune is over thirty. the inner solar system, everything inside
• 38:26
  Speaker 1: the asteroid belt occupies the first inch and a half
• 38:30
  Speaker 1: of that model. The outer solar system occupies the remaining 30.
• 38:35
  Speaker 1: Most of the solar system by any physical measure is
• 38:38
  Speaker 1: out there. This creates a problem that the Hohmann transfer
• 38:41
  Speaker 1: orbit cannot fully solve. The Hohmann transfer is the most
• 38:45
  Speaker 1: fuel efficient path between two orbits, and it works well
• 38:49
  Speaker 1: enough for the inner planets. For Mars, A Hohmann transfer
• 38:53
  Speaker 1: takes roughly 259 days. That is a serious mission, but
• 38:59
  Speaker 1: it is measurable in months. For Jupiter, a pure Hohmann
• 39:03
  Speaker 1: transfer from Earth would take close to three years. For Saturn,
• 39:08
  Speaker 1: over six. For Neptune, the math returns a number close
• 39:12
  Speaker 1: to 30 years. A 30-year Hohmann transfer to Neptune is
• 39:16
  Speaker 1: theoretically optimal in terms of fuel, but it is completely
• 39:21
  Speaker 1: useless in practice. Mo spacecraft lasts 30 years in transit.
• 39:26
  Speaker 1: Mo mission budget or human patience accommodates it. Something else
• 39:30
  Speaker 1: was needed. The deeper problem is that spacecraft lose speed
• 39:34
  Speaker 1: as they climb outward from the Sun. This is not
• 39:37
  Speaker 1: a flaw in the spacecraft. It is orbital mechanics. As
• 39:41
  Speaker 1: a spacecraft moves away from the Sun, the Sun's gravity
• 39:45
  Speaker 1: pulls it back. And the spacecraft trades kinetic energy for
• 39:49
  Speaker 1: potential energy, the same way a ball thrown upwards slows
• 39:53
  Speaker 1: as it climbs. By the time a spacecraft crosses the
• 39:57
  Speaker 1: asteroid belt and approaches Jupiter, it has bled off a
• 40:00
  Speaker 1: significant fraction of the velocity it had when it left Earth.
• 40:04
  Speaker 1: To keep going outward to Saturn, and then Uranus, and
• 40:08
  Speaker 1: then Neptune, requires more energy than a single rocket burn
• 40:12
  Speaker 1: can provide at launch. The energy has to come from
• 40:16
  Speaker 1: somewhere else. The answer came from a graduate student named
• 40:19
  Speaker 1: Michael Minovich, who in the early 1960s was working at
• 40:24
  Speaker 1: JPL on trajectory calculations. Minovich worked out the mathematics of
• 40:29
  Speaker 1: what happens when a spacecraft passes close to a planet
• 40:32
  Speaker 1: in motion. If the geometry is right, the planet's gravity
• 40:37
  Speaker 1: captures the spacecraft briefly, bends its path, and releases it
• 40:41
  Speaker 1: moving faster than it arrived. The planet slows down by
• 40:45
  Speaker 1: an amount too small to measure. The spacecraft speeds up
• 40:49
  Speaker 1: by an amount that can mean the difference between reaching
• 40:52
  Speaker 1: a destination and not reaching it at all. Minovich showed
• 40:56
  Speaker 1: this was not just a theoretical curiosity, it was a
• 41:00
  Speaker 1: practical tool. A few years later, Gary Flandro extended the
• 41:05
  Speaker 1: idea further. He was calculating trajectories for missions to the
• 41:09
  Speaker 1: outer planets and noticed something about the orbital positions of
• 41:12
  Speaker 1: the giant planets in the late 1970s. They would be
• 41:16
  Speaker 1: arrayed in a configuration that occurred roughly once every 175 years.
• 41:22
  Speaker 1: A spacecraft could fly to Jupiter, use Jupiter's gravity to
• 41:26
  Speaker 1: gain speed towards Saturn, use Saturn's gravity to gain speed
• 41:31
  Speaker 1: toward Uranus, and continue outward to Neptune. One launch. Four planets,
• 41:37
  Speaker 1: achieved through a chain of gravity assists that each added
• 41:41
  Speaker 1: speed the rocket alone could never have provided. This was
• 41:45
  Speaker 1: the gravity assist chain, and it transformed what was possible.
• 41:50
  Speaker 1: The trade-off is real, though. Gravity assists are not free.
• 41:55
  Speaker 1: To use a planet's gravity for a speed boost, the
• 41:58
  Speaker 1: spacecraft has to be in the right place at exactly
• 42:01
  Speaker 1: the right time, which means the trajectory is not a
• 42:05
  Speaker 1: direct line. It curves, it loops, it sometimes requires flying
• 42:10
  Speaker 1: toward Venus before heading toward Jupiter, or flying past Earth
• 42:14
  Speaker 1: twice before departing for Saturn. These indirect paths add time
• 42:19
  Speaker 1: and demand precision that compounds across years. If a spacecraft
• 42:23
  Speaker 1: is slightly off course during a gravity assist flyby, the
• 42:27
  Speaker 1: error does not just affect that leg of the journey.
• 42:30
  Speaker 1: It propagates forward, growing larger with each subsequent planetary encounter.
• 42:35
  Speaker 1: The teams managing these missions have to track trajectories with
• 42:39
  Speaker 1: extraordinary care and issue course corrections years before the corrections
• 42:44
  Speaker 1: are actually needed. What the Gravity Assist Chain enabled, fundamentally,
• 42:50
  Speaker 1: was a way to reach the outer solar system without
• 42:53
  Speaker 1: building rockets that would otherwise have had to be enormous
• 42:56
  Speaker 1: beyond any practical limit. The energy required to send a
• 43:00
  Speaker 1: spacecraft from Earth to Neptune in any reasonable time frame
• 43:04
  Speaker 1: carried entirely by rocket propellant at launch, would have demanded
• 43:09
  Speaker 1: a vehicle larger than anything ever constructed. There is no
• 43:13
  Speaker 1: version of the outer solar system exploration story that does
• 43:17
  Speaker 1: not run through Minovich and Flandreau and the mathematical framework
• 43:21
  Speaker 1: they built in the 1960s. Without gravity assists, realistic missions
• 43:26
  Speaker 1: to Jupiter would require launch vehicles so large they were
• 43:30
  Speaker 1: beyond anything humanity had ever seriously considered building. and everything
• 43:35
  Speaker 1: past Jupiter would simply be unreachable. Jupiter is bright enough
• 43:40
  Speaker 1: to see without a telescope, steady and white in the
• 43:44
  Speaker 1: night sky, the largest planet in the solar system, and
• 43:47
  Speaker 1: it takes between 18 months and 6 years to reach it,
• 43:51
  Speaker 1: depending on how you choose to go. That range is
• 43:54
  Speaker 1: worth sitting with for a moment. 18 months, 6 years,
• 43:59
  Speaker 1: both numbers describe real missions to the same planet. The
• 44:03
  Speaker 1: difference between them is not a matter of better technology
• 44:06
  Speaker 1: or faster rockets. It is a matter of what the
• 44:09
  Speaker 1: mission was designed to do, what energy was available, and
• 44:12
  Speaker 1: how much time the engineers were willing to trade for fuel.
• 44:16
  Speaker 1: Jupiter sits about 484 million miles from the Sun on average.
• 44:22
  Speaker 1: From Earth, depending on where the two planets are in
• 44:25
  Speaker 1: their orbits, the distance ranges from roughly 365 million miles
• 44:28
  Speaker 1: at closest approach. to more than 600 million miles when
• 44:34
  Speaker 1: they are on opposite sides of the solar system. Those
• 44:37
  Speaker 1: numbers are hard to feel in any visceral way. Here
• 44:41
  Speaker 1: is one that might help. If you shrank the distance
• 44:44
  Speaker 1: from Earth to the Moon down to a single inch,
• 44:47
  Speaker 1: the distance from Earth to Jupiter would stretch nearly the
• 44:50
  Speaker 1: length of a football field. The Moon, by that same scale,
• 44:54
  Speaker 1: is sitting in your palm. Jupiter is somewhere near the
• 44:58
  Speaker 1: far end zone. and yet it glows in the night
• 45:01
  Speaker 1: sky bright enough to cast a faint shadow on the
• 45:03
  Speaker 1: ground when conditions are right. The first spacecraft to reach
• 45:07
  Speaker 1: Jupiter were the Pioneer probes. Pioneer 10 launched in March
• 45:11
  Speaker 1: of 1972 and reached Jupiter in December of 1973, a
• 45:12
  Speaker 1: journey of about 21 months. Pioneer 11 followed, arriving in
• 45:15
  Speaker 1: December of 1974 after a similar transit. These were fast
• 45:27
  Speaker 1: missions by the standards of what came after, built on
• 45:31
  Speaker 1: relatively direct trajectories, carrying modest science payloads, and designed primarily
• 45:37
  Speaker 1: to prove that a spacecraft could survive the asteroid belt
• 45:40
  Speaker 1: crossing between Mars and Jupiter, which at the time was
• 45:43
  Speaker 1: considered a genuine hazard. No one knew whether the density
• 45:47
  Speaker 1: of material in the belt would destroy a spacecraft passing
• 45:50
  Speaker 1: through it. Pioneer 10 answered that question and kept going.
• 45:55
  Speaker 1: Then came Voyager. Voyager 1 launched in September of 1977
• 46:01
  Speaker 1: and reached Jupiter in March of 1979, a transit of
• 46:05
  Speaker 1: about 18 months. Voyager 2 launched two weeks earlier but
• 46:10
  Speaker 1: followed a slower trajectory and arrived at Jupiter in July
• 46:14
  Speaker 1: of 1979, roughly 23 months after launch. Both missions were
• 46:20
  Speaker 1: fast by any later standard. They were designed for speed
• 46:24
  Speaker 1: and trajectory efficiency above almost everything else, because the planetary
• 46:29
  Speaker 1: alignment that made the Grand Tour possible demanded it. There
• 46:33
  Speaker 1: was no room in the mission plan for long detours
• 46:36
  Speaker 1: or extended gravity assist sequences. The window was open, and
• 46:40
  Speaker 1: the spacecraft had to fly through it. Voyager 2's images
• 46:44
  Speaker 1: of Jupiter's moon Io, volcanic and furious, arrived at Earth
• 46:49
  Speaker 1: in 1979. less than two years after the spacecraft had
• 46:54
  Speaker 1: left Cape Canav. Errol. Then something changed. The next mission
• 47:00
  Speaker 1: to Jupiter was not a flyby. It was an orbiter.
• 47:04
  Speaker 1: And an orbiter is a fundamentally different kind of commitment.
• 47:08
  Speaker 1: Galileo launched in October of 1989. It did not reach
• 47:12
  Speaker 1: Jupiter until December of 1995. Six years and two months
• 47:17
  Speaker 1: of travel time. to a planet the Voyagers had reached
• 47:20
  Speaker 1: in 18 months a decade earlier. The difference was almost
• 47:24
  Speaker 1: entirely a matter of energy. Galileo was heavier than the
• 47:27
  Speaker 1: Voyager spacecraft, carried more science instruments, and was designed to
• 47:32
  Speaker 1: slow down enough to be captured by Jupiter's gravity and
• 47:36
  Speaker 1: spend years in orbit rather than streaking past. Slowing down
• 47:40
  Speaker 1: requires the same energy as speeding up, and Galileo did
• 47:45
  Speaker 1: not have the launch vehicle to get there directly. mission
• 47:49
  Speaker 1: planners gave it a different kind of engine, the gravity
• 47:53
  Speaker 1: of other planets. Galileo's trajectory included a flyby of Venus
• 47:58
  Speaker 1: in February of 1990, an Earth flyby in December of 1990,
• 48:03
  Speaker 1: a second Earth flyby in December of 1992, and a
• 48:08
  Speaker 1: pass through the asteroid belt that included a close look
• 48:11
  Speaker 1: at the asteroid Gaspra in 1991 and Ida in 1993.
• 48:17
  Speaker 1: Each of those encounters bent and accelerated the spacecraft, adding
• 48:21
  Speaker 1: velocity that rockets alone could not have provided. The two
• 48:25
  Speaker 1: Earth flybys are the strangest part of that list. Galileo
• 48:30
  Speaker 1: flew away from Jupiter, came back around past Earth and
• 48:34
  Speaker 1: then flew toward Jupiter again, gaining speed from Earth's own
• 48:38
  Speaker 1: orbital momentum in the process. It was an indirect path
• 48:42
  Speaker 1: by any measure, but without it, Galileo never reaches Jupiter
• 48:46
  Speaker 1: at all. What Galileo found when it arrived was worth
• 48:50
  Speaker 1: the six-year wait. The spacecraft spent nearly eight years in
• 48:55
  Speaker 1: Jupiter orbit, revealing the detailed structure of the planet's atmosphere,
• 49:00
  Speaker 1: detecting an ocean of liquid water beneath the surface of Europa,
• 49:05
  Speaker 1: mapping the volcanic activity on Io in unprecedented detail, and
• 49:09
  Speaker 1: watching pieces of comet Shoemaker-Levy 9 slam into Jupiter's cloud
• 49:14
  Speaker 1: tops in 1994 while still in transit. The Science Galileo
• 49:20
  Speaker 1: returned reshaped what planetary scientists understood about the outer solar system.
• 49:25
  Speaker 1: All of that came from a spacecraft that needed six
• 49:28
  Speaker 1: years just to get into position to start working. Now
• 49:31
  Speaker 1: consider Jupiter's size. It is not just the largest planet
• 49:36
  Speaker 1: in the solar system. It is larger than all the
• 49:39
  Speaker 1: other planets combined. You could fit every other planet inside
• 49:43
  Speaker 1: Jupiter and still have space remaining. Its Great Red Spot,
• 49:48
  Speaker 1: a storm system that has been churning for at least
• 49:50
  Speaker 1: three centuries, is wider than Earth. Its magnetic field is
• 49:55
  Speaker 1: the largest structure in the solar system after the Sun itself,
• 49:59
  Speaker 1: stretching in some directions nearly to Saturn's orbit. Four of
• 50:03
  Speaker 1: its moons are large enough that if they orbited the
• 50:05
  Speaker 1: Sun rather than Jupiter, they would be classified as planets
• 50:09
  Speaker 1: in their own right. Europa alone has become one of
• 50:12
  Speaker 1: the most compelling targets in the search for life beyond Earth,
• 50:16
  Speaker 1: carrying more liquid water under its ice shell shell than
• 50:19
  Speaker 1: exists in all of Earth's oceans combined. Jupiter also functions
• 50:24
  Speaker 1: as the solar system's most powerful gravitational tool. For any
• 50:29
  Speaker 1: mission trying to reach Saturn, Uranus, Neptune or beyond, Jupiter's
• 50:35
  Speaker 1: gravity is almost too useful to ignore. Voyager 2 gained
• 50:39
  Speaker 1: 6 miles per second from its Jupiter flyby alone. 6
• 50:44
  Speaker 1: miles per second is not a small number. That is
• 50:47
  Speaker 1: more than 21,000 miles per hour of velocity added to
• 50:52
  Speaker 1: the spacecraft in a single encounter with no propellant burned.
• 50:56
  Speaker 1: To achieve the same velocity increase with a rocket, you
• 51:00
  Speaker 1: would need an amount of fuel so large it would
• 51:02
  Speaker 1: make the spacecraft practically impossible to launch. Jupiter's gravity did
• 51:07
  Speaker 1: it for free in exchange for a slight and infinitesimal
• 51:11
  Speaker 1: change in the planet's own orbital momentum. This is why
• 51:15
  Speaker 1: nearly every mission to the outer solar system either uses
• 51:19
  Speaker 1: Jupiter as a waypoint or is carefully designed around the
• 51:23
  Speaker 1: possibility of doing so. Cassini used Jupiter for a gravity
• 51:27
  Speaker 1: assist on its way to Saturn. New Horizons used it
• 51:31
  Speaker 1: on the way to Pluto. Voyager 2 used it on
• 51:34
  Speaker 1: the way to Saturn and then onto Uranus and Neptune.
• 51:37
  Speaker 1: Jupiter is not just a destination, it is the solar
• 51:41
  Speaker 1: system's most important junction. Juno launched in August of 2011
• 51:47
  Speaker 1: and was built for a single purpose, studying Jupiter's interior structure,
• 51:52
  Speaker 1: magnetic field and atmospheric dynamics from orbit. It carried no
• 51:57
  Speaker 1: instruments for studying other planets. It had no science objectives
• 52:01
  Speaker 1: during the cruise phase. Its entire mission plan was oriented
• 52:05
  Speaker 1: toward Jupiter and nothing else. And it still took nearly
• 52:09
  Speaker 1: five years to get there. Juno used an Earth Gravity
• 52:13
  Speaker 1: Assist in October of 2013, swinging back past its home
• 52:18
  Speaker 1: planet two years into the mission, to gain the speed
• 52:21
  Speaker 1: it needed to cross the remaining distance to Jupiter. It
• 52:25
  Speaker 1: entered Jupiter orbit on July 4th, 2016. Five years for
• 52:30
  Speaker 1: a spacecraft carrying nothing but instruments pointed at its destination.
• 52:36
  Speaker 1: No detours for science along the way. No extended observation
• 52:40
  Speaker 1: campaigns at intermediate targets. Just the physics of getting from
• 52:45
  Speaker 1: here to there, and the physics required five years. That
• 52:50
  Speaker 1: is what Jupiter asks of any mission designed to stay.
• 52:53
  Speaker 1: The flyby missions got there in 18 months to two years,
• 52:57
  Speaker 1: by sacrificing depth for speed, accepting a brief encounter in
• 53:02
  Speaker 1: exchange for a faster arrival. The orbiters paid in time.
• 53:07
  Speaker 1: Galileo paid six years. Juno paid five. Both of them
• 53:12
  Speaker 1: then spent years doing science that no flyby could have accomplished.
• 53:16
  Speaker 1: The trade is real, and the trade has always been
• 53:19
  Speaker 1: worth it, but it is never cheap. Juno launched in
• 53:23
  Speaker 1: 2011 and did not slip into Jupiter orbit until July
• 53:27
  Speaker 1: of 2016. Five years of traveling across the solar system
• 53:31
  Speaker 1: to reach a destination that on a clear night you
• 53:34
  Speaker 1: can pick out with your eyes. When Cassini left Earth
• 53:38
  Speaker 1: in October of 1997, the scientists who designed it would
• 53:43
  Speaker 1: not see it reach Saturn for almost seven years. That
• 53:47
  Speaker 1: number deserves a moment. Seven years. A child born the
• 53:52
  Speaker 1: week of Cassini's launch would be in second grade by
• 53:56
  Speaker 1: the time the spacecraft finally arrived at its destination. The
• 54:00
  Speaker 1: researchers who spent their careers building the instruments aboard it,
• 54:05
  Speaker 1: the radar, the spectrometers, the imaging systems, had time to
• 54:10
  Speaker 1: publish other papers, attend their children's graduations, move between institutions.
• 54:16
  Speaker 1: grow visibly older before the mission they had given years
• 54:19
  Speaker 1: of their lives to finally started returning science from Saturn.
• 54:24
  Speaker 1: This is what outer planet exploration actually looks like from
• 54:27
  Speaker 1: the inside. Not a launch and an arrival, but a long,
• 54:32
  Speaker 1: quiet wait, measured in years. Saturn sits roughly 886 million
• 54:37
  Speaker 1: miles from Earth on average. The Sun is 93 million
• 54:39
  Speaker 1: miles away. Saturn is nearly 10 times farther from the
• 54:47
  Speaker 1: Sun than we are. That means that even at the
• 54:50
  Speaker 1: speeds a spacecraft can sustain, reaching it is not a
• 54:54
  Speaker 1: matter of months. It is a matter of years regardless
• 54:58
  Speaker 1: of how you approach the problem. But the deeper issue
• 55:01
  Speaker 1: for Cassini was not simply distance. The issue was energy.
• 55:05
  Speaker 1: To launch a spacecraft directly to Saturn in 1997 and
• 55:10
  Speaker 1: actually enter orbit around the planet rather than fly past
• 55:14
  Speaker 1: it would have required a rocket so large it was
• 55:17
  Speaker 1: not practically buildable. The fuel mass alone would have made
• 55:21
  Speaker 1: the launch vehicle effectively impossible. There was no direct path
• 55:25
  Speaker 1: that fell within the limits of what rocket technology could deliver.
• 55:29
  Speaker 1: So the mission designers did something that sounds almost impossible
• 55:34
  Speaker 1: when you first hear it. They sent Cassini the long
• 55:37
  Speaker 1: way around. The route Cassini took to Saturn covered roughly 3.5
• 55:41
  Speaker 1: billion kilometers. and required exactly four planetary flybys, each one
• 55:49
  Speaker 1: precisely timed. Each one extracting a specific amount of gravitational
• 55:54
  Speaker 1: energy from a planet that had no idea it was
• 55:57
  Speaker 1: being used as a slingshot. The first encounter came in
• 56:01
  Speaker 1: April of 1998, about six months after launch, when Cassini
• 56:06
  Speaker 1: swung past Venus. The geometry was arranged so that Venus's
• 56:10
  Speaker 1: gravity bent the spacecraft's path, and transferred a fraction of
• 56:14
  Speaker 1: the planet's orbital momentum to the spacecraft, increasing its speed
• 56:19
  Speaker 1: without burning a drop of fuel. Then Cassini continued outward,
• 56:24
  Speaker 1: looped back, and flew past Venus again in June of 1999.
• 56:29
  Speaker 1: Two visits to the same planet, 14 months apart, each
• 56:34
  Speaker 1: one pulling a little more energy from the planet's orbital motion.
• 56:37
  Speaker 1: Venus didn't noticeably slow down. Cassini accelerated. the technique exploited
• 56:43
  Speaker 1: a fundamental principle of orbital mechanics, that gravity acts the
• 56:48
  Speaker 1: same on all objects regardless of their mass, but momentum
• 56:52
  Speaker 1: transfer depends on the relative speeds involved. A small spacecraft
• 56:56
  Speaker 1: moving past a massive planet gains speed while the planet
• 57:00
  Speaker 1: loses an imperceptibly tiny fraction of its own orbital velocity.
• 57:05
  Speaker 1: Two months later, in August of 1999, Cassini did something
• 57:10
  Speaker 1: that still feels strange to think about. It flew past Earth.
• 57:15
  Speaker 1: A spacecraft that had been launched from Earth two years
• 57:18
  Speaker 1: earlier came back around and used our own planet as
• 57:21
  Speaker 1: a gravitational boost, skimming close enough to be tracked by
• 57:26
  Speaker 1: ground observers before continuing outward toward Jupiter. The Earth flyby
• 57:31
  Speaker 1: added another increment of speed. Then, in December of 2000,
• 57:37
  Speaker 1: Cassini swung past Jupiter. the most powerful gravitational engine in
• 57:42
  Speaker 1: the solar system, received the largest single velocity boost of
• 57:47
  Speaker 1: the entire journey. Jupiter's gravity is immense. The flyby accelerated
• 57:53
  Speaker 1: Cassini significantly and redirected it onto the final arc towards Saturn.
• 57:59
  Speaker 1: From Jupiter, Cassini traveled for another three and a half
• 58:02
  Speaker 1: years before arriving at Saturn in July 2004. Each of
• 58:08
  Speaker 1: those four planetary encounters had to be timed with a
• 58:11
  Speaker 1: precision that is difficult to overstate. The deviation of a
• 58:15
  Speaker 1: few seconds in the timing of a flyby, a trajectory
• 58:19
  Speaker 1: error of a few kilometers at the moment of closest approach,
• 58:23
  Speaker 1: would compound across the remaining years of the journey into
• 58:26
  Speaker 1: an error large enough to miss Saturn entirely, or arrive
• 58:30
  Speaker 1: at the wrong angle for orbital insertion. The navigation teams
• 58:34
  Speaker 1: running these calculations were working with equations that had to
• 58:38
  Speaker 1: hold true across billions of kilometers and years of elapsed time.
• 58:43
  Speaker 1: When Cassini executed its Saturn orbit insertion burn on July 1st, 2004,
• 58:49
  Speaker 1: crossing through a gap in the rings while firing its
• 58:52
  Speaker 1: main engine to slow down enough to be captured by
• 58:55
  Speaker 1: Saturn's gravity, the spacecraft was performing a maneuver that had
• 58:59
  Speaker 1: been planned in precise detail years before anyone knew whether
• 59:04
  Speaker 1: Cassini would even survive the interplanetary cruise. It survived! The
• 59:09
  Speaker 1: burn worked. After six years and nine months, the spacecraft
• 59:14
  Speaker 1: was in orbit. The achievement was a triumph of navigation
• 59:18
  Speaker 1: precision and engineering foresight, a complex maneuver executed flawlessly after
• 59:24
  Speaker 1: years of travel through the vacuum of space. What followed
• 59:28
  Speaker 1: was 13 years of discovery, that changed what we knew
• 59:31
  Speaker 1: about the outer solar system in ways nobody had fully anticipated.
• 59:36
  Speaker 1: Saturn's rings, viewed from orbit rather than from Earth's distance,
• 59:41
  Speaker 1: turned out to be structured in ways that existing models
• 59:44
  Speaker 1: had not predicted. Thousands of individual ringlets, gaps, waves, braided
• 59:50
  Speaker 1: features that reflected the gravitational influences of nearby moons. The
• 59:55
  Speaker 1: rings were dynamic, not static. The level of detail Cassini
• 1:00:00
  Speaker 1: returned redefined the basic science of planetary ring systems. These
• 1:00:05
  Speaker 1: observations revealed that Saturn's rings were far more complex than
• 1:00:09
  Speaker 1: mere collections of ice and rock. They were intricate systems
• 1:00:13
  Speaker 1: shaped by gravitational interactions, collisions, and the presence of small
• 1:00:18
  Speaker 1: moonlets embedded within them. The planet itself held surprises. At
• 1:00:23
  Speaker 1: Saturn's North Pole, The atmosphere organizes itself into a persistent
• 1:00:28
  Speaker 1: hexagonal shape, a six-sided storm system, each side of which
• 1:00:33
  Speaker 1: is longer than the diameter of Earth. The hexagon rotates
• 1:00:37
  Speaker 1: with the planet. It has apparently been there for decades
• 1:00:41
  Speaker 1: at minimum, possibly much longer. Nobody had predicted it. Nobody
• 1:00:46
  Speaker 1: had a clean theoretical explanation for it when Cassini first
• 1:00:49
  Speaker 1: confirmed its existence in detail. It became one of those
• 1:00:53
  Speaker 1: discoveries that generates as many questions as it answers. Titan,
• 1:00:58
  Speaker 1: Saturn's largest moon, had long been known to have an atmosphere.
• 1:01:03
  Speaker 1: Cassini revealed what was actually inside that atmosphere. The Huygens probe,
• 1:01:08
  Speaker 1: the European half of the mission, descended through Titan's orange
• 1:01:13
  Speaker 1: haze in January 2005 and transmitted data from the surface
• 1:01:17
  Speaker 1: for over an hour. What it found below the clouds
• 1:01:21
  Speaker 1: was a world shaped by liquid methane the way Earth
• 1:01:24
  Speaker 1: is shaped by water. Rivers of methane. Lakes of liquid
• 1:01:29
  Speaker 1: methane sitting in basins near the poles. Rain. Erosion. A
• 1:01:34
  Speaker 1: hydrological cycle running on a different chemical than any world
• 1:01:38
  Speaker 1: humans had studied before. Titan turned out to be the
• 1:01:42
  Speaker 1: only body in the solar system other than Earth with
• 1:01:44
  Speaker 1: stable liquid on its surface. That liquid was not water.
• 1:01:49
  Speaker 1: Enceladus was smaller and, before Cassini, relatively unremarkable on paper.
• 1:01:55
  Speaker 1: Then Cassini flew close enough to detect something emerging from
• 1:01:59
  Speaker 1: the Moon's south pole. Geysers! Jets of water vapour and
• 1:02:04
  Speaker 1: ice particles erupting from long fractures in the ice shell,
• 1:02:08
  Speaker 1: called tiger stripes. Spraying material into space, Cassini flew directly
• 1:02:14
  Speaker 1: through those plumes and sampled them. The water contained organic molecules.
• 1:02:19
  Speaker 1: Beneath Enceladus's frozen surface, there was a liquid ocean, kept
• 1:02:24
  Speaker 1: warm by tidal heating from Saturn's gravity. That ocean was
• 1:02:28
  Speaker 1: venting into space in real time. Enceladus immediately became one
• 1:02:33
  Speaker 1: of the most scientifically significant objects in the solar system.
• 1:02:38
  Speaker 1: Not because of its size or its orbit, but because
• 1:02:41
  Speaker 1: of what Cassini found pouring out of it. None of
• 1:02:44
  Speaker 1: that science was available until the spacecraft reached Saturn. And
• 1:02:49
  Speaker 1: reaching Saturn required six years and nine months of travel
• 1:02:52
  Speaker 1: across 3.5 billion kilometers. Four gravity assists from three different planets.
• 1:02:59
  Speaker 1: A navigation precision that had to hold across the entire journey.
• 1:03:04
  Speaker 1: A mission architecture built around the hard fact that no
• 1:03:07
  Speaker 1: rocket available in 1997 could send a spacecraft directly to
• 1:03:12
  Speaker 1: Saturn with enough fuel left over to slow down and
• 1:03:16
  Speaker 1: enter orbit. The route Cassini took was not a compromise.
• 1:03:20
  Speaker 1: It was the only viable option. Everything Cassini revealed about Saturn,
• 1:03:26
  Speaker 1: its rings, its moons, the geysers of Enceladus, the methane
• 1:03:30
  Speaker 1: lakes of Titan, came from a spacecraft that needed six
• 1:03:34
  Speaker 1: years and nine months and four planetary gravity assists just
• 1:03:38
  Speaker 1: to get close enough to start collecting seating data. Voyager
• 1:03:44
  Speaker 1: 2 flew past Uranus in January of 1986, after a
• 1:03:48
  Speaker 1: journey of eight and a half years from Cape Canaveral.
• 1:03:52
  Speaker 1: That was the only visit. No probe has returned since.
• 1:03:56
  Speaker 1: No orbiter has followed. No lander has been proposed that
• 1:04:00
  Speaker 1: ever made it off a whiteboard and into a budget
• 1:04:02
  Speaker 1: request that survived contact with reality. Of all the major
• 1:04:06
  Speaker 1: planets in the solar system, Uranus is the one that
• 1:04:09
  Speaker 1: humanity looked at once, briefly, from a moving spacecraft that
• 1:04:14
  Speaker 1: never slowed down and then left behind entirely. The distance
• 1:04:18
  Speaker 1: alone explains part of it. Uranus sits roughly 1.7 billion
• 1:04:24
  Speaker 1: miles from Earth at its nearest approach. That is about
• 1:04:27
  Speaker 1: twice the distance to Saturn. And Saturn already required Cassini
• 1:04:32
  Speaker 1: nearly seven years of travel time, with four planetary gravity
• 1:04:36
  Speaker 1: assists along the way. Uranus's father, The physics do not
• 1:04:41
  Speaker 1: get more forgiving as you move outward. They get worse.
• 1:04:44
  Speaker 1: Every additional billion miles requires more fuel, more time, more
• 1:04:49
  Speaker 1: precise trajectory planning, and a longer commitment from everyone involved.
• 1:04:54
  Speaker 1: The engineers, the scientists, the funding agencies, and the governments
• 1:04:59
  Speaker 1: that write the checks. Voyager 2 reached Uranus after launching
• 1:05:04
  Speaker 1: in August of 1977. It had already spent four and
• 1:05:08
  Speaker 1: a half years traveling to Jupiter, used Jupiter's gravity to
• 1:05:12
  Speaker 1: accelerate toward Saturn, spent another three and a half years
• 1:05:16
  Speaker 1: reaching Saturn, and then used Saturn's gravity to redirect toward Uranus.
• 1:05:22
  Speaker 1: By the time it arrived, it had been traveling for
• 1:05:24
  Speaker 1: eight and a half years and was operating on software
• 1:05:28
  Speaker 1: written nearly a decade earlier. The flyby itself lasted only
• 1:05:32
  Speaker 1: a matter of hours. Voyager 2 was moving too fast
• 1:05:36
  Speaker 1: to stop, too committed to its trajectory to linger. It
• 1:05:40
  Speaker 1: had Neptune waiting on the other side. But in those hours,
• 1:05:44
  Speaker 1: it found things nobody expected. The magnetic field was wrong.
• 1:05:49
  Speaker 1: Not slightly off, profoundly structurally wrong. On Earth, on Jupiter,
• 1:05:54
  Speaker 1: on Saturn, the magnetic poles sit roughly near the rotational poles,
• 1:05:59
  Speaker 1: tilted maybe a little, but recognizably aligned with the axis
• 1:06:03
  Speaker 1: the planet spins on. Uranus had a magnetic field tilted
• 1:06:06
  Speaker 1: nearly 60 degrees away from its rotational axis and offset
• 1:06:11
  Speaker 1: from the planet's center by a significant fraction of the
• 1:06:14
  Speaker 1: planet's radius. The field was lopsided. It wobbled as the
• 1:06:18
  Speaker 1: planet rotated. Nobody had predicted that, and nobody has fully
• 1:06:23
  Speaker 1: explained it since. Then there was the orientation of the
• 1:06:27
  Speaker 1: planet itself. Uranus does not spin upright like the other planets.
• 1:06:32
  Speaker 1: It rolls. Its rotational axis is tilted roughly 98 degrees
• 1:06:37
  Speaker 1: from vertical, which means it orbits the Sun essentially on
• 1:06:40
  Speaker 1: its side. Its poles face the Sun directly at certain
• 1:06:44
  Speaker 1: points in its 84-year orbit, leaving one hemisphere in continuous
• 1:06:49
  Speaker 1: daylight for decades and the other in continuous darkness. The
• 1:06:54
  Speaker 1: seasons on Uranus are 42 years long on each side.
• 1:06:58
  Speaker 1: During Voyager 2's flyby, The South Pole was pointed almost
• 1:07:02
  Speaker 1: directly at the Sun, which meant the spacecraft was approaching
• 1:07:06
  Speaker 1: a planet that looked, from one angle, like a pale
• 1:07:10
  Speaker 1: blue-green target, almost featureless. The extreme tilt made interpreting what
• 1:07:16
  Speaker 1: was even being observed a strange exercise. Voyager 2 also
• 1:07:21
  Speaker 1: found rings. They were faint, narrow and dark. Not the
• 1:07:26
  Speaker 1: brilliant architecture of Saturn's rings, but something quieter. harder to see.
• 1:07:31
  Speaker 1: It counted ten known moons during the flyby, and discovered
• 1:07:35
  Speaker 1: ten more. The moons themselves were odd. Miranda, one of
• 1:07:40
  Speaker 1: the smallest, had canyon systems so deep and terrain so
• 1:07:43
  Speaker 1: fractured that some researchers initially suspected a catastrophic collision had
• 1:07:48
  Speaker 1: broken it apart and reassembled it. The other moons showed
• 1:07:51
  Speaker 1: evidence of complex geological histories crammed into bodies too small.
• 1:07:56
  Speaker 1: By conventional thinking, to have generated the internal heat required
• 1:08:00
  Speaker 1: to produce them. All of that came from a few
• 1:08:03
  Speaker 1: hours of data collected by a spacecraft that did not stop,
• 1:08:08
  Speaker 1: gathered nearly 40 years ago, with instruments designed in the
• 1:08:12
  Speaker 1: early 1970s. That is the entirety of what humanity knows
• 1:08:16
  Speaker 1: about Uranus from direct observation. Everything else is inference, telescope
• 1:08:22
  Speaker 1: observation from Earth and from orbit, models built on incomplete information.
• 1:08:28
  Speaker 1: The reason no follow-up mission has launched is not indifference.
• 1:08:33
  Speaker 1: The planetary science community has ranked a Uranus orbiter among
• 1:08:37
  Speaker 1: its highest priorities for decades. The 2011 Planetary Science Decadal
• 1:08:42
  Speaker 1: Survey listed it as a flagship mission concept. The 2023
• 1:08:47
  Speaker 1: Decadal Survey went further and named a Uranus orbiter and
• 1:08:51
  Speaker 1: probe its top priority for the next large-scale planetary mission.
• 1:08:55
  Speaker 1: The scientific case is not in dispute Uranus and Neptune
• 1:08:59
  Speaker 1: represent a category of planet, ice giants, that appears to
• 1:09:03
  Speaker 1: be extremely common in planetary systems detected around other stars.
• 1:09:08
  Speaker 1: We have almost no understanding of how they work from
• 1:09:11
  Speaker 1: the inside because we have never spent significant time near one.
• 1:09:16
  Speaker 1: The obstacle is time, not political will. not scientific consensus,
• 1:09:21
  Speaker 1: not even cost alone, time. A mission designed to enter
• 1:09:26
  Speaker 1: orbit around Uranus using current propulsion technology and whatever gravity
• 1:09:31
  Speaker 1: assist sequence the trajectory allows takes somewhere between 13 and
• 1:09:35
  Speaker 1: 15 years from launch to arrival. That number does not
• 1:09:39
  Speaker 1: compress easily. The flight path to Uranus typically requires a
• 1:09:43
  Speaker 1: Jupiter gravity assist, which constrains the launch window. Miss the
• 1:09:48
  Speaker 1: window and you wait. Use the wrong trajectory and the
• 1:09:51
  Speaker 1: spacecraft either does not have enough energy to reach Uranus
• 1:09:55
  Speaker 1: at all, or arrives moving too fast to be captured
• 1:09:58
  Speaker 1: into orbit. Getting into orbit around Uranus, rather than flying
• 1:10:03
  Speaker 1: past it the way Voyager 2 did, requires arriving slowly
• 1:10:07
  Speaker 1: enough for the planet's gravity to hold on. That demands
• 1:10:10
  Speaker 1: either a very long, carefully planned trajectory, or a propulsion
• 1:10:15
  Speaker 1: system capable of significant breaking burns at arrival. And breaking
• 1:10:20
  Speaker 1: burns require fuel, and fuel adds mass. And mass has
• 1:10:24
  Speaker 1: consequences for everything else about the mission design. A mission
• 1:10:28
  Speaker 1: funded and approved in the next few years would launch
• 1:10:31
  Speaker 1: sometime in the late 2020s or early 2030s, depending on
• 1:10:36
  Speaker 1: the trajectory options available. From there, the spacecraft travels. Engineers
• 1:10:42
  Speaker 1: grow older. Scientists who wrote the original instrument proposals move
• 1:10:46
  Speaker 1: into different stages of their careers. Graduate students who join
• 1:10:50
  Speaker 1: the team during the proposal phase finish their PhDs, complete
• 1:10:55
  Speaker 1: postdoctoral fellowships, get faculty positions, and advise their own students
• 1:11:00
  Speaker 1: before the spacecraft ever enters Uranus orbit. The mission is
• 1:11:05
  Speaker 1: a commitment that spans professional lifetimes. What would an orbiter find?
• 1:11:10
  Speaker 1: The magnetic field alone is enough to justify the mission.
• 1:11:14
  Speaker 1: Understanding why it is so tilted, whether the geometry reflects
• 1:11:18
  Speaker 1: something unusual about Uranus's internal structure, whether the interior is
• 1:11:23
  Speaker 1: fundamentally different from Saturn and Jupiter, whether the ice and
• 1:11:27
  Speaker 1: water and ammonia believed to exist in its deep interior
• 1:11:31
  Speaker 1: behave in ways that explain the strange field. None of
• 1:11:34
  Speaker 1: that can be answered from Earth. The rings, the moons,
• 1:11:38
  Speaker 1: the atmospheric chemistry, the question of what actually drives Uranus's
• 1:11:42
  Speaker 1: weather in a planet that seems to radiate almost no
• 1:11:45
  Speaker 1: internal heat, all of it remains open. The earliest any
• 1:11:50
  Speaker 1: realistic Uranus orbiter mission could arrive is the mid-2040s, which
• 1:11:55
  Speaker 1: means a generation of planetary scientists is spending entire careers
• 1:12:00
  Speaker 1: waiting for the data from a single mission to a
• 1:12:03
  Speaker 1: single planet. Neptune is almost 2.8 billion miles from Earth
• 1:12:08
  Speaker 1: at its closest approach, a number that is genuinely difficult
• 1:12:12
  Speaker 1: to make feel real. Uranus already stretched the imagination. Neptune
• 1:12:17
  Speaker 1: makes Uranus look close. If you drove a car at
• 1:12:20
  Speaker 1: highway speed without stopping, without sleeping, without ever slowing down,
• 1:12:26
  Speaker 1: you would arrive at Neptune in roughly 5,000 years. The
• 1:12:30
  Speaker 1: number stops meaning anything almost immediately. What it actually means,
• 1:12:35
  Speaker 1: in practical terms, is that reaching Neptune requires a level
• 1:12:39
  Speaker 1: of patience and precision that no other planet demands quite
• 1:12:43
  Speaker 1: so severely. And yet humanity has been there. once. Voyager
• 1:12:49
  Speaker 1: 2 is the only spacecraft in history to have visited Neptune.
• 1:12:53
  Speaker 1: Not one mission since has come close. No orbiter, no flyby,
• 1:12:59
  Speaker 1: no probe. Neptune has been seen up close exactly once,
• 1:13:04
  Speaker 1: during a window of days in August of 1989, and
• 1:13:08
  Speaker 1: everything we know about it in any real detail traces
• 1:13:12
  Speaker 1: back to that single encounter. The spacecraft that made it
• 1:13:16
  Speaker 1: possible launched on September 5th, 1977, 12 years before it arrived.
• 1:13:22
  Speaker 1: The engineers who designed it had to aim at a
• 1:13:24
  Speaker 1: target that would not be in position for more than
• 1:13:27
  Speaker 1: a decade. They had to calculate where Neptune would be
• 1:13:30
  Speaker 1: in August of 1989, before the Voyager program had completed
• 1:13:35
  Speaker 1: its first planetary flyby. The math had to be essentially
• 1:13:39
  Speaker 1: perfect from the start. The journey itself was a chain.
• 1:13:43
  Speaker 1: Voyager 2 did not travel to Neptune in any direct sense.
• 1:13:48
  Speaker 1: It travelled to Jupiter first, arriving in July of 1979.
• 1:13:54
  Speaker 1: Jupiter's gravity bent its path and accelerated it significantly, redirecting
• 1:14:00
  Speaker 1: the spacecraft towards Saturn. Saturn arrival came in August of 1981.
• 1:14:06
  Speaker 1: Another gravity assist, another course change, another boost in speed.
• 1:14:12
  Speaker 1: From Saturn, Voyager 2 curved outward toward Uranus, arriving in
• 1:14:17
  Speaker 1: January of 1986. Each planetary encounter was not just a
• 1:14:22
  Speaker 1: scientific opportunity. It was a necessary mechanical step. Without Jupiter's gravity,
• 1:14:28
  Speaker 1: the spacecraft could not have built enough speed to reach
• 1:14:32
  Speaker 1: Saturn in any reasonable time frame. Without Saturn, Uranus. Without Uranus, Neptune.
• 1:14:39
  Speaker 1: The planets themselves were the engine. The rocket that launched
• 1:14:43
  Speaker 1: Voyager 2 provided the initial push, but it was the
• 1:14:47
  Speaker 1: solar system's own architecture, four gravity assists strung across 12 years,
• 1:14:53
  Speaker 1: that actually carried the spacecraft to the edge of the
• 1:14:56
  Speaker 1: planetary system. By the time Voyager 2 reached Neptune, it
• 1:15:01
  Speaker 1: had been travelling for 4,388 days. The people who had
• 1:15:06
  Speaker 1: designed and built it were 12 years older. Some had
• 1:15:10
  Speaker 1: moved on to other projects. Some had retired. The spacecraft
• 1:15:15
  Speaker 1: itself had crossed more than 4.4 billion miles of space
• 1:15:19
  Speaker 1: and was arriving at a planet that had never been
• 1:15:22
  Speaker 1: seen from anywhere closer than Earth's best telescopes. No one
• 1:15:26
  Speaker 1: knew exactly what was there. What they found was strange. Neptune,
• 1:15:31
  Speaker 1: at first glance, looks like the kind of planet that
• 1:15:34
  Speaker 1: should be relatively calm. It is an ice giant, cold
• 1:15:38
  Speaker 1: and distant. receiving roughly 900 times less sunlight than Earth does.
• 1:15:44
  Speaker 1: There is no obvious reason to expect violent weather, and
• 1:15:47
  Speaker 1: yet Neptune has the fastest recorded winds of any planet
• 1:15:51
  Speaker 1: in the solar system. Wind speeds measured during the Voyager
• 1:15:55
  Speaker 1: 2 flyby exceeded 1,200 miles per hour. The planet is
• 1:16:00
  Speaker 1: driven by some internal heat source, radiating more energy outward
• 1:16:06
  Speaker 1: than it receives from the Sun and that energy translates
• 1:16:09
  Speaker 1: into atmospheric violence on a scale that has no equivalent
• 1:16:13
  Speaker 1: anywhere in the inner solar system. The most visible feature
• 1:16:17
  Speaker 1: Voyager 2 captured was the Great Dark Spot, a storm
• 1:16:22
  Speaker 1: system embedded in Neptune's southern hemisphere, roughly the size of Earth,
• 1:16:27
  Speaker 1: rotating counterclockwise with a period of about 16 days. The
• 1:16:31
  Speaker 1: structure resembled Jupiter's Great Red Spot in basic form. A
• 1:16:35
  Speaker 1: persistent oval vortex carved out of the planet's upper atmosphere.
• 1:16:40
  Speaker 1: The difference was that when the Hubble Space Telescope looked
• 1:16:43
  Speaker 1: for it in 1994, the great dark spot was gone.
• 1:16:48
  Speaker 1: A new dark spot had appeared in the northern hemisphere instead.
• 1:16:51
  Speaker 1: Neptune's storms are not permanent fixtures. They form, they dominate
• 1:16:56
  Speaker 1: the atmosphere for years, and then they dissipate. Whatever dynamics
• 1:17:01
  Speaker 1: govern the planet's weather, operate on timescales that have so
• 1:17:05
  Speaker 1: far been impossible to study properly, because studying them requires
• 1:17:10
  Speaker 1: a spacecraft in the vicinity of Neptune for more than
• 1:17:13
  Speaker 1: a few days. Triton was the other discovery that changed
• 1:17:17
  Speaker 1: how scientists thought about the outer solar system. Neptune's largest
• 1:17:21
  Speaker 1: moon is roughly 1,680 miles in diameter, smaller than Earth's moon,
• 1:17:28
  Speaker 1: and it orbits Neptune in the wrong direction. Every other
• 1:17:32
  Speaker 1: large moon in the solar system orbits in the same
• 1:17:34
  Speaker 1: rotational direction as its parent planet. Triton goes backwards. This
• 1:17:40
  Speaker 1: retrograde orbit is not something that forms naturally from a
• 1:17:43
  Speaker 1: disk of material coalescing around a young planet. It is
• 1:17:47
  Speaker 1: the signature of a captured object, something that originated elsewhere
• 1:17:52
  Speaker 1: and was pulled into Neptune's gravitational grip. The leading explanation
• 1:17:56
  Speaker 1: is that Triton was once a Kuiper belt object. A
• 1:18:00
  Speaker 1: body from the vast reservoir of icy debris that lies
• 1:18:03
  Speaker 1: beyond Neptune's orbit, captured billions of years ago through a
• 1:18:08
  Speaker 1: gravitational interaction that has never been fully reconstructed. A moon
• 1:18:12
  Speaker 1: that used to be its own world. The implications reach far.
• 1:18:17
  Speaker 1: If Triton is captured, it tells you something about how
• 1:18:20
  Speaker 1: chaotic the early outer solar system was, and how objects
• 1:18:24
  Speaker 1: that formed in one region ended up in completely different ones.
• 1:18:28
  Speaker 1: Triton also has geysers. Voyager 2 photographed dark streaks on
• 1:18:33
  Speaker 1: the surface rising from vents, plumes of nitrogen gas and
• 1:18:38
  Speaker 1: dark material shooting several miles up into the thin atmosphere
• 1:18:42
  Speaker 1: before being carried downwind. A moon at the outer edge
• 1:18:46
  Speaker 1: of the planetary system, where sunlight is almost absent, where
• 1:18:50
  Speaker 1: surface temperatures fall to negative 391 degrees Fahrenheit is geologically
• 1:18:55
  Speaker 1: active the energy that drives those geysers is not well
• 1:18:59
  Speaker 1: understood Triton's orbit is slowly decaying in roughly 3.6 billion
• 1:19:06
  Speaker 1: years tidal forces will break it apart and Neptune will
• 1:19:11
  Speaker 1: gain a ring system comparable to Saturn's. All of this
• 1:19:15
  Speaker 1: from a flyby that lasted a matter of days. The
• 1:19:19
  Speaker 1: communication challenge during that flyby was its own kind of problem.
• 1:19:24
  Speaker 1: Light takes approximately four hours to travel from Neptune to Earth.
• 1:19:29
  Speaker 1: A radio signal sent from ground controllers to Voyager 2
• 1:19:32
  Speaker 1: during the Neptune encounter took four hours to arrive. The
• 1:19:37
  Speaker 1: spacecraft's response Whatever data it was transmitting back took another
• 1:19:42
  Speaker 1: four hours to cross the same distance in the other direction.
• 1:19:46
  Speaker 1: Every instruction had an eight-hour round-trip delay built in. If
• 1:19:50
  Speaker 1: something had gone wrong during the flyby, any corrective command
• 1:19:55
  Speaker 1: sent from Earth would have arrived long after the critical
• 1:19:58
  Speaker 1: moment had passed. The entire sequence had to be pre-programmed, autonomous,
• 1:20:04
  Speaker 1: and trusted completely. The team on Earth watched the data
• 1:20:08
  Speaker 1: come back, not in real time, but in deep delay,
• 1:20:12
  Speaker 1: receiving confirmation of things that had already happened four hours earlier.
• 1:20:16
  Speaker 1: No mission has been sent to Neptune since. The reasons
• 1:20:20
  Speaker 1: are not mysterious. Any spacecraft designed to enter orbit around
• 1:20:24
  Speaker 1: Neptune rather than simply fly past it faces a transit
• 1:20:28
  Speaker 1: time of 12 to 15 years using current propulsion technology
• 1:20:33
  Speaker 1: and any realistic gravity assist sequence. A mission approved and
• 1:20:37
  Speaker 1: funded today, with favorable launch windows, would not arrive until
• 1:20:42
  Speaker 1: the late 2030s at the earliest, and more likely the
• 1:20:46
  Speaker 1: early 2040s. The mission planning is complicated further by the
• 1:20:50
  Speaker 1: fact that Neptune's position in its orbit shifts the available
• 1:20:55
  Speaker 1: gravity assist geometry. The alignments that made Voyager 2's trajectory
• 1:21:00
  Speaker 1: possible were specific to the 1970s. Different geometry means different routes,
• 1:21:06
  Speaker 1: different transit times, different fuel requirements. A Neptune orbiter proposal
• 1:21:11
  Speaker 1: has circulated through planetary science priority surveys for years. It
• 1:21:16
  Speaker 1: has never made it to final approval and funding. What
• 1:21:19
  Speaker 1: that means scientifically is difficult to overstate. Neptune is a
• 1:21:24
  Speaker 1: major planet. It is a world with violent weather, a
• 1:21:28
  Speaker 1: captured moon, active geysers and a magnetic field tilted 47
• 1:21:33
  Speaker 1: degrees from its rotational axis a way that no current
• 1:21:36
  Speaker 1: model fully explains. Scientists know all of this from a
• 1:21:41
  Speaker 1: single flyby conducted 35 years ago with instruments that were
• 1:21:46
  Speaker 1: designed in the 1970s. The resolution of the best Voyager
• 1:21:51
  Speaker 1: 2 images of Neptune's surface features is roughly equivalent to
• 1:21:55
  Speaker 1: what a backyard telescope can resolve for Mars. Entire categories
• 1:22:00
  Speaker 1: of questions about Neptune and Triton remain open, Not because
• 1:22:04
  Speaker 1: the science is too difficult, but because the spacecraft is
• 1:22:07
  Speaker 1: not there. Voyager 2 is still out there, past Neptune,
• 1:22:12
  Speaker 1: past Pluto, moving through interstellar space, and the signal it
• 1:22:16
  Speaker 1: sends back now takes more than 18 hours to reach
• 1:22:20
  Speaker 1: us at the speed of light. New Horizons launched on
• 1:22:23
  Speaker 1: January 19, 2006, carrying the fastest initial velocity of any
• 1:22:29
  Speaker 1: deep space mission NASA had ever launched. and it still
• 1:22:33
  Speaker 1: took nine and a half years to reach Pluto. That
• 1:22:37
  Speaker 1: number deserves a moment. Nine and a half years. The
• 1:22:42
  Speaker 1: spacecraft left Earth moving at over 36,000 miles per hour
• 1:22:46
  Speaker 1: relative to the planet. Faster off the launch pad than
• 1:22:50
  Speaker 1: anything NASA had ever sent into deep space. It crossed
• 1:22:53
  Speaker 1: the orbit of the moon in nine hours. Nine hours
• 1:22:57
  Speaker 1: compared to the three days it took Apollo. And still,
• 1:23:01
  Speaker 1: by the time it finally arrived at Pluto in the
• 1:23:04
  Speaker 1: summer of 2015, the engineers who built it had aged
• 1:23:07
  Speaker 1: nearly a decade. Some had retired. Children had been born
• 1:23:12
  Speaker 1: after launch and started high school before the mission reached
• 1:23:15
  Speaker 1: its target. The distance is the reason. Pluto sits, on average,
• 1:23:20
  Speaker 1: roughly 3 billion miles from Earth. That number is technically
• 1:23:24
  Speaker 1: accurate and practically useless. Three billion miles is not a
• 1:23:29
  Speaker 1: number the human brain can hold. What the brain can
• 1:23:32
  Speaker 1: hold is this. If you drove a car at highway
• 1:23:35
  Speaker 1: speed without stopping, it would take you roughly 6,000 years
• 1:23:39
  Speaker 1: to get there. New Horizons, the fastest thing of its kind,
• 1:23:43
  Speaker 1: needed nine and a half. The spacecraft did not travel
• 1:23:46
  Speaker 1: alone exactly. It had help. In February of 2007, about
• 1:23:51
  Speaker 1: 13 months after launch, New Horizons swung past Jupiter. The
• 1:23:56
  Speaker 1: flyby was not incidental, it was essential. Jupiter's gravity grabbed
• 1:24:01
  Speaker 1: the spacecraft and flung it forward, adding roughly 9,000 miles
• 1:24:05
  Speaker 1: per hour to its already extraordinary speed. That single encounter
• 1:24:10
  Speaker 1: shaved years off the journey. Without the Jupiter gravity assist,
• 1:24:15
  Speaker 1: New Horizons would have needed at least three additional years
• 1:24:18
  Speaker 1: to reach Pluto, arriving in 2018 rather than 2015. The
• 1:24:24
  Speaker 1: mission designers knew this before launch. The Jupiter flyby was
• 1:24:29
  Speaker 1: built into the plan from the beginning, time to the month,
• 1:24:32
  Speaker 1: with the spacecraft's trajectory calculated to extract the maximum possible
• 1:24:37
  Speaker 1: energy from the encounter. After Jupiter, New Horizons entered what
• 1:24:42
  Speaker 1: the mission team called the cruise phase. For long stretches
• 1:24:46
  Speaker 1: of that cruise, the spacecraft was placed into electronic hibernation.
• 1:24:51
  Speaker 1: Not all of it, all at once. The team maintained
• 1:24:54
  Speaker 1: basic systems and checked in periodically. But the instruments were
• 1:24:59
  Speaker 1: powered down, the processes slowed, and the spacecraft essentially slept
• 1:25:04
  Speaker 1: its way across the outer solar system. This was not laziness.
• 1:25:08
  Speaker 1: It was engineering discipline. Every hour, a system runs in
• 1:25:12
  Speaker 1: deep space as an hour of wear it will never
• 1:25:15
  Speaker 1: get back, and the mission needed those systems functional and
• 1:25:19
  Speaker 1: precise when they finally mattered. Nine years later, during a
• 1:25:23
  Speaker 1: flyby that would last less than a single day, the
• 1:25:26
  Speaker 1: approach phase began in January 2015, six months before the flyby.
• 1:25:33
  Speaker 1: At that point, Pluto was still just a smudge in
• 1:25:36
  Speaker 1: New Horizons cameras, brighter than it had been, but unresolved.
• 1:25:42
  Speaker 1: Over the following months, the image sharpened. Features appeared. A
• 1:25:47
  Speaker 1: bright region in the southern hemisphere became visible weeks before arrival.
• 1:25:52
  Speaker 1: Nobody knew what it was yet. July 14th, 2015. The flyby.
• 1:25:59
  Speaker 1: New horizons passed within 7,800 miles of Pluto's surface, which
• 1:26:05
  Speaker 1: in interplanetary terms is extremely close. The closest approach happened
• 1:26:10
  Speaker 1: at about 7.49 in the morning, eastern time. the spacecraft
• 1:26:15
  Speaker 1: was not listening to anyone from Earth at that moment.
• 1:26:18
  Speaker 1: It couldn't be. Signals from New Horizons at that distance
• 1:26:22
  Speaker 1: take about four and a half hours to reach Earth,
• 1:26:25
  Speaker 1: traveling at the speed of light. A command sent from
• 1:26:28
  Speaker 1: Mission Control would take four and a half hours to arrive.
• 1:26:32
  Speaker 1: Any response from the spacecraft would take another four and
• 1:26:35
  Speaker 1: a half hours to come back. That is a nine-hour
• 1:26:38
  Speaker 1: round trip for a single instruction. If something had gone
• 1:26:42
  Speaker 1: wrong during the flyby, The spacecraft would have been on
• 1:26:45
  Speaker 1: its own. There was no possibility of intervention. The entire
• 1:26:50
  Speaker 1: flyby sequence was pre-programmed. Every instrument activation, every image, every measurement,
• 1:26:58
  Speaker 1: all of it scripted weeks in advance and uploaded to
• 1:27:02
  Speaker 1: the spacecraft's memory. The team on Earth had done everything
• 1:27:05
  Speaker 1: they could do. Then they let go and waited. New
• 1:27:10
  Speaker 1: Horizons flew past Pluto autonomously, alone. 3 billion miles away,
• 1:27:16
  Speaker 1: following instructions written by people who would not know if
• 1:27:19
  Speaker 1: those instructions had worked until the data started arriving hours later.
• 1:27:24
  Speaker 1: What the spacecraft sent back was extraordinary. Tombor Regio, named
• 1:27:30
  Speaker 1: for Clyde Tombor, who discovered Pluto in 1930, turned out
• 1:27:34
  Speaker 1: to be a vast heart-shaped plane of nitrogen ice roughly
• 1:27:38
  Speaker 1: the size of Texas and Oklahoma combined. It is one
• 1:27:42
  Speaker 1: of the most recognizable features in the solar system now,
• 1:27:45
  Speaker 1: immediately iconic, though no human eye had ever seen it
• 1:27:49
  Speaker 1: before July of 2015. Surrounding it were mountain ranges made
• 1:27:54
  Speaker 1: of water ice, some peaks rising more than 11,000 feet
• 1:27:58
  Speaker 1: above the surrounding plains, comparable to the Rocky Mountains. These
• 1:28:02
  Speaker 1: were not expected. Pluto is small and cold and was
• 1:28:06
  Speaker 1: assumed to be geologically dead frozen in whatever state it
• 1:28:10
  Speaker 1: had settled into billions of years ago. The mountains suggested
• 1:28:14
  Speaker 1: something more active, a world still processing heat, still changing.
• 1:28:19
  Speaker 1: The atmosphere surprised everyone as well. Pluto has a thin
• 1:28:24
  Speaker 1: envelope of nitrogen, and when New Horizons looked back at
• 1:28:27
  Speaker 1: the planet from the far side, it photographed that atmosphere
• 1:28:31
  Speaker 1: backlit by the Sun. What it found was a faint
• 1:28:35
  Speaker 1: blue haze, the same scattering of light that makes Earth's
• 1:28:38
  Speaker 1: sky blue, produced here by a completely different chemistry at
• 1:28:42
  Speaker 1: the edge of the solar system. The colour was unmistakable. Charon,
• 1:28:47
  Speaker 1: Pluto's largest moon, had a red polar cap. The origin
• 1:28:51
  Speaker 1: of that cap is still debated. The leading explanation involves
• 1:28:55
  Speaker 1: methane escaping from Pluto's atmosphere, drifting to Charon, freezing at
• 1:29:00
  Speaker 1: the pole, and then being chemically altered by ultraviolet radiation
• 1:29:05
  Speaker 1: into reddish compounds called tholins over millions of years. Even
• 1:29:10
  Speaker 1: the Moon has a story that no one had predicted.
• 1:29:13
  Speaker 1: After the flyby, New Horizons did not stop. It could
• 1:29:17
  Speaker 1: not stop. There was no mechanism to slow it down,
• 1:29:21
  Speaker 1: and no reason to. It continued outward into the Kuiper Belt,
• 1:29:25
  Speaker 1: the vast ring of frozen objects beyond Neptune. In January 2019,
• 1:29:30
  Speaker 1: it flew past a small contact binary object called Arrokoth,
• 1:29:35
  Speaker 1: two lobes of ancient material barely 22 miles across, joined
• 1:29:39
  Speaker 1: at a narrow neck. It was the most distant object
• 1:29:43
  Speaker 1: a spacecraft had ever closely examined. Arrokoth turned out to
• 1:29:48
  Speaker 1: be pristine, undisturbed by collisions or heating for essentially the
• 1:29:53
  Speaker 1: entire history of the solar system, a relic from the
• 1:29:56
  Speaker 1: formation of the planets themselves. New Horizons is still operating.
• 1:30:02
  Speaker 1: It is still moving outward at roughly 36,000 miles per hour,
• 1:30:06
  Speaker 1: sending back data from regions of the solar system no
• 1:30:09
  Speaker 1: probe has ever reached. Its power supply is slowly fading.
• 1:30:14
  Speaker 1: By the 2040s, the spacecraft will likely fall silent. At
• 1:30:18
  Speaker 1: that point, it will be so far from Earth that
• 1:30:21
  Speaker 1: its signals, traveling at the speed of light, will need
• 1:30:24
  Speaker 1: nearly a full day to make the crossing. The mission
• 1:30:27
  Speaker 1: that began with a launch in 2006 and reached its
• 1:30:31
  Speaker 1: target in 2015 will have spent decades in the dark,
• 1:30:36
  Speaker 1: still transmitting, still moving, long after the last of its
• 1:30:40
  Speaker 1: designers have grown old. New Horizons is still out there,
• 1:30:45
  Speaker 1: still transmitting, and by the time it finally falls silent
• 1:30:49
  Speaker 1: sometime in the 2040s, its signals will take nearly a
• 1:30:53
  Speaker 1: full day to cross the distance back to Earth. In
• 1:30:56
  Speaker 1: the mid-1960s, a young scientist named Gary Flandro at NASA's
• 1:31:02
  Speaker 1: Jet Propulsion Laboratory noticed something in routine trajectory calculations that
• 1:31:08
  Speaker 1: would eventually become one of the most important discoveries in
• 1:31:12
  Speaker 1: the history of space exploration. Flandro was working on a
• 1:31:16
  Speaker 1: fairly routine assignment, identifying potential trajectories for future deep space missions,
• 1:31:23
  Speaker 1: when the numbers revealed something unusual. In the late 1970s, Jupiter, Saturn,
• 1:31:30
  Speaker 1: Uranus and Neptune would drift into a rough alignment that
• 1:31:34
  Speaker 1: happens only once every 175 years. Not a perfect straight line,
• 1:31:40
  Speaker 1: nothing so clean as that, but close enough that a
• 1:31:43
  Speaker 1: spacecraft launched at precisely the right moment could use each
• 1:31:47
  Speaker 1: planet's gravity as a slingshot toward the next, hopping from
• 1:31:51
  Speaker 1: giant to giant across the outer solar system in a
• 1:31:55
  Speaker 1: single continuous journey. Without that alignment, reaching Neptune from Earth
• 1:32:00
  Speaker 1: required a direct shot across billions of miles, carrying enough
• 1:32:05
  Speaker 1: fuel to slow down at the far end. No rocket
• 1:32:08
  Speaker 1: that existed or that could realistically be built was capable
• 1:32:11
  Speaker 1: of that. With the alignment, the planets themselves became the engine.
• 1:32:16
  Speaker 1: Flandreau ran the numbers carefully. The window opened in 1977.
• 1:32:21
  Speaker 1: It would stay open for only a few years. After that,
• 1:32:24
  Speaker 1: it would close, and nothing would reopen it for more
• 1:32:27
  Speaker 1: than a century and a half. The concept that emerged
• 1:32:30
  Speaker 1: from those calculations had an appropriately grand name, the Grand Tour.
• 1:32:35
  Speaker 1: NASA's initial vision was ambitious. A fleet of spacecraft would
• 1:32:40
  Speaker 1: launch in sequence, visiting all four outer planets, dropping probes,
• 1:32:45
  Speaker 1: conducting extended observations, documenting worlds that no human instrument had
• 1:32:50
  Speaker 1: ever seen up close. Preliminary designs called for specialised spacecraft
• 1:32:56
  Speaker 1: designed around the mission's scale. The projected costs were significant.
• 1:33:01
  Speaker 1: Then the budget reality arrived. The early 1970s were not
• 1:33:05
  Speaker 1: generous years for NASA. Apollo had wound down. Political appetite
• 1:33:11
  Speaker 1: for large-scale space spending had cooled. The original Grand Tour concept,
• 1:33:16
  Speaker 1: as designed, was cancelled. The spacecraft were too expensive. The
• 1:33:21
  Speaker 1: mission was too elaborate. What survived was a stripped-down version.
• 1:33:26
  Speaker 1: Two spacecraft instead of four. Simpler in design, cheaper to build,
• 1:33:31
  Speaker 1: carrying a reduced but still scientifically formidable set of instruments.
• 1:33:36
  Speaker 1: They were called Voyager 1 and Voyager 2. Voyager 2
• 1:33:40
  Speaker 1: launched first on August 20th, 1977. Voyager 1 launched 16
• 1:33:47
  Speaker 1: days later on September 5th, 1977, on a faster trajectory
• 1:33:52
  Speaker 1: that would allow it to arrive at Jupiter first, despite
• 1:33:55
  Speaker 1: leaving Earth second. The twin spacecraft were remarkably similar in design,
• 1:34:01
  Speaker 1: yet each carried distinct scientific instruments tailored to their specific missions.
• 1:34:06
  Speaker 1: Both were equipped to study atmospheric composition, magnetic fields and
• 1:34:11
  Speaker 1: particle radiation in unprecedented detail. The decision to build two
• 1:34:16
  Speaker 1: spacecraft instead of the original multi-spacecraft fleet provided a critical
• 1:34:22
  Speaker 1: operational advantage. If one encountered catastrophic failure, the entire mission
• 1:34:28
  Speaker 1: would not be lost. The engineering team designed both Voyagers
• 1:34:32
  Speaker 1: to operate with complete autonomy for extended periods, a necessity
• 1:34:37
  Speaker 1: given the growing communication delays as they travelled outward. Each
• 1:34:42
  Speaker 1: spacecraft carried a golden record with sounds, images and greetings
• 1:34:47
  Speaker 1: from Earth in 55 different languages, a message from humanity
• 1:34:51
  Speaker 1: intended for any intelligence that might someday find it in
• 1:34:55
  Speaker 1: the depths of interstellar space. the practical meaning of that
• 1:34:59
  Speaker 1: alignment is worth sitting with for a moment. Voyager 2's
• 1:35:03
  Speaker 1: journey to Neptune took 12 years. It covered roughly 4.4
• 1:35:08
  Speaker 1: billion miles. At every planetary encounter along the way, the
• 1:35:13
  Speaker 1: spacecraft arrived moving faster than it had at the previous one,
• 1:35:17
  Speaker 1: having borrowed speed from a planet's orbital momentum. Jupiter added
• 1:35:21
  Speaker 1: roughly 6 miles per second to Voyager 2's velocity relative
• 1:35:26
  Speaker 1: to the Sun. That single encounter compressed what would otherwise
• 1:35:29
  Speaker 1: have been additional decades of travel time into something manageable.
• 1:35:34
  Speaker 1: Without Jupiter, there was no Saturn flyby on any reasonable timeline.
• 1:35:39
  Speaker 1: Without Saturn, there was no Uranus. Without Uranus, Neptune was
• 1:35:44
  Speaker 1: simply unreachable. The whole mission was a chain, and every
• 1:35:49
  Speaker 1: link depended on the one before it. The alignment was
• 1:35:52
  Speaker 1: not a convenience. It was the mission's entire structural foundation.
• 1:35:58
  Speaker 1: Voyager 1 reached Jupiter in March of 1979, roughly 18
• 1:36:02
  Speaker 1: months after launch. What it found there was staggering. Active
• 1:36:07
  Speaker 1: volcanoes on Io, the detailed structure of the Jovian cloud bands,
• 1:36:13
  Speaker 1: the thin ring system around Jupiter that nobody had confirmed
• 1:36:16
  Speaker 1: was there. Then Voyager 1 continued to Saturn, arriving in
• 1:36:21
  Speaker 1: November of 1980. It threaded through the ring system, made
• 1:36:25
  Speaker 1: close passes of several moons, and transmitted imagery that scientists
• 1:36:29
  Speaker 1: were still analyzing years later. At Saturn, Voyager 1's mission
• 1:36:34
  Speaker 1: to the outer planets ended. Mission planners had made a
• 1:36:37
  Speaker 1: deliberate choice. Instead of continuing toward Uranus and Neptune, Voyager
• 1:36:43
  Speaker 1: 1 would be directed into a close flyby of Titan,
• 1:36:46
  Speaker 1: Saturn's largest moon. Titan had an atmosphere. That made it
• 1:36:51
  Speaker 1: scientifically irresistible. But the geometry required for a close Titan
• 1:36:55
  Speaker 1: approach flung Voyager 1 upward, out of the plane of
• 1:36:59
  Speaker 1: the solar system entirely, on a trajectory that made Uranus
• 1:37:04
  Speaker 1: and Neptune impossible. Voyager 1 traded the rest of the
• 1:37:08
  Speaker 1: Grand Tour for a better look at one moon. Voyager
• 1:37:12
  Speaker 1: 2 was not given that constraint. It continued on its
• 1:37:15
  Speaker 1: ordained path, reaching Saturn in August of 1981, Uranus in
• 1:37:19
  Speaker 1: January of 1986, and Neptune in August of 1989. Twelve
• 1:37:26
  Speaker 1: years after leaving Earth, the spacecraft arrived at the edge
• 1:37:30
  Speaker 1: of the known solar system and revealed Neptune in detail
• 1:37:34
  Speaker 1: for the first time. A storm the size of Earth
• 1:37:38
  Speaker 1: churned in the planet's southern hemisphere. Triton, Neptune's largest moon,
• 1:37:43
  Speaker 1: erupted with geysers of nitrogen ice. The encounter lasted only
• 1:37:48
  Speaker 1: a few hours. Then Voyager 2 continued past Neptune, past Pluto,
• 1:37:54
  Speaker 1: into the region beyond the planets. Voyager 1, moving faster
• 1:37:59
  Speaker 1: and on a steeper outward trajectory, crossed into interstellar space
• 1:38:04
  Speaker 1: in 2012. The announcement came after years of careful analysis.
• 1:38:09
  Speaker 1: Scientists were debating precisely where the boundary between the Sun's
• 1:38:13
  Speaker 1: influence and interstellar space actually fell, and the data from
• 1:38:17
  Speaker 1: Voyager 1 was their primary instrument for finding it. The
• 1:38:21
  Speaker 1: spacecraft detected a sharp change in the density and direction
• 1:38:25
  Speaker 1: of the particles surrounding it, consistent with leaving the heliosphere
• 1:38:29
  Speaker 1: and entering the interstellar medium. It was the first human-made
• 1:38:33
  Speaker 1: object to do so. As of now, Voyager 1 sits
• 1:38:37
  Speaker 1: roughly 24 billion miles from Earth. The signal it sends back,
• 1:38:42
  Speaker 1: travelling at the speed of light, takes more than 22
• 1:38:44
  Speaker 1: hours to arrive. It is the most distant object humanity
• 1:38:49
  Speaker 1: has ever built, and it is still out there, still transmitting,
• 1:38:54
  Speaker 1: still returning data about the environment beyond the solar system.
• 1:38:59
  Speaker 1: Voyager 2 crossed into interstellar space in 2018, roughly six
• 1:39:04
  Speaker 1: years after its twin. Both spacecraft run on radioisotope thermoelectric generators,
• 1:39:11
  Speaker 1: devices that convert the heat from decaying plutonium into electricity.
• 1:39:16
  Speaker 1: Those generators have been slowly losing power for nearly 50 years.
• 1:39:21
  Speaker 1: NASA periodically shuts down instruments and heaters to conserve energy
• 1:39:25
  Speaker 1: and extend the missions. Engineers have estimated that meaningful scientific
• 1:39:30
  Speaker 1: data may continue returning into the early 2030s. After that,
• 1:39:35
  Speaker 1: the generators will no longer produce enough electricity to power
• 1:39:38
  Speaker 1: the transmitters, and contact will be lost. The spacecraft will
• 1:39:42
  Speaker 1: continue moving outward into interstellar space carrying their golden records
• 1:39:47
  Speaker 1: and their instruments in permanent silence, still drifting at roughly 35,000
• 1:39:53
  Speaker 1: miles per hook. Our unreachable and unretrievable. What is remarkable
• 1:40:00
  Speaker 1: about this is not just what the Voyager missions accomplished,
• 1:40:04
  Speaker 1: but what made them possible. An alignment of planets that
• 1:40:08
  Speaker 1: occurs once every 175 years, identified by one scientist doing
• 1:40:14
  Speaker 1: routine calculations, exploited within a precise window measured in months.
• 1:40:19
  Speaker 1: If Flandreau had made his discovery ten years later, it
• 1:40:23
  Speaker 1: would already have been too late. If the launch had
• 1:40:26
  Speaker 1: slipped by a few years for budget reasons, the window
• 1:40:29
  Speaker 1: would have closed. The outer planets would have scattered back
• 1:40:32
  Speaker 1: into their usual uncooperative positions, and everything from Uranus onward
• 1:40:37
  Speaker 1: would have remained unexplored, possibly for another century. The missions
• 1:40:42
  Speaker 1: did not just require technology and funding and political will,
• 1:40:46
  Speaker 1: they required the planets to cooperate, which they do only
• 1:40:50
  Speaker 1: on their own terms and tea. Air Owned Schedule No
• 1:40:55
  Speaker 1: mission has returned to Uranus or Neptune since Voyager 2
• 1:40:58
  Speaker 1: flew past them. The travel times required using any currently
• 1:41:02
  Speaker 1: operational propulsion system are measured in more than a decade
• 1:41:06
  Speaker 1: each way, and no such mission has been funded. The
• 1:41:10
  Speaker 1: data humanity has on both planets still comes primarily from
• 1:41:14
  Speaker 1: those brief flybys in 1986 and 1989. The window that
• 1:41:19
  Speaker 1: made Voyager possible, the planetary alignment that occurs roughly once
• 1:41:25
  Speaker 1: every 175 years, closed in the early 1980s, and the
• 1:41:29
  Speaker 1: next one will not open until approximately 2153. On December 24th, 2024,
• 1:41:37
  Speaker 1: The Parker Solar Probe reached 430,000 miles per hour, the
• 1:41:42
  Speaker 1: fastest any human-made object has ever traveled. That number deserves
• 1:41:47
  Speaker 1: a moment. At that speed you could circle the entire
• 1:41:51
  Speaker 1: Earth in under four minutes. You could cross the continental
• 1:41:54
  Speaker 1: United States in roughly 20 seconds. It is a velocity
• 1:41:59
  Speaker 1: so far beyond anything in ordinary human experience that comparisons
• 1:42:03
  Speaker 1: almost stop working. And yet, Even at 430,000 miles per hour,
• 1:42:09
  Speaker 1: the solar system barely flinches. At that speed, reaching Mars
• 1:42:14
  Speaker 1: at average distance would still take around four months. Jupiter
• 1:42:18
  Speaker 1: would be nearly two years away. Neptune, the outermost planet,
• 1:42:23
  Speaker 1: would require close to 15 years of travel. The number
• 1:42:27
  Speaker 1: that broke every speed record in history is still not
• 1:42:30
  Speaker 1: fast enough to make the outer solar system feel close.
• 1:42:34
  Speaker 1: This is the part that should genuinely disturb anyone who
• 1:42:37
  Speaker 1: thinks carefully about it. The distances involved are not just large,
• 1:42:42
  Speaker 1: they are large in a way that exposes the limits
• 1:42:45
  Speaker 1: of speed itself as a solution. There is something important
• 1:42:49
  Speaker 1: to understand about how the Parker Solar Probe reached that velocity.
• 1:42:54
  Speaker 1: It did not achieve 430,000 miles per hour by firing
• 1:42:58
  Speaker 1: its engines harder than any spacecraft before it. It achieved
• 1:43:02
  Speaker 1: that speed by falling, The probe executed a close pass
• 1:43:06
  Speaker 1: around the Sun on December 24th, dropping to within about 3.8
• 1:43:12
  Speaker 1: million miles of the solar surface. And as it fell
• 1:43:15
  Speaker 1: toward that closest point, the Sun's gravity accelerated it the
• 1:43:19
  Speaker 1: same way gravity accelerates anything falling toward a massive object.
• 1:43:24
  Speaker 1: The probe was essentially in free fall at an extreme
• 1:43:28
  Speaker 1: depth inside the Sun's gravitational well. When it swung back out,
• 1:43:32
  Speaker 1: it was moving at record speed. But that speed was temporary.
• 1:43:37
  Speaker 1: As it climbed away from the Sun, gravity bled the
• 1:43:40
  Speaker 1: speed back off again. The Parker Solar Probe is not
• 1:43:43
  Speaker 1: a fast spacecraft in any sustained sense. It is a
• 1:43:47
  Speaker 1: spacecraft that gets briefly, extraordinarily fast at one specific point
• 1:43:52
  Speaker 1: in its orbit, and then slows back down. This distinction
• 1:43:55
  Speaker 1: matters enormously. The speed that makes headlines is not the
• 1:44:00
  Speaker 1: speed that would get a spacecraft to Neptune. For long
• 1:44:03
  Speaker 1: interplanetary journeys, what matters is sustained velocity maintained across hundreds
• 1:44:08
  Speaker 1: of millions or billions of miles. And the record holders
• 1:44:12
  Speaker 1: for that are not the Parker Solar Probe. They are
• 1:44:15
  Speaker 1: the Voyager spacecraft and New Horizons. Voyager 1 has been
• 1:44:20
  Speaker 1: travelling for nearly 50 years and is moving at roughly 38,000
• 1:44:25
  Speaker 1: miles per hour. New Horizons cross the distance to Pluto
• 1:44:29
  Speaker 1: at around 36,000 miles per hour. sustained over nine and
• 1:44:33
  Speaker 1: a half years. For comparison, the Apollo astronauts made their
• 1:44:37
  Speaker 1: journey to the Moon at about 25,000 miles per hour.
• 1:44:41
  Speaker 1: So the genuine sustained speed record for deep space crews
• 1:44:45
  Speaker 1: is sitting somewhere in the range of 35,000 to 40,000
• 1:44:49
  Speaker 1: mil S per hour, a number that is remarkable by
• 1:44:54
  Speaker 1: any terrestrial standard and still produces travel times measured in
• 1:44:58
  Speaker 1: decades for the outer planets. What would Parker's peak speed
• 1:45:03
  Speaker 1: actually change if a spacecraft could somehow maintain it across
• 1:45:08
  Speaker 1: an entire interplanetary journey? The math is instructive. Voyager 2
• 1:45:14
  Speaker 1: reached Neptune in 12 years traveling at roughly 35,000 miles
• 1:45:19
  Speaker 1: per hour. A spacecraft holding 430,000 miles per hour continuously
• 1:45:25
  Speaker 1: would cover the same distance in just over a year.
• 1:45:28
  Speaker 1: That is a genuine, dramatic improvement. But it requires something
• 1:45:33
  Speaker 1: that does not exist. A propulsion system capable of maintaining
• 1:45:37
  Speaker 1: that level of thrust not for a few seconds, not
• 1:45:40
  Speaker 1: for a single orbital pass, but for years. Chemical rockets
• 1:45:45
  Speaker 1: burn for minutes. The speeds they produce are fixed at
• 1:45:48
  Speaker 1: engine cutoff. After that, a spacecraft coasts. It does not accelerate.
• 1:45:54
  Speaker 1: It simply carries whatever velocity it was given at launch
• 1:45:57
  Speaker 1: and gradually loses some of it climbing out of the
• 1:46:00
  Speaker 1: Sun's gravity well. This is the ceiling. The problem with
• 1:46:05
  Speaker 1: reaching the outer solar system faster is not that engineers
• 1:46:08
  Speaker 1: haven't thought about speed. It is that no propulsion system
• 1:46:12
  Speaker 1: currently in operational use can push a spacecraft continuously for
• 1:46:17
  Speaker 1: the years that would be required to make a meaningful
• 1:46:19
  Speaker 1: difference in transit time to Neptune or Uranus. You can
• 1:46:23
  Speaker 1: build a faster rocket, and that gives you a somewhat
• 1:46:26
  Speaker 1: faster coast. Voyager was faster than Pioneer, and Pioneer was
• 1:46:31
  Speaker 1: faster than the spacecraft before it. Each generation gained something,
• 1:46:36
  Speaker 1: but each generation was still coasting, and coasting across 3
• 1:46:40
  Speaker 1: billion miles takes a long time, no matter how fast
• 1:46:43
  Speaker 1: you start. There is a related problem on the deceleration side.
• 1:46:48
  Speaker 1: Getting to Neptune quickly means arriving at Neptune quickly. which
• 1:46:53
  Speaker 1: means arriving with enormous velocity. A spacecraft travelling at sustained
• 1:46:58
  Speaker 1: high speed and carrying no way to slow down will
• 1:47:01
  Speaker 1: simply fly through the Neptune system and keep going. Every
• 1:47:05
  Speaker 1: mission that has entered orbit around a planet, Galileo at Jupiter,
• 1:47:09
  Speaker 1: Cassini at Saturn, Juno, Messenger, had to slow down on arrival.
• 1:47:14
  Speaker 1: Slowing down requires fuel, or an atmosphere to break against,
• 1:47:18
  Speaker 1: or both. Neptune has no solid surface and only a
• 1:47:22
  Speaker 1: thin upper atmosphere. Slowing an incoming spacecraft enough to achieve
• 1:47:26
  Speaker 1: orbit requires either a massive fuel load or years of
• 1:47:30
  Speaker 1: gentle deceleration, neither of which pairs well with the goal
• 1:47:33
  Speaker 1: of getting there faster. So the honest accounting looks like this.
• 1:47:38
  Speaker 1: The fastest object humanity has ever built hit 430,000 miles
• 1:47:44
  Speaker 1: per hour for a brief period while falling around the Sun. Sustained.
• 1:47:49
  Speaker 1: Practical interplanetary cruise speeds for the best missions ever flown
• 1:47:54
  Speaker 1: have been closer to 35,000 to 40,000 miles per hour.
• 1:47:59
  Speaker 1: At those speeds, Neptune takes 12 years. At Parker's peak speed,
• 1:48:05
  Speaker 1: Neptune would still take somewhere in the range of 13
• 1:48:08
  Speaker 1: to 15 years by most practical calculations that account for
• 1:48:13
  Speaker 1: the Sun's gravity slowing a spacecraft as it travels outward.
• 1:48:17
  Speaker 1: The improvement is real, but not transform. The fastest thing
• 1:48:22
  Speaker 1: we have ever built would still take more than a
• 1:48:24
  Speaker 1: decade to reach Neptune, and that is the most honest
• 1:48:28
  Speaker 1: summary of where interplanetary travel currently stands. Every travel time
• 1:48:33
  Speaker 1: in this conversation is built on the same foundation– chemical rockets.
• 1:48:38
  Speaker 1: The same basic propulsion technology that humanity has been using
• 1:48:42
  Speaker 1: since the first ballistic missiles of the 1940s. A chemical
• 1:48:47
  Speaker 1: rocket burns fuel, expands gas, and pushes itself forward by
• 1:48:52
  Speaker 1: throwing mass backward. That is the entire principle. It has
• 1:48:56
  Speaker 1: not changed in 80 years. And for reaching orbit, for
• 1:49:00
  Speaker 1: escaping Earth's gravity, chemical propulsion is genuinely impressive. The energy
• 1:49:06
  Speaker 1: density of liquid hydrogen and liquid oxygen is extraordinary by
• 1:49:11
  Speaker 1: any measure that mattered to engineers in the 20th century.
• 1:49:14
  Speaker 1: But the solar system does not care about what impressed
• 1:49:18
  Speaker 1: engineers in the 20th century. The distances are simply too large,
• 1:49:22
  Speaker 1: and chemical rockets reach their limits fast. The core problem
• 1:49:27
  Speaker 1: is that a chemical rocket can only burn for minutes.
• 1:49:31
  Speaker 1: It fires, it accelerates, and then it coasts. Everything after
• 1:49:36
  Speaker 1: the burn is freefall, the spacecraft falling along a curve
• 1:49:40
  Speaker 1: dictated by gravity, unable to speed up further unless it
• 1:49:43
  Speaker 1: fires again. And firing again means carrying more fuel, which
• 1:49:48
  Speaker 1: means the rocket has to be larger to lift that fuel,
• 1:49:51
  Speaker 1: which means you need still more fuel to lift the
• 1:49:54
  Speaker 1: larger rocket. Engineers call this the tyranny of the rocket equation.
• 1:49:59
  Speaker 1: Every kilogram of fuel you add to a spacecraft costs
• 1:50:02
  Speaker 1: you more fuel to carry it. The math punishes you
• 1:50:05
  Speaker 1: for ambition. This is why Hohmann transfers are the dominant strategy.
• 1:50:10
  Speaker 1: Not because they are fast, but because they minimize the
• 1:50:13
  Speaker 1: fuel required to get anywhere at all. Chemical propulsion optimized
• 1:50:18
  Speaker 1: for efficiency rather than speed is the only version of
• 1:50:23
  Speaker 1: chemical propulsion that works across interplanetary distances. Ion propulsion breaks
• 1:50:29
  Speaker 1: that logic. Instead of burning a chemical fuel, An ion
• 1:50:33
  Speaker 1: engine takes a propellant, usually xenon gas, and ionizes it,
• 1:50:38
  Speaker 1: stripping electrons away to create charged particles, then accelerates those
• 1:50:43
  Speaker 1: particles through an electric field and ejects them at speeds
• 1:50:47
  Speaker 1: roughly 10 times higher than a chemical exhaust plume. The
• 1:50:51
  Speaker 1: thrust produced is tiny. At full power, the Dawn spacecraft's
• 1:50:55
  Speaker 1: ion engines generated about 90 millinewtons of force, roughly the
• 1:51:00
  Speaker 1: weight of a small sheet of paper resting on your hand.
• 1:51:04
  Speaker 1: That is not a misprint. But because the exhaust velocity
• 1:51:08
  Speaker 1: is so much higher, the engine extracts far more useful
• 1:51:12
  Speaker 1: work from every kilogram of propellant. And because the thrust
• 1:51:16
  Speaker 1: never stops, the spacecraft keeps accelerating. Over weeks, the small
• 1:51:21
  Speaker 1: push adds up. Over months, A spacecraft on ion propulsion
• 1:51:26
  Speaker 1: can reach speeds that would have required enormous fuel loads
• 1:51:29
  Speaker 1: from a chemical system. Dawn used three ion engines to
• 1:51:33
  Speaker 1: orbit Vesta in 2011, then break out of that orbit
• 1:51:37
  Speaker 1: and travel to Ceres, arriving in 2015. No chemical rocket
• 1:51:42
  Speaker 1: mission could have done both. The flexibility was the point.
• 1:51:46
  Speaker 1: Ion propulsion can redirect a spacecraft in ways that chemical
• 1:51:50
  Speaker 1: systems cannot afford. The limitation is that ion engines require
• 1:51:54
  Speaker 1: electrical power to run, which means large solar panels are
• 1:51:59
  Speaker 1: a nuclear power source, and the thrust is too gentle
• 1:52:02
  Speaker 1: to escape a planet's gravity well on its own. Ion
• 1:52:06
  Speaker 1: propulsion is a deep space technology, not a launch technology.
• 1:52:10
  Speaker 1: But for the cruise phase of long missions, the years
• 1:52:14
  Speaker 1: of travel between planets, it changes what is achievable. A
• 1:52:18
  Speaker 1: Mars mission using advanced ion propulsion could potentially cut transit
• 1:52:22
  Speaker 1: times to three or four months. That alone would transform
• 1:52:26
  Speaker 1: the calculus for crewed missions. Nuclear thermal propulsion pushes further.
• 1:52:32
  Speaker 1: The concept was seriously developed in the United States under
• 1:52:36
  Speaker 1: a program called NERVA, Nuclear Engine for Rocket Vehicle Application,
• 1:52:41
  Speaker 1: which ran from the late 1950s through 1972. The idea
• 1:52:46
  Speaker 1: is to use a nuclear reactor to heat a propellant,
• 1:52:49
  Speaker 1: usually hydrogen, to extremely high temperatures, then eject it through
• 1:52:53
  Speaker 1: a nozzle. No combustion, just heat. Because the exhaust temperatures
• 1:52:58
  Speaker 1: achievable with a reactor are roughly twice what chemical combustion
• 1:53:01
  Speaker 1: can reach, the exhaust velocity doubles and the efficiency roughly
• 1:53:06
  Speaker 1: doubles with it. Engineers project that a nuclear thermal rocket
• 1:53:10
  Speaker 1: could cut a Mars transit from eight months to somewhere
• 1:53:13
  Speaker 1: around four. The program was cancelled in 1972 as NASA's
• 1:53:18
  Speaker 1: budget collapsed after Apollo. The technology worked. The engines were
• 1:53:23
  Speaker 1: tested on the ground. They were never flown. Nuclear pulse
• 1:53:27
  Speaker 1: propulsion is something else entirely. Project Orion, studied seriously in
• 1:53:33
  Speaker 1: the late 1950s and early 1960s, proposed a spacecraft that
• 1:53:38
  Speaker 1: would detonate a series of small nuclear charges behind a
• 1:53:41
  Speaker 1: massive pusher plate. Absorbing each shock through a system of
• 1:53:45
  Speaker 1: hydraulic dampeners and converting the blast into forward momentum, the
• 1:53:50
  Speaker 1: numbers were startling. An Orion vehicle could potentially reach Mars
• 1:53:55
  Speaker 1: in weeks rather than months, and some designs suggested transit
• 1:53:59
  Speaker 1: times to the outer planets measured in months rather than decades.
• 1:54:03
  Speaker 1: Freeman Dyson, one of the physicists involved, described it as
• 1:54:07
  Speaker 1: the most capable large spacecraft ever seriously designed. It was
• 1:54:12
  Speaker 1: never built. The Partial Nuclear Test Ban Treaty of 1963,
• 1:54:18
  Speaker 1: which prohibited nuclear detonations in space, effectively ended it. The
• 1:54:23
  Speaker 1: physics was sound. The politics were not. Solar sails operate
• 1:54:28
  Speaker 1: on a completely different principle. Using the pressure of sunlight
• 1:54:32
  Speaker 1: itself as a propellant-free source of thrust, photons carry momentum,
• 1:54:38
  Speaker 1: and when they reflect off a large, thin surface, they
• 1:54:41
  Speaker 1: transfer that momentum to the sail. JAXA's ICARUS mission, launched
• 1:54:46
  Speaker 1: in 2010, demonstrated the technology in deep space, unfurling a
• 1:54:52
  Speaker 1: 14-meter sail and confirming measurable photon-driven acceleration. The thrust is
• 1:54:57
  Speaker 1: even smaller than an ion engine and falls off as
• 1:55:00
  Speaker 1: the spacecraft moves away from the sun. A solar sail
• 1:55:04
  Speaker 1: heading toward the outer planets becomes less effective the farther
• 1:55:08
  Speaker 1: it travels. Toward the inner solar system, toward Mercury or Venus,
• 1:55:13
  Speaker 1: solar sails become more interesting. But for reaching Jupiter or
• 1:55:17
  Speaker 1: Saturn in any reduced timeframe, sunlight is too faint to
• 1:55:21
  Speaker 1: do meaningful work. The most radical concept currently under discussion
• 1:55:26
  Speaker 1: is laser propulsion. The Breakthrough Starshot Initiative has proposed accelerating
• 1:55:31
  Speaker 1: extremely small, lightweight probes, chip-scale instruments attached to gossamer sails,
• 1:55:39
  Speaker 1: using a ground-based laser array powerful enough to push them
• 1:55:42
  Speaker 1: to 20% of the speed of light. At that velocity,
• 1:55:46
  Speaker 1: such a probe could reach the nearest star in roughly
• 1:55:49
  Speaker 1: 20 years. The same principle applied within the solar system
• 1:55:54
  Speaker 1: would reduce travel times to outer planets from years to hours.
• 1:55:58
  Speaker 1: But the probes would be microscopic. No scientific instrument of
• 1:56:02
  Speaker 1: any complexity could survive the acceleration. No crewed vehicle comes
• 1:56:07
  Speaker 1: anywhere close to being compatible with the concept. The technology
• 1:56:11
  Speaker 1: required for the ground-based laser array does not yet exist
• 1:56:15
  Speaker 1: at the necessary scale. Breakthrough Starshot is a serious research program,
• 1:56:22
  Speaker 1: but its practical applications to planetary exploration remain entirely theoretical.
• 1:56:28
  Speaker 1: What is actually flying today, on real missions, is chemical
• 1:56:32
  Speaker 1: propulsion and ion propulsion. Nuclear thermal has never left the ground.
• 1:56:38
  Speaker 1: Nuclear pulse was never built. Solar sails have demonstrated thrust
• 1:56:43
  Speaker 1: but not planetary missions. Laser propulsion exists on paper. The
• 1:56:48
  Speaker 1: gap between what is theoretically possible and what is operationally
• 1:56:52
  Speaker 1: available is measured in decades of engineering, funding and political will.
• 1:56:58
  Speaker 1: Until one of those technologies makes it off a drawing
• 1:57:01
  Speaker 1: board and onto a launch pad, the answer to how
• 1:57:04
  Speaker 1: long it takes to reach any planet will continue to
• 1:57:06
  Speaker 1: be measured not in weeks, but in years. If you
• 1:57:10
  Speaker 1: tried to plan a mission to every planet in the
• 1:57:12
  Speaker 1: solar system, one after another from closest to furthest, the
• 1:57:17
  Speaker 1: total time that comes back is not comfortable. Start with Mercury,
• 1:57:22
  Speaker 1: the closest planet to the Sun, the one that looks
• 1:57:25
  Speaker 1: like the obvious first stop, a direct flyby, the kind
• 1:57:29
  Speaker 1: Mariner 10 managed in 1973 takes roughly 147 days. But
• 1:57:36
  Speaker 1: a flyby gives you hours of useful data. If you
• 1:57:39
  Speaker 1: want real science, you need orbit. MESSENGER needed seven years.
• 1:57:44
  Speaker 1: BepiColombo needed eight. The reason is the gravity well. Mercury
• 1:57:49
  Speaker 1: sits deep inside the Sun's pull, moving fast, and to
• 1:57:53
  Speaker 1: match its speed, you have to shed an enormous amount
• 1:57:56
  Speaker 1: of energy through a chain of planetary flybys before you
• 1:57:59
  Speaker 1: can even attempt orbital insertion. So the first planet alone,
• 1:58:04
  Speaker 1: if you are doing it properly, consumes the better part
• 1:58:06
  Speaker 1: of a decade before you are ready to move on.
• 1:58:10
  Speaker 1: Venus is next. The transit time, measured cleanly, runs between
• 1:58:15
  Speaker 1: 100 and 150 days. That part is fast. But Venus
• 1:58:20
  Speaker 1: is not just a destination. It is a tool, a
• 1:58:23
  Speaker 1: gravitational waypoint that spacecraft have been borrowing from for 50 years.
• 1:58:29
  Speaker 1: Cassini swung past Venus twice before heading towards Saturn. Parker
• 1:58:34
  Speaker 1: Solar Probe used it seven times. The question of how
• 1:58:37
  Speaker 1: long Venus takes depends entirely on what you are using
• 1:58:40
  Speaker 1: it for and where you need to go next. If
• 1:58:43
  Speaker 1: you are continuing outward through the solar system, you may
• 1:58:46
  Speaker 1: find that Venus is most valuable as a speed adjustment,
• 1:58:50
  Speaker 1: and adjusting speed through gravity takes time. It takes months
• 1:58:54
  Speaker 1: of carefully timed approach trajectories. It takes geometry that only
• 1:58:59
  Speaker 1: lines up under specific conditions. Mars comes after that. The
• 1:59:04
  Speaker 1: average Hohmann transfer runs about 259 days, just over eight months.
• 1:59:10
  Speaker 1: Perseverance covered it in roughly seven. That sounds manageable until
• 1:59:14
  Speaker 1: you account for the launch window. Earth and Mars are
• 1:59:17
  Speaker 1: lined for efficient transfers only once every 26 months. Miss
• 1:59:21
  Speaker 1: that window by a week and you wait more than
• 1:59:24
  Speaker 1: two years for the planets to come back around. A
• 1:59:27
  Speaker 1: mission building an itinerary across the entire solar system cannot
• 1:59:31
  Speaker 1: simply leave Mars when it is finished. It has to
• 1:59:34
  Speaker 1: leave when the orbital geometry permits, or the energy cost
• 1:59:38
  Speaker 1: to continue becomes prohibitive. So Mars adds not just eight
• 1:59:43
  Speaker 1: months of transit, but potentially another year or more of
• 1:59:47
  Speaker 1: waiting in place before the next leg becomes feasible. Then
• 1:59:51
  Speaker 1: the scale shifts. Past the asteroid belt, the solar system
• 1:59:56
  Speaker 1: stops operating on the timescales that feel like recognizable human planning.
• 2:00:01
  Speaker 1: Jupiter sits more than five times as far from the
• 2:00:04
  Speaker 1: Sun as Earth. The fastest missions reached it in about
• 2:00:08
  Speaker 1: 18 months. Galileo, which actually entered orbit, took six years.
• 2:00:14
  Speaker 1: Juno took five. If the goal is a serious orbiting mission,
• 2:00:18
  Speaker 1: the minimum realistic figure is somewhere in that five to
• 2:00:21
  Speaker 1: six year range. A flyby shaves time, but a flyby
• 2:00:25
  Speaker 1: gives you a few hours of data at closest approach,
• 2:00:29
  Speaker 1: and then the planet is behind you and receding. The
• 2:00:32
  Speaker 1: Voyager spacecraft were flybys, and they changed our understanding of
• 2:00:37
  Speaker 1: the solar system permanently. But they answered certain questions while
• 2:00:41
  Speaker 1: leaving others completely open. Saturn had seven more years. Cassini's trajectory.
• 2:00:49
  Speaker 1: which remains the definitive example of how you reach Saturn
• 2:00:52
  Speaker 1: with current propulsion technology, consumed six years and nine months
• 2:00:57
  Speaker 1: from launch to orbital insertion. That included two Venus flybys,
• 2:01:02
  Speaker 1: one Earth flyby, and one Jupiter flyby. Each of those
• 2:01:06
  Speaker 1: encounters had to hit its timing within seconds. Each one
• 2:01:10
  Speaker 1: added months to the calendar. There is no shortcut available
• 2:01:14
  Speaker 1: with chemical propulsion. The energy required to reach Saturn directly
• 2:01:19
  Speaker 1: without gravity assists would demand a rocket beyond anything ever built.
• 2:01:24
  Speaker 1: So you take the seven years, you build in the
• 2:01:26
  Speaker 1: planetary encounters, you accept that everyone who designed the spacecraft
• 2:01:31
  Speaker 1: will be considerably older by the time it starts returning science.
• 2:01:36
  Speaker 1: Uranus sits roughly twice as far from Earth as Saturn does.
• 2:01:40
  Speaker 1: Voyager 2 reached it in eight and a half years
• 2:01:43
  Speaker 1: after leaving Earth. But Voyager 2 was on a trajectory
• 2:01:47
  Speaker 1: optimized by an extraordinarily rare planetary alignment. Current estimates for
• 2:01:53
  Speaker 1: a dedicated Uranus orbiter, using the best gravity assist options
• 2:01:57
  Speaker 1: available in the near future, land in the range of
• 2:02:00
  Speaker 1: 13 to 15 years of transit time. There is no
• 2:02:05
  Speaker 1: mission currently flying toward Uranus. There is no mission in
• 2:02:09
  Speaker 1: approved development flying toward Uranus. The planet has had one
• 2:02:13
  Speaker 1: visitor in the entire history of space exploration, and that
• 2:02:17
  Speaker 1: visitor spent a matter of hours in its vicinity before
• 2:02:20
  Speaker 1: continuing on. Neptune takes 12 years at minimum. Voyager 2
• 2:02:26
  Speaker 1: required exactly that. Launched in 1977, arriving in 1989, having
• 2:02:32
  Speaker 1: used Jupiter, Saturn and Uranus as successive slingshots along the way.
• 2:02:37
  Speaker 1: That planetary alignment that made Voyager 2's trajectory possible occurs
• 2:02:42
  Speaker 1: once every 175 years. Outside of those rare windows, a
• 2:02:48
  Speaker 1: mission to Neptune demands either a very long time or
• 2:02:52
  Speaker 1: a very large rocket, and usually both. No orbiter has
• 2:02:56
  Speaker 1: ever been sent. No lander has ever been proposed in
• 2:03:00
  Speaker 1: any serious funded capacity. Neptune remains, in practical mission planning terms,
• 2:03:06
  Speaker 1: almost unreachable with current technology on any timeline that fits
• 2:03:10
  Speaker 1: within a single human career. Add those numbers together. Mercury,
• 2:03:15
  Speaker 1: perhaps 8 years. Venus adds a few months to the
• 2:03:18
  Speaker 1: baseline but reshapes every trajectory that follows. Mars adds 8
• 2:03:23
  Speaker 1: months of transit and potentially 2 years of waiting. Jupiter
• 2:03:27
  Speaker 1: adds at minimum 5 years. Saturn adds 7. Uranus adds
• 2:03:32
  Speaker 1: 13 to 15. Neptune adds 12. A sequential mission touching
• 2:03:38
  Speaker 1: every planet as an orbiter. with realistic waiting periods at
• 2:03:42
  Speaker 1: each destination, runs well past 60 years of total elapsed time.
• 2:03:47
  Speaker 1: Even a stripped-down sequential flyby mission, no orbital insertion at
• 2:03:53
  Speaker 1: any planet, designed purely for speed, passes each world once
• 2:03:58
  Speaker 1: and moves on. That version, optimized aggressively, still takes more
• 2:04:03
  Speaker 1: than 30 years to reach Neptune. The numbers are not
• 2:04:06
  Speaker 1: going to improve meaningfully until the fundamental technology changes, and
• 2:04:11
  Speaker 1: the technology has not changed in any essential way since
• 2:04:15
  Speaker 1: Mariner 4 became the first spacecraft to reach Mars in 1965.
• 2:04:21
  Speaker 1: The solar system looks small in the diagrams you find
• 2:04:24
  Speaker 1: in textbooks. Eight planets arranged in a tidy row, everything labeled,
• 2:04:30
  Speaker 1: distances implied, but never felt. The planets sit there as
• 2:04:34
  Speaker 1: coloured dots, a few inches apart, and your eye moves
• 2:04:38
  Speaker 1: across the whole system in a second. What those diagrams
• 2:04:42
  Speaker 1: cannot show you is that the gap between Earth and Neptune,
• 2:04:46
  Speaker 1: rendered to actual scale, would leave most of the page blank.
• 2:04:50
  Speaker 1: The planets would be invisible. There would be nothing to
• 2:04:53
  Speaker 1: see but empty space. Every mission described in this video
• 2:04:58
  Speaker 1: required engineers and scientists to spend years of their working
• 2:05:02
  Speaker 1: lives staring at that emptiness. They sent a spacecraft across
• 2:05:06
  Speaker 1: it and then waited. Waited for signals traveling at the
• 2:05:10
  Speaker 1: speed of light to make the crossing. Minutes for the
• 2:05:13
  Speaker 1: inner planets, hours for the outer ones. If something went wrong,
• 2:05:18
  Speaker 1: the spacecraft was already dead before any response could arrive.
• 2:05:22
  Speaker 1: The teams on the ground knew this. They built their
• 2:05:25
  Speaker 1: machines to survive alone, to make decisions without them, and
• 2:05:30
  Speaker 1: then they let go. The human dimension of this is
• 2:05:33
  Speaker 1: worth sitting with. The engineers who designed Cassini in the
• 2:05:37
  Speaker 1: mid-1980s were, in some cases, retired by the time it
• 2:05:41
  Speaker 1: entered Saturn orbit in 2004. They spent careers on a
• 2:05:45
  Speaker 1: single machine and watched it succeed from a distance measured
• 2:05:49
  Speaker 1: in years. The scientists who built Voyager 2 were young
• 2:05:53
  Speaker 1: researchers in the early 1970s. They were middle-aged when it
• 2:05:57
  Speaker 1: reached Neptune in 1989. Some of them did not live
• 2:06:01
  Speaker 1: to see it cross into interstellar space in 2018, still transmitting,
• 2:06:06
  Speaker 1: still moving outward. The distances are not abstract numbers in
• 2:06:10
  Speaker 1: a table. They are a measure of how much human
• 2:06:13
  Speaker 1: time it costs to cross them. Careers. Decades. In some cases,
• 2:06:18
  Speaker 1: entire lives. None of that has changed. The same orbital
• 2:06:23
  Speaker 1: mechanics that governed Mariner 10 govern BepiColombo today. The same
• 2:06:29
  Speaker 1: vast stretches of nothing that Voyager crossed are still out there, unchanged,
• 2:06:34
  Speaker 1: indifferent to how much faster our computers have become. The
• 2:06:38
  Speaker 1: planets are still out there, still moving on their orbits,
• 2:06:43
  Speaker 1: still separated from us by distances that defeat any intuitive
• 2:06:47
  Speaker 1: sense of scale, and the fact that we have reached
• 2:06:49
  Speaker 1: all of them even briefly, even once, remains one of
• 2:06:53
  Speaker 1: the stranger things humanity has ever managed to do.