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Time Dilation & Cosmic Mysteries: Unpacking Your Space Queries · Space Nuts: Astronomy Insights & Cosmic Discoveries

• 0:00
  Andrew Dunkley: Hi there. Thanks for joining us again. This
• 0:02
  is Space Nuts, a Q and A edition where we,
• 0:05
  uh, get questions, we look at them and go,
• 0:08
  no clue. No, I haven't got any idea what
• 0:10
  we're doing. So we're not doing that today.
• 0:13
  Professor Fred Watson: Actually, we will.
• 0:14
  Andrew Dunkley: Uh, we've got a question from Ron, uh, asking
• 0:17
  about, uh, astronomy in regard to
• 0:20
  a super fast moving planet. How would that
• 0:22
  work if you were living on it, is what he's
• 0:24
  saying. Uh, Jeff wants to know about photon
• 0:27
  pressure. Um, Brian is asking
• 0:30
  about, um, what a black hole
• 0:33
  covers up. Interesting.
• 0:35
  And when is a moon no longer a
• 0:38
  moon? Bob asks. We'll see if we can answer
• 0:41
  all of those questions on today's episode of
• 0:43
  space nuts.
• 0:45
  Professor Fred Watson: 15 seconds.
• 0:46
  Guidance is internal. 10,
• 0:48
  9. Ignition sequence start.
• 0:51
  Professor Fred Watson: Space nuts.
• 0:52
  Professor Fred Watson: 5, 4, 3, 2.
• 0:53
  1, 2, 3, 4, 5, 5, 4, 3,
• 0:56
  2, 1.
• 0:57
  Andrew Dunkley: Space nuts.
• 0:58
  Professor Fred Watson: Astronauts report it feels good.
• 1:01
  Andrew Dunkley: He's back for more. His name is Professor
• 1:03
  Fred Watson Watson, astronomer at large.
• 1:05
  Hello, Fred Watson.
• 1:06
  Professor Fred Watson: Hello, Andrew. Uh, it's good to be back for
• 1:09
  more.
• 1:09
  Andrew Dunkley: It is. I'm trying to figure out why your
• 1:12
  parents gave you the first name Professor.
• 1:14
  That seems a little.
• 1:17
  Professor Fred Watson: I thought so too. It could have been, sir.
• 1:22
  Andrew Dunkley: I was probably not as bad as those parents
• 1:24
  in. I think it was Sweden or somewhere who
• 1:26
  named their kid Megadeth. Well,
• 1:29
  yes, and the government tried
• 1:32
  to stop them and they went to court and they
• 1:34
  won.
• 1:35
  Professor Fred Watson: Yes, I believe I remember that.
• 1:37
  Andrew Dunkley: Yeah, Quite a while back now. But m.
• 1:40
  That's beside the point. Uh, we've got, uh,
• 1:43
  answers to questions or we've got questions
• 1:45
  we're hoping we can answer.
• 1:48
  When I say we, I'm talking about you.
• 1:53
  Professor Fred Watson: No pressure.
• 1:54
  Andrew Dunkley: No, none at all. Um, this first question is
• 1:57
  brilliant, though. Uh, hi there. From upstate
• 1:59
  New York. How hope all is well,
• 2:02
  uh, as you approach spring. It's not, Ron.
• 2:06
  Hay fever sucks. I,
• 2:09
  I played 36 holes of golf this week in a, in
• 2:11
  a major local tournament. And, um,
• 2:15
  I, I paid for it dearly.
• 2:18
  Um, and I still am. But,
• 2:21
  uh, yeah, it's a tough time of year for
• 2:23
  people who suffer hay fever here. I came
• 2:26
  second, by the way. Anyway, um, sorry,
• 2:29
  this is a bit wordy. He says with the
• 2:31
  information just released on the star S300,
• 2:34
  uh, one orbiting Sagittarius A being one of
• 2:36
  the fastest stars in the Milky Way. And he
• 2:39
  sent us a link about that. Moving at 8% the
• 2:42
  speed of light. That sounds like a lot.
• 2:43
  25,000 kilometres per second. I was
• 2:46
  wondering how this would affect an astronomer
• 2:49
  on a hypothetical planet, um,
• 2:52
  around that region, um, relative
• 2:55
  to an Outside observer, their clocks would
• 2:57
  uh, slow dramatically due to the high
• 2:59
  velocity and uh, the depth in
• 3:02
  the black hole's gravitational well at
• 3:05
  um, I can't say that word. Peri.
• 3:08
  Professor Fred Watson: Periapsis.
• 3:09
  Andrew Dunkley: Yes. Uh, then speed up as they
• 3:12
  reach apoapsis.
• 3:15
  Yes. Uh, would that not cause their
• 3:18
  measurements of time related events like
• 3:20
  orbital periods of nearby planets or
• 3:23
  stars to vary during their orbit
• 3:26
  of the black hole? If so, it's crazy to
• 3:29
  think how this would skew their science until
• 3:31
  they learned about relativity. Thanks very
• 3:34
  much for a great podcast. Always enlightening
• 3:37
  Ron. Thank you Ron.
• 3:38
  I love this question, it's really
• 3:41
  clever.
• 3:42
  Professor Fred Watson: Well it is. Um, and um,
• 3:45
  uh, Ron's exactly right.
• 3:49
  This thing is in
• 3:51
  a very very unusual place within
• 3:56
  orbiting Sagittarius, a star which is the 4
• 3:58
  million solar mass black hole at the centre
• 4:01
  of our galaxy. So um, exactly
• 4:04
  as he says, both the relativistic
• 4:06
  speed and um, by that I mean speed close to
• 4:09
  the speed of light. 8% is close to the speed
• 4:12
  of light compared with most of the speeds
• 4:14
  that we uh, encounter uh, in astronomy.
• 4:18
  Uh, um, it is uh,
• 4:20
  that that relativistic speed changes,
• 4:23
  gives you time dilation as does the
• 4:26
  gravitational well that um, this
• 4:29
  star orbits. I
• 4:32
  didn't um, Let me just have a look at
• 4:34
  that uh, link. I have looked at it before,
• 4:38
  um, but I can't remember how elliptical its
• 4:41
  orbit is. Um, it's
• 4:44
  basically very elongated and so
• 4:46
  at what is what Ron uh,
• 4:49
  correctly calls periapsis, um
• 4:53
  the apse closest to the galactic
• 4:56
  centre.
• 4:57
  Apsidies are uh, the points in an orbit
• 5:00
  which are either ah, the nearest or the
• 5:02
  furthest periapsis nearest apoapsis
• 5:04
  furthest away. Um, we talk about perigee and
• 5:07
  apogee for satellites around the Earth.
• 5:10
  Um, that's the way the
• 5:13
  terminology works. So um, you're
• 5:15
  going to have different phenomena uh
• 5:18
  depending on where you are because the
• 5:20
  velocity will vary as you
• 5:22
  go around this elongated orbit. That's
• 5:25
  Kepler's laws. They still hold at these
• 5:28
  uh, high gravitational intensities.
• 5:30
  Um, um, so the time
• 5:33
  dilation phenomena uh, and
• 5:36
  the sorts of things that you would experience
• 5:38
  are exactly ah as Ron said there'd be another
• 5:41
  phenomenon as well. Um, I mean
• 5:45
  being near a black hole is gonna be weird uh
• 5:47
  as to what your sky looks like because the
• 5:49
  black hole is relatively nearby, there's
• 5:52
  stuff swirling around it with the accretion
• 5:54
  disc. It's kind of hard to imagine yourself
• 5:56
  in that situation. But at
• 5:59
  relativistic speeds like that, uh, it
• 6:01
  also changes the position of
• 6:04
  objects in your sky.
• 6:07
  Um, so if you imagine something hurtling
• 6:09
  along at, uh, a significant
• 6:12
  fraction of the speed of light, uh, the stars
• 6:15
  that you see tend to
• 6:18
  the positions you'll see will, um, be.
• 6:21
  They're erroneous, but the stars will look as
• 6:24
  though they're nearer to the direction in
• 6:26
  which you're heading than they really are. So
• 6:29
  there's a tendency, especially as you
• 6:30
  approach getting near the speed of light,
• 6:32
  basically all the stars that you'd see will
• 6:34
  be bunched up, uh, fairly close together in
• 6:37
  front of you, um, because that's the way
• 6:40
  their light is changing itself.
• 6:42
  Andrew Dunkley: I reckon, uh, being on a planet in a
• 6:44
  situation like that, uh, orbiting a fast
• 6:47
  moving star, your astrophotography would be
• 6:49
  awesome.
• 6:51
  Professor Fred Watson: Yeah, it would.
• 6:52
  Except, well, probably not. I mean,
• 6:56
  the star trails would all be in the wrong
• 6:58
  place. Yeah, the stars, yeah, but no, great
• 7:00
  question. And Ron, you're on the money there
• 7:03
  too.
• 7:04
  Andrew Dunkley: Yeah. And um, by comparison, I suppose we can
• 7:06
  draw on a science fiction movie,
• 7:08
  Interstellar, where they visited
• 7:11
  Miller's planet, which was near Gargantua,
• 7:14
  uh, which is, um, a black
• 7:17
  hole in the film. And for every
• 7:19
  hour they were on the surface of
• 7:22
  the planet, seven years passed on Earth. So,
• 7:25
  uh, that's the sort of gravitational effect
• 7:27
  we're talking about. But, um, in terms
• 7:29
  of the speed of a planet,
• 7:32
  um, in accordance with the fast moving star,
• 7:36
  it's a different kettle of fish, isn't it?
• 7:38
  Um, you're observing everything out of sync,
• 7:41
  I suppose would be the best way to describe
• 7:42
  it.
• 7:43
  Professor Fred Watson: Yes, that's right, exactly.
• 7:45
  Andrew Dunkley: So quite interesting. Thanks for the
• 7:48
  question, Ron, and great to hear from you.
• 7:54
  Professor Fred Watson: Also space nuts.
• 7:56
  Andrew Dunkley: Uh, our next question, Fred Watson, comes
• 7:58
  from Jeff, uh, also in the United States.
• 8:00
  He's in Colorado. Uh, hello
• 8:02
  Andrew and Fred Watson. Thanks for the
• 8:04
  amazing shows. Oh, have we done one? Have we.
• 8:08
  Uh, the sun emits millions of billions of
• 8:11
  photons every second and it's pretty
• 8:14
  much average as a star.
• 8:17
  Uh, could the radiation pressure from all the
• 8:19
  universe's photons exert enough force to
• 8:22
  affect the expansion of the universe? Now
• 8:25
  there's an interesting theory, Fred Watson.
• 8:27
  Professor Fred Watson: Uh, there is, yes. Uh, it's
• 8:30
  um, so 10 to the 45. 10 to
• 8:33
  the power. 45 photons per second coming from
• 8:35
  the sun. Um, I like that number. It's quite a
• 8:38
  lot. It is
• 8:41
  so that, uh,
• 8:45
  that basically that, um,
• 8:48
  flux of photons, if I can put it that way,
• 8:51
  is for every star is an
• 8:53
  outward flux, um, for the universe
• 8:56
  as a whole, it's kind of going to cancel out.
• 8:59
  Um, but it is something
• 9:02
  that is taken into account,
• 9:06
  um, in the study of
• 9:09
  cosmology, the behaviour of the universe as a
• 9:11
  whole. And so in particular, you know,
• 9:14
  the potentially nice idea that this could
• 9:16
  account for dark energy, uh, which is the
• 9:19
  energy of the universe itself. That
• 9:22
  is, um, two quite different
• 9:25
  things. Uh, dark energy is
• 9:28
  independent of any kind of photon pressure.
• 9:31
  Uh, and so, um, it's basically the
• 9:34
  idea of all the photons floating around the
• 9:36
  universe. Uh, I think they are the most
• 9:39
  common subatomic particle in the universe, if
• 9:42
  I remember rightly. And neutrinos are the
• 9:45
  second. I think that was one we covered a
• 9:47
  couple of weeks ago.
• 9:48
  Andrew Dunkley: Yep.
• 9:49
  Professor Fred Watson: Uh, so, um, they're accounted for
• 9:52
  in our understanding of the expansion of the
• 9:54
  universe. And it doesn't explain dark energy.
• 9:57
  Dark energy is still an issue. Uh, it's,
• 10:00
  uh, something just connected with space
• 10:02
  itself. It will be there. Even if the
• 10:04
  universe was completely dark, you'd still
• 10:07
  have dark energy, we believe. And
• 10:10
  we don't know what it is.
• 10:12
  Andrew Dunkley: No, we don't. So probably
• 10:15
  not contributing to the expansion of the
• 10:17
  universe.
• 10:18
  Professor Fred Watson: Yes, that's correct.
• 10:20
  Andrew Dunkley: And we don't really know what is
• 10:22
  contributing.
• 10:23
  Professor Fred Watson: Well, the main contribution is the Big Bang
• 10:25
  itself. Yeah, uh,
• 10:28
  Andrew Dunkley: it's just a continuing explosion of some
• 10:31
  kind.
• 10:32
  Professor Fred Watson: Um, so the
• 10:35
  Big Bang was what gave the
• 10:37
  universe the impetus to start expanding.
• 10:40
  Uh, and, uh, for the first.
• 10:43
  Well, uh, it actually, the expansion
• 10:46
  rate was very, very rapid at the beginning of
• 10:49
  this period that we call inflation, which
• 10:51
  lasted for a gazillionth of a second.
• 10:54
  Uh, and then it sort of settled down to
• 10:56
  something a bit more reasonable.
• 10:58
  Professor Fred Watson: Uh,
• 11:00
  Professor Fred Watson: but about 5 billion years ago,
• 11:03
  that's when the expansion started
• 11:05
  accelerating. Uh, and that's where we
• 11:08
  think the effect of dark energy really
• 11:11
  kicked in. Because dark energy was,
• 11:14
  uh, strong enough to overcome the
• 11:16
  gravitational pull of everything in the
• 11:17
  universe, which is trying to slow the
• 11:19
  expansion down gravitationally. And that's
• 11:22
  what we used to think would happen. And you
• 11:23
  get the big crunch. Uh, but yeah, when
• 11:26
  dark energy was. Oh, when the accelerated
• 11:29
  expansion was discovered back in
• 11:30
  1998,
• 11:33
  uh, that's when we realised there was
• 11:36
  something, um, lacking in our understanding
• 11:39
  of the universe. Something that is there,
• 11:41
  this energy that seems to
• 11:44
  impel the universe to be continuously
• 11:46
  accelerating. Uh, um, accelerating
• 11:49
  faster than it was before. And that's what's
• 11:51
  happening now? M. Yeah, I think
• 11:54
  just accelerating. It's the acceleration of
• 11:56
  the universe. The acceleration itself may be
• 11:58
  slowing down, but it is
• 12:01
  still going, I'm getting myself.
• 12:02
  Andrew Dunkley: Yeah, it's very confusing. It expanded
• 12:04
  initially. When the Big Bang happened, it
• 12:06
  expanded. Uh, am I right in saying fast?
• 12:08
  Faster than the speed of light for a period
• 12:11
  of time.
• 12:12
  Professor Fred Watson: Yes. I mean, space, um, can do
• 12:15
  whatever it likes.
• 12:16
  It's not limited to the speed of light.
• 12:17
  Andrew Dunkley: Then it's down now, started
• 12:20
  accelerating again. But now we think that
• 12:23
  acceleration is slowing down
• 12:26
  possibly.
• 12:28
  Professor Fred Watson: Ah, that's still an observation that's yet to
• 12:30
  be confirmed. That the acceleration is
• 12:32
  decreasing, but it's still accelerating.
• 12:34
  That's the thing. It's not, you know, the
• 12:36
  expansion of the universe is not slowing
• 12:38
  down. Uh, still
• 12:41
  accelerating, but the acceleration might be
• 12:43
  slowing down. It's.
• 12:46
  Yeah, it's a double derivative for those.
• 12:48
  Andrew Dunkley: It is, isn't it?
• 12:49
  Professor Fred Watson: Yeah, exactly.
• 12:50
  Andrew Dunkley: Yeah. Uh, one of our live viewers,
• 12:53
  uh, has suggested that the universe could
• 12:56
  be a doughnut shape. He doesn't have anything
• 12:58
  to prove that he's. It's just a hunch. I,
• 13:02
  uh, we. We can prove certain things, can't
• 13:04
  we?
• 13:05
  Professor Fred Watson: Yeah, Um, I mean, it's.
• 13:09
  You can have a doughnut. You can have, uh,
• 13:11
  what you might call, uh, the geometry of a
• 13:14
  doughnut, uh, without the thing actually
• 13:16
  looking like a doughnut. If I put it that
• 13:19
  way. Yeah. Um, uh, this is all
• 13:22
  to do with, for example, the geometry that we
• 13:24
  all deal with in everyday life is Euclidean
• 13:27
  geometry. Where parallel lines don't meet.
• 13:29
  But there are other geometries, uh, which
• 13:31
  could affect the shape of the universe. And
• 13:33
  doughnuts, I think, do come into
• 13:36
  consideration.
• 13:36
  Andrew Dunkley: Well, there you go, Moose. Well done.
• 13:40
  See, people just.
• 13:41
  They just think and ideas
• 13:44
  come forward. And sometimes they're worth
• 13:46
  investigating, indeed.
• 13:49
  Um, thank you for the question. I think it
• 13:52
  was from Jeff in Colorado.
• 13:53
  Professor Fred Watson: It was.
• 13:54
  Andrew Dunkley: This is Space Nuts, a Q A edition with Andrew
• 13:57
  Dunkley and Professor Fred Watson Watson.
• 14:00
  Professor Fred Watson: Hey, that's one of the better sims, believe
• 14:03
  me. We've had a couple of cool cardiac arrest
• 14:05
  down here too. Pete. Up
• 14:08
  here.
• 14:08
  Professor Fred Watson: Space Nuts.
• 14:11
  Andrew Dunkley: Uh, our next question comes from the Gold
• 14:13
  Coast. Not the one in, uh, Africa, the one in
• 14:15
  Australia. Uh, from Brian. He says.
• 14:18
  Hi. Long time listener, first question. Thank
• 14:21
  you, Brian. Good to hear from you. Sorry.
• 14:22
  About black holes.
• 14:23
  Professor Fred Watson: Ah.
• 14:23
  Andrew Dunkley: Uh, uh, since we cannot see behind a black
• 14:26
  hole, does it cover up the cosmic microwave
• 14:29
  background radiation. Or does that light get
• 14:32
  lensed around the black hole?
• 14:35
  Professor Fred Watson: Um. Uh, yes,
• 14:38
  it does. Uh, to an
• 14:41
  extent.
• 14:42
  Professor Fred Watson: Um,
• 14:45
  Professor Fred Watson: there are funny effects
• 14:47
  that come into play on the
• 14:50
  background radiation. And
• 14:54
  basically this sent my thinking,
• 14:57
  uh, to something I really read about for a
• 14:59
  long time. And, um,
• 15:02
  I've now lost it. Yes, here it is. Something
• 15:05
  called the Sunyaev
• 15:07
  Zeldovich effect, which
• 15:10
  is the way, um,
• 15:13
  the spectrum of the cosmic
• 15:15
  microwave background is
• 15:18
  modified as the Light
• 15:21
  passes through galaxy clusters.
• 15:23
  It comes about through a phenomenon called
• 15:25
  inverse Compton scattering. Uh,
• 15:29
  so that the radiation of the background
• 15:33
  is uh, modified. If that light passes through
• 15:36
  a galaxy cluster. I
• 15:39
  can't remember which way it works, whether
• 15:42
  it's. I, uh, think it is
• 15:45
  a decrease in the intensity
• 15:48
  of the background. It might be an
• 15:51
  increase. I might have that. It's a long time
• 15:53
  since I've thought about this. But that's the
• 15:55
  sort of thing that. That is taken into
• 15:58
  account when people consider
• 16:01
  uh, um. Basically
• 16:04
  the cosmic microwave background as we see it
• 16:07
  from planet Earth. Uh, and
• 16:10
  so, um, phenomena like black holes.
• 16:14
  Uh, Brian's question is
• 16:17
  a good one because there would be a
• 16:19
  modification of the radiation, um, and
• 16:23
  lensing would be part of that. You've got a
• 16:25
  black hole that uh, intensely
• 16:28
  modifies the gravitational field around it
• 16:31
  that makes it act like a lens. So that's
• 16:34
  going to happen with not just uh, the light
• 16:37
  of any stars behind it, but the
• 16:39
  gravitational, uh, sorry, the background
• 16:41
  radiation itself, the microwave background.
• 16:44
  Um, it's an interesting, um,
• 16:47
  question actually. I'd like to follow up a
• 16:49
  little bit more on that one, Brian, and maybe
• 16:51
  we'll get back to you. Uh, again, just to the
• 16:53
  extent, you know, is that a way of
• 16:56
  determining how many black holes there are in
• 16:58
  space? Uh, because we don't really have.
• 17:00
  We've got a few estimates based on
• 17:03
  um, our uh, knowledge of stellar
• 17:06
  evolution. Uh, we've got a few estimates
• 17:09
  based on galaxies themselves. Um,
• 17:12
  I guess, um,
• 17:15
  thinking aloud a bit and maybe this is the
• 17:17
  answer to the question, uh,
• 17:20
  the black hole itself.
• 17:23
  Let's just think about the black holes that
• 17:25
  uh, are uh, in the centres of
• 17:28
  galaxies, the supermassive black holes.
• 17:31
  So that uh, you might have a supermassive
• 17:34
  black hole that's a few billion times the
• 17:36
  solar mass, but that's still
• 17:39
  probably small compared with the mass of the
• 17:41
  galaxy itself. So the lensing effect
• 17:45
  is going to be what you would see from the
• 17:48
  galaxy and the black hole at the middle.
• 17:51
  Uh, and I think that's probably the likely
• 17:53
  answer to the question. Um, so I might
• 17:56
  not need to follow up on that,
• 17:59
  having kind of worked it out. Uh,
• 18:02
  and that lensing effect of galaxies is indeed
• 18:05
  taken into account when we look at
• 18:08
  the cosmic um, microwave background
• 18:10
  radiation. So yes is the answer.
• 18:13
  Andrew Dunkley: Okay, Easy peasy.
• 18:15
  Uh, and uh, thank you for the question, um,
• 18:18
  Brian. And uh, yeah, hope all is well in that
• 18:21
  horrible, horrible part of the world
• 18:24
  known as the Gold Coast. It's well named
• 18:27
  because it's got beautiful beaches,
• 18:29
  beautiful, uh, climate. Don't um, know
• 18:32
  about the people. No, I'm kidding. Queensland
• 18:34
  is a lovely. Wow.
• 18:38
  Like I mentioned last episode, my best friend
• 18:41
  Dave, he's a Queenslander. So there you are.
• 18:43
  Professor Fred Watson: Don't worry, Brian. He doesn't like
• 18:45
  Yorkshiremen either.
• 18:47
  Andrew Dunkley: Uh, I like everybody, I do.
• 18:50
  Ah, our final question, Fred Watson, comes
• 18:53
  from Bob. I don't know where Bob's from, but
• 18:55
  hello, Bob, wherever you are. Uh, how small
• 18:58
  and misshapen can a moon be and
• 19:01
  still with a straight face
• 19:04
  be called a moon?
• 19:05
  That's an interesting question. Brings Mars
• 19:07
  into the equation here because its moons are,
• 19:09
  uh, certainly not what you would consider
• 19:12
  moon shaped. We think of our moon as a
• 19:15
  sphere, and that's generally how it goes. And
• 19:18
  a lot of the other ones in the solar system
• 19:20
  are spherical. Uh, Mars is an
• 19:22
  exception. Um, we think they're captured
• 19:24
  asteroids, don't we? But, uh, yeah, when is a
• 19:26
  moon not a moon? I think is the precid
• 19:28
  version of the question.
• 19:31
  Professor Fred Watson: Yeah. Um, so this
• 19:34
  could be my old mate Bob Argyle, uh,
• 19:36
  who's in Cambridge. Ah, but
• 19:39
  I don't know. So, Bob, if this is you, drop
• 19:42
  us a line. Um,
• 19:45
  yeah, there's a very good friend, uh, going
• 19:48
  back a long way. We used to work together at
• 19:50
  the Royal Greenwich Observatory. Bob
• 19:52
  originated, if it's the same Bob, in, uh,
• 19:55
  Newcastle on Tyne, up there in the north of
• 19:57
  England. And I used to live there too. So,
• 20:00
  yeah, still got some records that he sold me.
• 20:05
  Um, okay. We've done all that before because
• 20:07
  I think Bob's asked us questions before and,
• 20:10
  um, it probably isn't the same, Bob,
• 20:13
  but it's a great question. Um,
• 20:16
  how small and misshapen can a moon be and
• 20:19
  still with a straight face be called a moon?
• 20:21
  Um, pretty small, actually. Uh,
• 20:24
  certainly. Um, some of
• 20:26
  the moons of Saturn, uh,
• 20:30
  their diameters are measured, you know,
• 20:33
  in single figure kilometres.
• 20:36
  Um, uh, we're kind of getting
• 20:39
  on that way. You mentioned the moons of Mars,
• 20:41
  Andrew.
• 20:42
  Professor Fred Watson: Yes.
• 20:43
  Professor Fred Watson: Phobos, 23 kilometres across or
• 20:45
  thereabouts. Deimos, the smaller one. I think
• 20:47
  it's about 15 kilometres and as you said,
• 20:50
  probably captured asteroids. But the fact
• 20:53
  that it's in orbit around, that these are in
• 20:55
  orbit around Mars makes them moons. They are
• 20:57
  satellites of Mars. Uh, and I think they get
• 21:00
  smaller when you look at Saturn. Some of the
• 21:02
  moons sort of wed into the
• 21:05
  ring system. I think, as I said,
• 21:08
  the diameters measured in single kilometres,
• 21:11
  but we still call them moons. They add to the
• 21:13
  total of. I think it's 297 at the moment. The
• 21:16
  total number of moons known around Saturn.
• 21:19
  Uh, so, um, yeah. How small can a moon be
• 21:22
  and still with a straight face be called a
• 21:24
  moon? Pretty damn small. A few kilometres.
• 21:26
  Yeah, maybe even a few metres.
• 21:29
  If you're talking about, um, you know,
• 21:32
  debris around a planet, there might be
• 21:34
  objects that size in orbit around Earth.
• 21:36
  Although we do tend to know now about near
• 21:39
  Earth objects, whether they're um, asteroids
• 21:41
  or moons. We think that
• 21:45
  our um, big moon is the only natural
• 21:47
  satellite of Earth. We've discussed some of
• 21:50
  that stuff as well. We do, sorry. There are
• 21:53
  objects that accompany Earth, uh, temporarily
• 21:56
  in its orbit.
• 21:56
  Andrew Dunkley: Well, I was about to mention 1, uh, 2025
• 21:59
  PN7 which was discovered in
• 22:02
  Aug. 2025 by the Pan Starrs
• 22:05
  Observatory. It's referred to as a
• 22:08
  quasi moon. Uh, and
• 22:10
  it's like 19 metres across,
• 22:13
  62ft.
• 22:14
  Professor Fred Watson: Yeah, yeah.
• 22:15
  Andrew Dunkley: So it's, it's, you know, it's only
• 22:18
  about the size of a house.
• 22:20
  Professor Fred Watson: Yeah, that's right.
• 22:22
  Andrew Dunkley: So, um, but, but it won't stay.
• 22:26
  Professor Fred Watson: That's correct. Um, and it's only
• 22:29
  a quasi moon. It's actually in orbit around
• 22:31
  the sun. Uh, a number of objects like that,
• 22:34
  they'.
• 22:35
  Andrew Dunkley: It's caught up with us at the moment, isn't
• 22:37
  it?
• 22:37
  Professor Fred Watson: That's right. When you look at its orbit, you
• 22:40
  realise that uh, uh,
• 22:43
  uh, it's in orbit around the sun, but it
• 22:45
  always hangs around the Earth as well.
• 22:47
  But it's not in orbit around the Earth. Uh, I
• 22:50
  think that's what defines a quasi moon.
• 22:53
  Andrew Dunkley: Here's a question. Um, with
• 22:56
  Mars probably capturing its moons
• 23:00
  because of their odd potato shapes, is it
• 23:03
  possible sometime that Earth could capture,
• 23:05
  uh, an asteroid and claim a second moon?
• 23:09
  Professor Fred Watson: Uh, yes. Yeah, that's right, it is.
• 23:11
  Andrew Dunkley: Wow.
• 23:12
  Professor Fred Watson: And it may have happened in the past as well.
• 23:14
  Andrew Dunkley: Yeah. Okay. Very
• 23:16
  interesting.
• 23:17
  Professor Fred Watson: All right. The um, thing about the Earth
• 23:19
  compared with Mars, Andrew though, is that
• 23:22
  what might inhibit that capture on
• 23:25
  Earth is our uh, main moon because
• 23:28
  it's compared with our uh, planet,
• 23:31
  it's a massive object. Uh,
• 23:33
  it's 1-80th of the mass of Earth.
• 23:36
  Uh, the mass of Phobos and Deimos together
• 23:39
  is, you know, basically
• 23:42
  infinitesimal compared with the mass of Mars.
• 23:45
  So the moon itself, its gravity is
• 23:48
  going to be something that, um, any
• 23:50
  wandering asteroid will feel, uh,
• 23:53
  alongside the gravity of Earth and that might
• 23:55
  tend to kick it out relatively quickly from
• 23:58
  being in orbit around the Earth.
• 24:00
  Andrew Dunkley: Okay, very cool.
• 24:02
  Um, Gosh, we went through those fast.
• 24:06
  Professor Fred Watson: We did. Great questions too.
• 24:08
  Andrew Dunkley: They were indeed, yes. If, uh, you have
• 24:11
  questions for us, um, please send them in.
• 24:13
  We'd love to hear from you. You, uh, can jump
• 24:15
  on our website to get more information about
• 24:17
  how to do that. Well, I'll tell you. Click
• 24:19
  the AMA button at the top of the
• 24:22
  screen and you can send text and audio
• 24:25
  questions. Don't forget to tell us who you
• 24:27
  are and maybe where you're from. That'd be
• 24:29
  nice. We love to know where our audience is.
• 24:32
  We had someone from Yemen the other day,
• 24:33
  which was terrific. Uh, but, uh,
• 24:36
  nice to have an international flavour in our,
• 24:39
  in our shows. And thanks, uh, to our live
• 24:41
  viewers as well, who pop questions in once
• 24:44
  in a while, which is terrific. And thank you,
• 24:47
  Fred Watson. Uh, we'll catch you on the next
• 24:48
  episode of Space Nuts.
• 24:50
  Professor Fred Watson: Great pleasure, Andrew, and thanks again to
• 24:53
  all the listeners sending in those fabulous
• 24:54
  questions.
• 24:55
  Andrew Dunkley: Indeed. And thanks to Huw in the studio.
• 24:58
  Couldn't be with us today because he's a
• 25:00
  quasi Huw. And from me, Andrew
• 25:03
  Dunkley, thanks for your company. We'll see
• 25:05
  you on the next episode of Space Nuts.
• 25:07
  Professor Fred Watson: Bye. Bye.
• 25:08
  Andrew Dunkley: Uh, you've been listening to the Space Nuts
• 25:11
  podcast, available at
• 25:14
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• 25:16
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• 25:19
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• 25:21
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• 25:24
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• 25:26
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