Latest / Talking Science / Comets and Asteroids – Visitors from the Deep Past

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Comets and Asteroids – Visitors from the Deep Past · Talking Science

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so i want you to imagine that you are building
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something enormous let's say um you're building
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a massive house right okay a huge house got it
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yeah and you spend months on it you do the foundation
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the framing the roof but when the house is finally
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finished You look around the yard and it is just
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completely littered with offcuts. Right. Like
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leftover bricks, bent nails deep in the grass,
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that kind of thing. Exactly. Piles of sawdust,
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bits of timber that were just, you know, a little
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too small to actually use in the framework. Well,
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when you look up at the night sky, our solar
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system is actually a lot like that cluttered
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yard. It really is. I mean, about four and a
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half billion years ago, you had this vast, chaotic
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cloud of gas and dust. And it collapsed, swirled
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into a giant disk, and the center ignited into
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our sun. And the rest of the material kind of
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clumped together to form the planets. Right.
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But just like building that house, not absolutely
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everything got used. There were billions of leftover
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scraps that were just... never gathered up into
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a finished world. Which brings us to the mission
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of our deep dive today. We are going to explore
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these ancient planetary offcuts. We always talk
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about the big finished products, the planets,
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the moons, the sun. But to truly understand the
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environment you live in, you have to look at
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the leftovers. Those drifting ancient time capsules
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that have basically been sitting in the deep
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freeze of space since the dawn of the solar system.
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Because they hold the original blueprints, right?
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Like the untouched materials. So we're going
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to untangle the differences between them, debunk
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some massive Hollywood myths, look at the existential
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threats they pose, and spoiler alert, how they
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might be responsible for our very existence.
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It's a wild story. And the best place to start
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is with the off - closest to home, the rocky
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ones, which we generally call asteroids. Right.
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So asteroids are essentially chunks of rock or
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metal. They vary wildly in size from a tiny pebble
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to a behemoth hundreds of kilometers across.
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And most of them hang out in the main asteroid
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belt, which is that wide zone between Mars and
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Jupiter. They do. And their composition actually
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tells us a lot about the early solar system's
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temperature. They formed closer to the sun inside
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what we call the frost line. The frost line,
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so it's too hot for ice. Exactly. It was simply
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too hot for volatile compounds like water or
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methane to remain solid. So the only things that
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could condense and survive the heat were heavy
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refractory materials, basically, metals and silicates.
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That's why asteroids are rocky. Okay, that makes
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sense. Before we go any further, I have to tackle
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the Hollywood myth of the asteroid belt because,
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honestly, it drives me completely crazy. Oh,
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I know exactly what you're going to say. Right.
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Because in basically every sci -fi movie, a spaceship
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flies into an asteroid field, alarms are blaring,
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the pilot is sweating, and they're frantically
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weaving through this dense field of tumbling
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boulders that are constantly smashing into each
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other. That is a massive exaggeration, isn't
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it? It is an entire cinematic invention. Space
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is, above all things, unf— Fathomably roomy.
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Roomy. I like that. Yeah, I mean, while there
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are millions of asteroids in the main belt, they
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are spread across such a staggering volume of
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space that the belt is practically empty. To
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give you a sense of scale, if you took every
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single asteroid in the main belt, right, and
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you crushed them all together into one single
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object, that object would still be smaller than
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our moon. Wait, really? The entire asteroid belt
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combined is smaller than the moon. Considerably
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smaller. It's actually only about 4 % of the
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moon's mass. That is mind -blowing. So if you
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were standing on the surface of an asteroid in
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the main belt, you very likely wouldn't even
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be able to see the next nearest asteroid without
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a high -powered telescope. Wow. The average distance
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between them is hundreds of thousands of kilometers.
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You would just be standing there completely alone
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in the dark. Okay, well, that completely reframes
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the picture for me, but it also brings up a pretty
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strange contradiction, I think. How so? Well,
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if there is enough material out there to form
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something, even if it's smaller than the moon,
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why didn't it clump together over 4 billion years?
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Like, why is it just millions of scattered shards
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instead of a dwarf planet? Yeah, that's where
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the unseen architectural force of the solar system
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comes in gravity. And in this case, the architect
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was Jupiter. The gas giant. Yeah, Jupiter is
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an absolute monster. As it orbits the sun, its
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immense gravitational pull acts almost like a
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whisk. It creates these things called orbital
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resonances. Orbital resonances, so it's like
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a rhythm. Exactly. Jupiter's gravity periodically
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lines up with the orbits of these rocks, giving
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them a regular, repeating gravitational kick.
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Oh, I see. So if the asteroid belt is a bunch
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of wet cement and bricks trying to set, Jupiter
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is like a giant restless dog constantly running
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through it, kicking up the materials and making
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sure it never actually solidifies. That is a
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great analogy. It captures the disruption perfectly.
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To form a planet by accretion, you need a relatively
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calm environment. Right, where things just sort
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of... gently drift together. Exactly. Where dust
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and rocks can gently bump into each other and
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stick, gradually building up mass and gravity.
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But Jupiter's resonant kicks accelerated those
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rocky chunks to super high velocities. So they
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were moving too fast to stick. Much too fast.
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When they intersected, they weren't gently bumping.
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They were slamming into each other at thousands
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of miles an hour. They shattered instead of clumping.
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Jupiter effectively weaponized the asteroids
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against each other. It just bullied them into
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staying shattered. Okay, so Jupiter's gravity
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kept the rocky scraps near us from forming a
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cohesive world. But what about the scraps that
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were pushed way, way out, like far past Jupiter,
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into the freezing dark edges of the solar system?
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Well, that isolated freezing environment is what
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gives us comets. Because they form so far from
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the sun, well beyond that frost line we just
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talked about, materials like water, ammonia,
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and methane. could freeze completely solid. I've
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heard them called dirty snowballs. Yeah, astronomers
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use that term a lot, and it's a fairly accurate
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description of a comet's core. It's a messy amalgamation
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of ice, frozen gases, dust, and dark rock. And
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their isolation is just staggering. I know they
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hang out in two main reservoirs. You've got the
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Kuiper belt, which is a thick wing of icy bodies
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just beyond Neptune. But then there's the Oort
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cloud, which is almost hard to even comprehend.
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The Oort cloud really requires a massive leap
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in how we visualize our solar system. It isn't
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a flat ring like the asteroid belt or the Kuiper
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belt. It is a vast spherical shell of icy debris
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that completely encapsulates. the entire solar
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system like a giant bubble right and it extends
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so far out into interstellar space that its outer
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edge reaches almost halfway to the nearest star
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That's insane. It is. A single comet originating
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from the outer Oort cloud might take millions
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of years to complete just one agonizingly slow
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orbit around the sun. Millions of years for one
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single lap, just drifting in the absolute deep
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freeze. But, you know, here's where it gets dynamic
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out there. A comet is basically a dark, invisible
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lump of ice doing absolutely nothing. But occasionally,
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a gravitational nudge sends one of these frozen
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lumps falling inward, right toward the inner
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solar system. And when it gets close to the sun...
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It puts on a massive show. This is where we see
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the transition from a dormant snowball to an
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active comet. As it crosses the orbit of Mars
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and approaches the sun, solar radiation rapidly
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heats the comet's surface. But wait, in the vacuum
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of space, liquid water can't exist, right? So
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it can't just melt. Exactly. The ice doesn't
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melt. It sublimates. It flashes directly from
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a solid ice into a gas. Just like dry ice at
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a Halloween party. Precisely like dry ice. And
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this violent transition releases this massive
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expanding cloud of gas and dust that totally
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shrouds the tiny solid core. Yeah. We call that
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glowing fuzzy envelope the coma. The coma. And
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streaming off of that, of course, is the famous
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comet tail. Now, I want to pause here for a second
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because the physics of that tail completely brought
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on my brain when I first learned about it. It's
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very counterintuitive. It really is. Because
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a comet's tail does not behave like a trail of
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smoke. behind a speeding train. No, it doesn't.
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And that's because our intuition is entirely
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based on atmospheric drag. You know, on Earth,
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if you run with a sparkler, the smoke and sparks
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trail behind you because you're pushing through
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air and the air pushes the smoke back. Right.
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But space is a vacuum. Exactly. There is no air
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resistance to drag the tail behind the comet.
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Instead, the tail is entirely dictated by the
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sun. And to be precise, a comet usually has two
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distinct tails. Two tails. Yes. As the ice sublimates
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and releases dust and gas, those two materials
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actually react differently. The heady dust particles
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form a glowing, slightly curved tail that is
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physically pushed outward by the radiation pressure
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of sunlight itself. Wait, sunlight actually exerts
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pressure. It does. Photons from the sun are literally
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pushing the dust away. But then there is the
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gas, which gets ionized by the sun's ultraviolet
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light. This creates an ion tail, which gets caught
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in the solar wind. And the solar wind is what,
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like a stream of particles? Yeah, charged particles
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constantly blowing out from the sun in all directions.
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So that ion tail shoots straight out, directly
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away from the sun, like a windsock in a hurricane.
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Okay, wait, wait, wait. So if both the dust tail
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and the ion tail are always being blown away
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from the sun, that means when the comet hits
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its closest point... swings around the sun, and
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starts heading back out into deep space, the
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tail is actually leading the way. Yes, visually
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it is. It's flying through space looking like
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it's going backwards. It's a bizarre visual,
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isn't it? As the comet retreats back toward the
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Kuiper Belt, the solar wind is still blowing
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from the center of the solar system outward,
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so it is constantly pushing that coma material
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ahead of the comet's rocky core. That is just
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incredible. It perfectly illustrates how solar
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radiation totally overrides the comet's own momentum.
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But as beautiful as these mechanisms are, we
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do have to talk about the darker side of this
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debris field. Ah, yes, the impacts. Right, because
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as long as these rocky and icy offcuts stay in
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their distant belts and clouds, they're just
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beautiful curiosities. But the story turns incredibly
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dramatic when one of those orbits crosses the
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path of a planet. And we know with absolute certainty
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that Earth's path has been crossed many times.
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The most famous one, of course, occurred 66 million
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years ago. R .I .P. the dinosaurs. Exactly. It
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ended the 150 million year reign of the non -avian
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dinosaurs. A rocky asteroid, roughly 10 kilometers
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across, struck the shallow sea in what is now
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the Yucatan Peninsula in Mexico. Okay, I have
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to push back on the scale here for a minute because
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I think it's something people really struggle
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to visualize. Sure. A 10 -kilometer rock is like
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the size of a small city. To a person, that is
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massive. But compared to the Earth, which is
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over 12 ,000 kilometers in diameter, a 10 -kilometer
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rock is quite literally a speck of dust on a
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bowling ball. That's a fair comparison. So how
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does a single localized impact from a speck manage
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to darken the skies of the entire planet and
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trigger a mass extinction? It just seems mathematically
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disproportionate. It does seem disproportionate
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until you factor in kinetic energy. The destructiveness
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of an impact isn't just about the mass of the
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object. It's heavily dependent on velocity. The
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equation for kinetic energy squares the velocity.
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Oh, wow. So speed is the real killer. Exactly.
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And this asteroid was likely traveling at over
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70 ,000 kilometers per hour. Yeah. When it slammed
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into the Earth's crust and halted in a fraction
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of a second, all of that orbital momentum was
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instantly converted into heat and shock waves.
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So it's not a physical puncture at all. It's
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a colossal explosion. A massive one. The energy
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released was roughly equivalent to 10 billion
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World War II -era atomic bombs detonating simultaneously.
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10 billion? I can't even fathom that. It's unfathomable.
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But the size and speed were actually only half
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the problem. The asteroid was also geologically
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incredibly unlucky in where it hit. Really? Location
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mattered that much? Oh, massively. The bedrock
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in the Yucatan Peninsula is rich in gypsum and
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anhydrite, which are minerals packed with sulfur.
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The immense heat of the impact instantly vaporized
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the asteroid and billions of tons of this bedrock.
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So it blasted all that vaporized rock and sulfur
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into the sky. Right. Pulverized rock, soot, and
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sulfur aerosols were blasted high into the stratosphere
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and even out into space. And because it's in
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the stratosphere, which is above the weather,
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it doesn't just rain out immediately, right?
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It spreads globally. Yes. Those sulfur aerosols
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enveloped the entire globe, creating a dense
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haze that reflected incoming sunlight back into
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space. The Earth's surface was plunged into a
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sudden, freezing darkness that lasted for years.
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And without sunlight, the plants die. Exactly.
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Photosynthesis ground to a halt. The base of
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the food web collapsed, taking the dinosaurs
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and roughly 75 % of all plant and animal species
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on Earth with it. And the wild part is, the evidence
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for this isn't just theoretical modeling. We
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can literally see the chemical fingerprint today.
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We can. Aside from mapping the buried 150 kilometer
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wide Chicxulub crater, geologists have found
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a distinct boundary line in rock formations all
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over the world, dating to exactly 66 million
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years ago. The K -T boundary, right? Yes, the
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K -T boundary. It's a thin layer of play that
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is highly enriched in iridium. Now, iridium is
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an element that is exceedingly rare on Earth's
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crust, but very abundant in asteroids. So it's
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the literal fallout ash of an apocalypse just
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preserved in stone all over the globe. Precisely.
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Which obviously instantly raises the most terrifying
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question for anyone listening. Could this happen
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to us? Could a city -sized rock be silently hurtling
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toward us right now? Well, the physics of the
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solar system haven't changed, so impacts are
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a natural part of our environment. But humanity
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occupies a unique position. Unlike the dinosaurs,
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we aren't just looking down at the grass. We
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are actively looking up. Right. We have transitioned
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from simply being at the mercy of the cosmos
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to actively surveying it. Astronomers are constantly
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running systematic surveys to catalog what they
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call near -Earth objects or NEOs. Exactly. They
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are continuously mapping the trajectories of
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these objects and projecting their orbits decades
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into the future. And the reassuring news is that
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the truly civilization -ending giants, the 10
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-kilometer behemoths, are relatively rare. And
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they're big enough to spot, right? Crucially,
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yes. They are bright enough to be easily spotted.
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NASA estimates we have found over 90 % of the
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near -Earth objects that size, and none of them
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are on a collision course for the foreseeable
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future. Okay, that is definitely a relief, though
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I imagine the smaller ones, the ones that could
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take out a city rather than a whole species,
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those are much harder to track. They are numerous
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and definitely harder to spot. But let's say
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we did spot one. Let's say 20 years from now,
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astronomers find a sizable rock with Earth's
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name on it. What do we actually do? I mean, we
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aren't calling Bruce Willis to drill into it
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with nuclear bombs, right? No, absolutely not.
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Exploding an asteroid is a terrible idea because
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then instead of one giant bullet heading for
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Earth, you just have a massive shotgun blast
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of radioactive debris heading for Earth. Not
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ideal. No. The actual strategy relies on orbital
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mechanics. And in 2022, humanity proved we could
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do it with a mission called DART. The Double
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Asteroid Redirection Test. I absolutely love
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this concept. We essentially played cosmic billiards.
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We aren't trying to blow it up. We just crashed
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a spacecraft into a rock to slightly nudge it
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off course. That's the basic premise. But how
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they measured the success is the truly ingenious
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part. Oh, because it's hard to tell if you moved
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it. Right. If you just crash a probe into a solitary
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asteroid orbiting the sun, a tiny shift in its
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massive multi -year orbit would take a very,
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very long time to measure from Earth. So NASA
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targeted a binary asteroid system. Okay, so there
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was a large asteroid called Didymos. Yes, and
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orbiting around it was a smaller moonlet called
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Dimorphos. So basically a rock orbiting a bigger
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rock. Precisely. NASA aimed a refrigerator -sized
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spacecraft. traveling at 13 ,000 miles per hour,
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and slammed it dead center into the smaller moonlet.
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Wow. And because Dimorphos was orbiting Didymos
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area 11 hours, astronomers on Earth could watch
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the moonlet pass in front of the larger asteroid,
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creating a mini eclipse. Oh, that's brilliant.
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By measuring the timing of those eclipses before
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and after the crash, they could immediately see
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if the moonlet's orbit had sped up or slowed
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down. Exactly. So they basically just checked
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its lap time. And it worked, didn't it? It did.
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The kinetic impact... physically shortened the
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moonlets orbit by over 30 minutes it transferred
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enough momentum to permanently alter a celestial
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body's path that is wild and if you scale that
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up if you hit an earthbound asteroid early enough
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a microscopic change in its trajectory will magnify
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over millions of miles of travel causing it to
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miss the earth entirely it is a profound milestone
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For the very first time in our planet's history,
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a species possesses the mechanical capability
16:57
to engineer its own survival against a cosmic
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impact. We actually learned how to move the scraps
17:04
around the yard. We did. It's an incredible achievement.
17:07
But you know, while we're figuring out how to
17:09
dodge the big rocks, Earth is actually constantly
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plowing through the tiny ones, right? And instead
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of causing a tragedy, they give us one of the
17:17
most beautiful light shows in nature. Yes. On
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a daily basis, the Earth's atmosphere sweeps
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up tons of cosmic dust and small pebbles. When
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you look up and see a shooting star, you're witnessing
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the violent end of a tiny piece of space debris
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plunging into the atmosphere at tens of thousands
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of miles an hour. And it's not just friction
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that makes them glow, right? It's something called
17:38
ram pressure. That's right. Because the rock
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is moving so fast that it violently compresses
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the air right in front of it. And when you compress
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gas that rapidly, it heats up to thousands of
17:48
degrees. That superheated air is what scorches
17:52
the rock and creates the glowing streak of a
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meteor. Exactly. Actually, terminology time,
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we should probably clear up the confusing naming
17:58
convention of these rocks because it always trips
18:00
people up. Good idea. A meteoroid is a small
18:04
chunk of rock or dust drifting in space. But
18:07
a meteor is when that chunk hits the atmosphere
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and burns up in a streak of light the shooting
18:12
star. And a meteorite is the surviving rock if
18:16
a piece is big enough to actually land on the
18:18
ground. So it's literally just a real estate
18:20
issue. Space, sky, or in your hand, location,
18:23
location, location dictates the name. That is
18:26
the perfect way to remember it. And the coolest
18:29
part of meteors is how they connect back to the
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comets we talked about earlier. Oh, right. The
18:33
meteor showers. Yes. This is a beautiful piece
18:36
of cosmic recycling. When a comet swings near
18:39
the sun and sublimates. Remember how we said
18:41
solar wind and radiation blow the dust and gas
18:44
away? Yeah, the two tails. Well, the heavier
18:46
dust particles tend to linger along the comet's
18:48
orbital path. The comet essentially litters its
18:51
entire elliptical lane with a tube of dirty debris.
18:55
And because the solar system is basically a giant,
18:57
predictable clockwork, sometimes Earth's orbit
19:00
directly intersects with one of those debris
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tubes. Exactly. When you hear about an annual
19:05
meteor shower, like the Perseids in August or
19:08
the Leonids in November, you are looking at the
19:10
calendar of Earth's highway intersections. Every
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year, at the exact same time, Earth drives headlong
19:17
into a dusty trail left behind by a specific
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comet's previous flyby. So we are basically driving
19:23
a car through a swarm of cosmic gnats. Those
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dozens of shooting stars lighting up the sky
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are quite literally the crumbs of an ancient
19:31
comet burning up in our air. That's a great visual.
19:33
And the parent comet might be totally invisible
19:35
to us, drifting back out in the deep freeze of
19:38
the Oort cloud, but its ghostly footprint is
19:40
still putting on a show for us here on Earth.
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It perfectly illustrates how dynamic the debris
19:45
field is. But, you know, as we wrap up this deep
19:47
dive, we really have to look at the most profound
19:50
mystery these scraps hold. Right, because it's
19:52
not just about how they light up our sky or even
19:55
how they threaten our future. It's about the
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active role they played at the very beginning
20:00
of Earth's history. Yes, because four billion
20:03
years ago, during a period called the Late Heavy
20:06
Bombardment, the solar system was incredibly
20:09
chaotic. The young Earth was getting constantly
20:12
hammered by comets and asteroids. And for a long
20:15
time, the scientific consensus was that icy comets
20:18
were the delivery vehicles that brought water
20:20
to a dry, molten Earth, slowly filling our oceans.
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Which, logically, makes a lot of sense. It made
20:27
perfect sense. Comets are full of ice. Earth
20:29
needed water. But recent advancements in studying
20:32
these offcuts have complicated that picture in
20:35
a really fascinating way. Because we aren't just
20:36
looking at them through telescopes anymore. Yeah,
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we've gone to them. Missions like OSIRIS -REx
20:41
and Hayabusa 2 have flown alongside them, landed
20:44
on them, them and physically scooped up pristine
20:46
samples from near -Earth asteroids, bringing
20:48
them back to labs. And when scientists analyzed
20:51
the chemical fingerprints of the water locked
20:53
inside those rocky asteroids, they found a startling
20:57
match, right? It comes down to isotopes. The
21:00
ratio of heavy water to normal water. Yes. Water
21:03
is H2O, but sometimes a hydrogen atom has an
21:06
extra neutron, making it heavy deuterium. The
21:10
ratio of deuterium to normal hydrogen acts like
21:13
a barcode. Okay. And when scientists measured
21:15
the barcode of the ice and comets, it didn't
21:18
match the water in Earth's oceans. But when they
21:20
measured the water locked inside rocky, carbon
21:22
-rich asteroids... The barcode was a near -perfect
21:25
match. Exactly. So the dry, rocky asteroids from
21:28
closer to home were actually the ones smuggling
21:30
the water in. It appears so. And not just water.
21:33
Those pristine asteroid samples were found to
21:36
contain complex organic molecules like amino
21:38
acids. The literal carbon -based building blocks
21:41
of biology. Yes. The implications are staggering.
21:45
During the brutal violence of the early solar
21:47
system, these ancient scraps were relentlessly
21:49
colliding with our planet. But in doing so, they
21:52
may have seeded the barren crust of the exact
21:54
chemical cocktail required for life to begin.
21:57
It is the ultimate poetic irony. The exact same
22:00
type of space rocks that darkened the skies,
22:02
triggered global winters, and ended the 150 million
22:06
year reign of the dinosaurs. Might have also
22:09
delivered the raw ingredients that made human
22:10
life possible in the first place. They are the
22:13
destroyers and the creators of worlds. Not bad
22:16
for a pile of leftover offcuts in the cosmic
22:19
yard. They really are the alpha and the omega
22:21
of our planetary story. Understanding them isn't
22:25
just about protecting our future from an impact.
22:27
It's about untangling the mystery of our own
22:30
genesis. I couldn't have said it better myself.
22:32
So here's a final provocative thought for you
22:34
to mull over. The next time you walk into your
22:36
kitchen and pour yourself a glass of water from
22:38
the tap, Or, hey, even just look down at the
22:41
skin on your own hands. Think about where those
22:43
molecules might have originally come from. It's
22:45
a crazy thought. Before they were a part of you,
22:47
before they were even part of the Earth, they
22:49
might have spent billions of years drifting in
22:51
the chaotic, freezing edges of the solar system.
22:54
You might just be built out of the very same
22:56
space scraps we've been talking about today.
22:59
Until next time on the Deep Dive, keep looking
23:02
up.