# Time Dilation & Cosmic Mysteries: Unpacking Your Space Queries

An episode of [Space Nuts: Astronomy Insights & Cosmic Discoveries](https://invisiblenetwork.ai/podcast/1080090608), 13 Oct 2026, 27 min.

Audio: https://dts.podtrac.com/redirect.mp3/pscrb.fm/rss/p/op3.dev/e/api.spreaker.com/download/episode/75746057/time_dilation_cosmic_mysteries_unpacking_your_space_queries_nordvpn.mp3

Sponsor Link: This episode of Space Nuts is brought to you by NordVPN. Protect your online privacy today by visiting www.nordvpn.com/spacenuts for an exclusive offer! Space Nuts: Fast-Moving Planets, Photon Pressure, and the Nature of Moons In this engaging Q&A episode of Space Nuts, hosts Andrew Dunkley and Professor Fred Watson tackle a series of intriguing listener questions. Kicking off with Ron's query about the implications of living on a super fast-moving planet orbiting Sagittarius A*, they delve into the fascinating world of time dilation and its effects on scientific observations. Next, Jeff from Colorado poses a thought-provoking question regarding photon pressure and its potential influence on the universe's expansion, leading to a discussion on dark energy and cosmic mysteries. Brian from the Gold Coast wonders about the visibility of cosmic microwave background radiation behind black holes, while Bob rounds off the episode with a query on the characteristics that define a moon. Join Andrew and Fred Watson as they navigate these cosmic curiosities with their trademark wit and insight. Key topics - The effects of high-speed planetary motion near a black hole on time and observational science. - Exploring photon pressure and its relationship to the universe's expansion and dark energy. - The cosmic microwave background radiation and its interaction with black holes. - Defining moons: How small and irregular can they be while still being classified as moons? Timestamps 00:00 - Introduction and overview of listener questions 01:20 - Ron’s question about living on a fast-moving planet and time dilation effects 15:30 - Jeff’s inquiry into photon pressure and its impact on cosmic expansion 25:00 - Brian’s question regarding cosmic microwave background radiation and black holes 35:15 - Bob’s exploration of what constitutes a moon 45:00 - Closing thoughts and listener engagement Become a supporter of this podcast: https://www.spreaker.com/podcast/space-nuts-astronomy-insights-cosmic-discoveries--2631155/support.

## Transcript

[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] Apple Podcasts, Spotify,
[25:16] iHeartRadio or your favourite podcast
[25:19] player. You can also stream on
[25:21] demand@bytes.com. this has been another
[25:24] quality podcast production from
[25:26] bytes.com.

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