# From Stopping Light to Space Junk — Your Questions Answered

An episode of [Space Nuts: Astronomy Insights & Cosmic Discoveries](https://invisiblenetwork.ai/podcast/1080090608), 13 Jul 2026, 33 min.

Audio: https://dts.podtrac.com/redirect.mp3/pscrb.fm/rss/p/op3.dev/e/api.spreaker.com/download/episode/72944388/226bd6e5_e0d6_4450_a611_0790758922fc.mp3

Sponsor Link: This episode is brought to you with the support of NordVPN - your first stop when it comes to online security and privacy. To check out our special money saving offer for Space Nuts liseners, visit www.nordvpn.com/spaenuts In this Q&A edition of Space Nuts, host Andrew Dunkley and astronomer Professor Fred Watson tackle intriguing audience questions ranging from the possibility of stopping a photon to the complexities of intertwining electromagnetic fields. They also discuss the speeds of colliding particles in the Large Hadron Collider and the growing issue of excess satellites in space. Join us for a fascinating exploration of these cosmic queries! Chapters: (00:00) Space Nuts aims to answer audience questions in a Q and A edition(01:04) Professor Fred Watson answers an audio question from Andrew Chunk(02:03) Kevin asks question regarding whether we have stopped a photon from moving(10:30) Fred: The fabric of space time consists of different fields(14:30) Stay safe online with our sponsor, NordVPN Space Nuts(16:28) Question comes from Andy from Cheshire, UK(22:52) There is growing problem of excess satellites in space and what to do(30:10) Mark: Everything you said, um, is possible(30:38) If you have questions for Space Nuts, send them in 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.
[0:01] Andrew Dunkley: This is Space Nuts. It's a Q and A edition.
[0:04] Uh, my name is Andrew Dunkley. Thanks for
[0:06] your company. In, uh, this episode we will
[0:08] endeavour to answer audience
[0:11] questions. Uh, Kevin wants to know about
[0:14] stopping a photon. Did that really happen?
[0:17] Ah, we've got a, uh, duo
[0:20] named Reynold and Brian wanting to ask about
[0:22] intertwining electromagnetic fields.
[0:26] Um, the speed of colliding particles in the
[0:29] Large Hadron Collider is a question we've
[0:32] received. And Mark is asking us
[0:35] about the excess number of satellites in
[0:38] space and what can be done about it. He's got
[0:40] an idea. We will see what that's all about
[0:43] on this episode of space nuts.
[0:46] Generic: 15 seconds. Guidance is internal.
[0:48] 10, 9. Ignition
[0:51] sequence start.
[0:52] Professor Fred Watson: Space nuts.
[0:53] Generic: 5, 4, 3. 2. 1. 2, 3, 4,
[0:56] 5, 5, 4, 3, 2, 1.
[0:58] Professor Fred Watson: Space nuts.
[0:59] Generic: Astronauts report it feels good.
[1:02] Andrew Dunkley: And he's back again for more.
[1:04] Uh, it is Professor Fred Watson Watson,
[1:06] Astronomer at large. Hello Fred Watson.
[1:08] Professor Fred Watson: Hello Andrew. Um, fancy seeing you here. Yes,
[1:12] in my study.
[1:13] Andrew Dunkley: Yes, I'm in mine as well. Although it's
[1:16] hard to see because the background's all
[1:18] blurred. I must have a setting
[1:21] that I changed in this thing and I can't
[1:23] figure it out how to undo it. But um, it
[1:25] doesn't really matter. You probably don't
[1:28] want to see all the junk at the back of my
[1:29] room anyway. It's not as good as your
[1:32] junk.
[1:32] Professor Fred Watson: Oh, it's good Chunk. My microscope, uh, there
[1:35] as well.
[1:35] Andrew Dunkley: Oh yeah, that's nice.
[1:37] Professor Fred Watson: If I see anything I need to look at closely,
[1:39] I can just turn around in my chair and have a
[1:41] look.
[1:41] Andrew Dunkley: Yeah, well, your age, that's probably.
[1:46] You walked into that one.
[1:47] Professor Fred Watson: I did deny. Yes.
[1:50] Andrew Dunkley: Um, shall we answer some questions?
[1:53] Professor Fred Watson: Uh, no, no, let's
[1:57] Andrew Dunkley: uh, let's go to our first question. It's an
[1:59] audio question and it comes from Kevin.
[2:03] Kevin: Hello, space notes. My name is Kevin. I'm
[2:06] from Las Vegas, Nevada and I finally have a
[2:08] question to ask you after listening to you
[2:11] guys from the beginning. It's regarding
[2:14] an article that I came across but didn't get
[2:16] to fully read on how we have
[2:18] officially docked a particle
[2:21] of light. Not just slowed it down but full
[2:24] on. Um, stop. My question is kind of a
[2:27] two part A, is this a
[2:30] legitimate thing? Have we stopped a, uh,
[2:32] photon from moving and B,
[2:36] if not, this can be posed as a what if
[2:38] question. But what's the consequences for
[2:41] a photon that come to a complete stop?
[2:43] Now, photons don't have rest mass. It's only
[2:46] in the mass of their energy. But does
[2:49] it Gain rest mass now that it is at a rest
[2:52] or is this one of those it enters
[2:56] and just ends up going back to the speed of
[2:58] light once whatever's holding it lets go?
[3:02] Um, Google doesn't quite give me the run
[3:04] around for a bunch of stuff so I figured I'd
[3:07] ask you guys. Love the show. Thank you for
[3:10] listening.
[3:11] Professor Fred Watson: Thank you.
[3:12] Andrew Dunkley: Kevin. Uh, I love this question. Uh, this is
[3:14] a subject that has come up uh, a few times
[3:17] over the years and it prompted me to do
[3:20] a bit of research. Uh, and I did find uh,
[3:23] an article on the Physics World website
[3:26] that uh, discusses this.
[3:27] Professor Fred Watson: Fred Watson Good.
[3:32] Uh, yes, that's right. Look, it's ah,
[3:36] it really is an interesting um, process.
[3:39] Um, but it's uh, it's,
[3:43] there's a bit of subterfuge here in the
[3:46] nomenclature
[3:47] Andrew Dunkley: because well that's a big word.
[3:50] Professor Fred Watson: Uh, there is two big words
[3:53] there. Don't know what either of them mean.
[3:57] There's a, you're almost
[4:00] playing with words here in a way because
[4:03] you do stop light. But it's not
[4:06] the individual photon
[4:09] that stops. It gets
[4:11] converted into something else,
[4:14] if I can put it that way. So you've got to
[4:16] start off with a Bose
[4:19] Einstein condensate. A
[4:21] condensate which is ultra
[4:24] cold atoms, they're a fraction of a
[4:27] degree above absolute zero.
[4:30] And the thing about one of these, they're
[4:33] usually called a bec, a Bose Einstein
[4:35] condensate. Um, it is
[4:38] basically a whole lot of atoms and usually
[4:41] it's sodium, uh, which um,
[4:44] are so cold that they behave
[4:47] like a single quantum object. So
[4:50] it's a bit like entanglement
[4:53] where you've got two quantum particles and
[4:55] they um, behave like a single particle.
[4:58] It's that. But in a, in
[5:01] a whole petri dish if you like, a lot
[5:04] of um, a lot of uh, these atoms are
[5:07] entangled effectively. So you've got this
[5:09] bec, the boson condensate. But
[5:12] then you've got to uh,
[5:14] you sort of excite it with
[5:17] a laser and then you send your
[5:20] photon in that you want to stop.
[5:23] And um, it basically
[5:26] the photon,
[5:30] it's not a photon anymore. It's now
[5:32] interacting with these super cold
[5:35] atoms, uh, in a way that
[5:39] effectively slows the transfer of energy
[5:42] down. So it's not the same photon that
[5:45] stopped. It becomes something else. It
[5:48] becomes um, uh.
[5:50] One um, document I read
[5:54] suggests it's actually
[5:56] converted into a matter
[5:59] based hologram, uh, uh,
[6:02] which is a slightly um, odd way of putting it
[6:05] but basically it tells you that
[6:08] you've changed the photon but
[6:11] uh, you can then basically,
[6:14] um, there's
[6:17] a separate laser that's exciting the BEC
[6:20] into this unusual state. If you turn that
[6:23] off, uh, the pulse doesn't
[6:26] just slow down. Sorry, the
[6:29] photon that you're trying to stop actually
[6:31] does stop when you turn this energy off.
[6:34] And what you've got is
[6:37] essentially,
[6:40] Kevin: uh,
[6:41] Professor Fred Watson: all the information, if I can put it that
[6:42] way, contained in the photon is
[6:45] transferred into this imprint in
[6:48] the bec, in the atoms of the
[6:51] Bose Einstein condensate. It
[6:54] becomes, as I said earlier, like a hologram.
[6:57] But then if you turn that, what's called the
[6:59] coupling laser back on, um, the light
[7:02] pulse is reconstructed
[7:05] and sets off again on its path. I haven't
[7:08] explained that very well, but that's
[7:11] basically what's happening.
[7:12] Andrew Dunkley: Okay, so
[7:15] Kevin's right. Uh, we
[7:17] have demonstrated that you can
[7:20] slow light down. I, uh, think when the storey
[7:23] first came out, they actually said they
[7:25] stopped it. Uh, but
[7:28] second, uh, part of his question was,
[7:31] does it reconstitute itself and get on with
[7:34] its journey? And the answer is yes, that's
[7:36] correct.
[7:36] Professor Fred Watson: Yeah. So this is. It's not, um, a
[7:39] particular, you know, it's not a specific
[7:41] piece of research. This. There's a whole lot
[7:43] of research going on. It's almost like
[7:46] becoming, um, uh, just a
[7:50] everyday tool of physicists to do this, to
[7:53] stop pulses of light, uh, and
[7:56] tinker around and see what they can learn
[7:58] from it. Making that grossly
[8:00] oversimplified. So I apologise to all my
[8:03] physicist friends. Um, but it's, um,
[8:06] almost a routine process to do this. Now. I
[8:09] think I'm right in saying that not just.
[8:12] Although I suspect it's only a few labs in
[8:14] the world that have got the equipment
[8:15] necessary, uh, to do it. Because
[8:18] it's not just your everyday microscope or
[8:20] anything like that. It's, uh, quite a
[8:22] specific piece of, uh, infrastructure,
[8:25] including the Bose Einstein condensate, which
[8:27] I think we're all actually made in the. Was
[8:30] it in the 1980s? Um, they were predicted by
[8:32] Bose and Einstein, two physicists. Uh,
[8:35] but I don't think we actually managed to make
[8:38] one until maybe 40 years ago. I might have
[8:40] that date wrong, but that sticks in my mind.
[8:43] Andrew Dunkley: Yeah, that's fascinating. I wonder why we're
[8:45] so keen to learn how to do this with light. I
[8:48] mean, what do we gain from it?
[8:51] Professor Fred Watson: Well, um, uh, it
[8:54] teaches you about the properties of the Bose
[8:57] Einstein condensate. And
[9:00] being able to stop a photon and store its
[9:02] energy is quite an
[9:05] interesting thing. Particularly if
[9:07] you think, well, maybe we can apply this to
[9:10] quantum computing. I think that's
[9:14] uh, one of the reasons why this is a hot
[9:16] topic, uh, that it does have
[9:19] applications for quantum,
[9:21] uh, information. It also,
[9:25] um, you know, it relates to
[9:28] our understanding of physics at the most
[9:30] basic level. Uh, it's, uh.
[9:32] Yes, it's extraordinary. I think it is a very
[9:35] useful line of research and, um.
[9:38] Sounds like it, I think. Yes, I think I
[9:41] should understand it better. That's the
[9:43] bottom line.
[9:44] Andrew Dunkley: Kevin might also be interested to know the
[9:46] revival process after you switch the laser
[9:49] back on is quite slow. It's not like it
[9:52] instantly goes back to its 300 million
[9:54] metres per second. Um, light speed,
[9:57] uh, takes a little bit, and
[10:00] I'm talking a little bit of time to, to sort
[10:03] of rev its engines back up again.
[10:05] Professor Fred Watson: Yeah, so, so that's not. I mean, photons
[10:08] in a vacuum always travel at that 300
[10:11] or 300,000 kilometres per second, the way we
[10:13] usually put it, 300 million kilometres
[10:16] per second. Um, uh, but that's only
[10:19] the speed in a vacuum. The speed in
[10:21] different, um, other media is
[10:24] different.
[10:26] Andrew Dunkley: Thanks for the question, Kevin. That's um,
[10:28] that's a really interesting one.
[10:30] Our next question, Fred Watson, comes from.
[10:33] Uh, Now I'm going to assume this is two
[10:35] people. And the reason I say that is because
[10:37] the other day we read a note from Rennie in
[10:39] California about, uh, one of his grandsons
[10:41] being inspired to perhaps study astronomy in
[10:44] the future. And these two fellows
[10:47] sport the same surname as Rennie. So I'm
[10:49] going to assume these are two people,
[10:52] Reynold and who've sent this question in.
[10:55] And if I'm wrong, I'm sorry, but, uh, I just
[10:58] got that gut feeling about it. They haven't
[11:00] actually said these are from two different
[11:02] people, but, um, uh, the fabric
[11:05] of space time consists of different
[11:07] fields. An example is the Higgs field,
[11:11] uh, electromagnetic field, et cetera.
[11:14] So my question is, theoretically, could any
[11:16] of these fields intertwine and become
[11:19] a new type of field, or could the
[11:21] intertwining effect a, uh, field
[11:24] to interfere with its behaviour?
[11:28] That's getting really into the,
[11:30] um, big complexities of,
[11:34] uh, studying
[11:38] these particles.
[11:41] It's the smallest level of anything really,
[11:43] isn't it?
[11:45] Professor Fred Watson: That's correct, yes. So we're talking about
[11:47] fundamental particles which equally, uh,
[11:50] well, can be seen as, um, uh,
[11:53] as disturbances
[11:56] or eddies if you like, in, in the field, in
[11:59] the force field. Uh, so, you
[12:01] know, whatever that force field is. But I
[12:04] think there's a fairly straightforward answer
[12:06] to this question though. Uh, um.
[12:09] Exactly. As Reynolds and Brian say, the
[12:12] fabric of space time consists of different
[12:13] fields, such as the Higgs field. And the
[12:16] Higgs boson is a disturbance within the Higgs
[12:18] field. But, um,
[12:21] uh, and so the question is, theoretically,
[12:23] could any of these fields intertwine and
[12:25] become a new type of field or could the
[12:28] intertwining affect a field to interfere with
[12:30] its behaviour? And the answer is yes to the
[12:33] first part. They don't exactly
[12:36] intertwine, they superimpose. And
[12:39] you've actually, um, Reynold and Brian
[12:41] already named one because the
[12:43] electromagnetic field is actually a
[12:46] superposition of the electric field and the
[12:49] magnetic field, which are themselves
[12:50] separate. And there are other, there are
[12:52] other superpositions as well.
[12:55] Um, uh, the weak
[12:58] nuclear force intertwines with
[13:01] the electromagnetic force to become the
[13:03] electroweak force, which is something we
[13:05] think was present in the early universe.
[13:09] Uh, so, uh,
[13:12] yes, it's interesting the way that these
[13:15] superpositions happen. So they're absolutely
[13:17] right. They can entwine, uh, and, uh,
[13:22] um, at least maybe intertwines the wrong
[13:24] word. But, uh, superimpose at least so that
[13:27] you have multiple fields becoming
[13:30] something different, a new type of field.
[13:32] Exactly as they say. Okay, yeah.
[13:34] Andrew Dunkley: Ah, it's a strange world, isn't it, when you
[13:36] get down to the. It is
[13:39] tiny, tiny objects and, um,
[13:42] Professor Fred Watson: strange in the big objects as well.
[13:45] Andrew Dunkley: I suppose so. I mean, if you
[13:48] really sit back and drink a few scotches and
[13:50] start looking up and thinking about it, your
[13:53] brain just explodes. It's probably the scotch
[13:55] more so than the problems of the universe.
[13:59] Um, it is so
[14:02] out there when you're, you know, just
[14:05] contemplating existence itself is one
[14:08] of the things I find myself thinking about
[14:10] from time to time. How is existence
[14:14] not, not just why, but how.
[14:18] Uh, it's all very weird.
[14:21] Uh, and thank you to Reynold and Brian for
[14:23] sending in that question. And, um,
[14:27] we wish you well. Uh, and please send some
[14:29] more.
[14:30] Uh, this is Space Nuts, a Q and A edition
[14:32] with Andrew Dunkley and Professor Fred Watson
[14:34] Watson.
[14:36] Andrew Dunkley: Let's take a short break from the show to
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[16:27] Kevin: Space Nuts.
[16:28] Andrew Dunkley: Uh, I think we've got another audio question.
[16:30] We seem to be on a bit of a, um, um,
[16:33] um, you know, particle
[16:36] type of bender at the moment with this
[16:39] episode. Uh, this, this question comes from
[16:41] Andy.
[16:43] Andy: Hi guys. Andy again, from uk,
[16:45] actually from Cheshire, just down the road
[16:47] from the beautiful Jodrell Bank. Although
[16:50] I've never forgiven them since they took out
[16:52] the planetarium. Um, just a quick question.
[16:55] Um, the lhc, um,
[16:58] we're told that it
[17:01] accelerates particles to
[17:04] very close to the speed of light, about 0.9 C
[17:06] or whatever the actual figure is.
[17:09] Um, but they also say that
[17:12] they're colliding particles at close to the
[17:14] speed of light. Now if they're colliding
[17:17] particles that they're accelerating in
[17:18] opposite directions, surely that means they
[17:21] should be the collisions. The impact
[17:24] should be at close to twice
[17:27] the speed of light. Um,
[17:30] if you just clear that one up, I'd be very
[17:32] happy. Um, I,
[17:36] I think I'm right and I think the collisions
[17:38] are happening at greater than the speed of
[17:39] light. But prove me wrong
[17:42] again, fantastic show. Speak to you soon.
[17:46] Andrew Dunkley: Thanks, Andy. Um, reminds me of all
[17:49] those,
[17:52] Andrew Dunkley: I
[17:52] Andrew Dunkley: suppose, when they're teaching you to drive
[17:53] and they're saying, um, look, you're
[17:56] driving along the highway at 100 kilometres
[17:58] an hour and a car's coming in the opposite
[18:00] direction at 100 kilometres an hour and you,
[18:03] uh, sadly, hit each other. The
[18:06] impact speed is 200 kilometres an hour. I
[18:08] guess that's what he's getting at.
[18:10] Professor Fred Watson: Exactly that, yes. Um,
[18:13] um, and it's a natural thing and it's a
[18:16] question that we often get, uh, because it's
[18:19] completely counterintuitive.
[18:22] Uh, exactly as, um, as Andy's saying. Uh,
[18:25] and yeah, Cheshire's lovely. He's right. And
[18:28] so is Jodrell Bank. Um, uh, uh,
[18:31] as Andy's saying, you're colliding these
[18:33] things. If I remember rightly, the, uh,
[18:36] proton, uh, speed
[18:39] within the Large Hadron Collider,
[18:42] I think it's
[18:42] 99.99998%
[18:46] of the speed of light. So that's how fast
[18:49] these things are going, almost the speed of
[18:51] light. And you've got two, uh, streams of
[18:54] them going in opposite directions. You bring
[18:56] them together at the various experiment
[18:58] points. Um, I've been to some of those. I've
[19:01] been in the cavity at the cavern, actually,
[19:03] where the compact muon solenoid lives.
[19:06] Uh, and that's where they collide. So
[19:08] shouldn't they collide at nearly twice the
[19:10] speed of light? And the answer is no,
[19:13] because then
[19:13] Andrew Dunkley: you ought to be no.
[19:14] Professor Fred Watson: Yeah, that only works in classical mechanics,
[19:18] uh, where, as you said, the velocities just
[19:20] add together. Uh, if these things were
[19:23] moving, you know, in the, what we call the
[19:26] classical realm, in other words, slow stuff,
[19:29] you would add the velocities together. Uh,
[19:31] but when you get to
[19:33] relativistic speeds, as we call them, speeds
[19:36] close to the speed of light, you have to
[19:39] account for two other relativistic
[19:42] factors, which are, uh, time dilation
[19:44] and length contraction. And both of those
[19:47] things are things, uh, that become very
[19:50] significant at, uh, nearly the speed of
[19:52] light. And so when you take those into
[19:54] account, you get a different formula. And
[19:58] I don't know whether listeners are going to
[19:59] turn off here, but, uh, I'm going to give you
[20:02] the formula. So in the
[20:04] classical case, if you've got two
[20:07] velocities, U and V, it's always U and V,
[20:10] not you and me, U and V. Um,
[20:13] uh, and yes, in classical case, U plus
[20:16] V is
[20:18] the closing speed, but in the relativistic
[20:21] case, the Closing speed is u
[20:24] +v divided by
[20:27] 1 over u
[20:29] times v over c squared.
[20:33] So u +v divided by 1 over
[20:36] UV over c squared. That's the
[20:38] relativistic formula. And when you put the
[20:41] numbers in, uh, you realise
[20:44] that you can never, uh, exceed the speed
[20:47] of light by this.
[20:50] Um, you just get, uh, an answer
[20:53] that's even closer to the speed of light than
[20:55] your two initial, uh, colliders.
[20:58] So, um, here's an example. Uh,
[21:01] you've got two things travelling,
[21:04] hitting each other or travelling towards each
[21:07] other at 0.8 of the speed of light.
[21:09] In the classical situation, they would be
[21:13] coming together at 1.6 times the speed of
[21:15] light. That will be their relative veloc. But
[21:17] when you do the relativistic calculation, uh,
[21:20] their Closing velocity is
[21:22] 0.975 times the
[21:25] speed of light.
[21:26] Andrea: Okay.
[21:31] Andrew Dunkley: Okay.
[21:33] Professor Fred Watson: I hope that makes sense. It's all
[21:36] about the weird things that happen when you
[21:38] get near the speed of light. You know, time
[21:40] dilation itself, time slowing down for,
[21:43] uh, you know, for the. For as
[21:45] a difference between the observer and the
[21:47] person moving at the speed of light and
[21:49] length contraction. These are all weird
[21:50] things. So it shouldn't be a surprise that
[21:53] they don't just. The velocities don't just
[21:55] add together, they combine in that
[21:56] relativistic sense. Sorry about the equation.
[21:59] It's an equation I quite like, which is why I
[22:01] threw it in there.
[22:04] Andrew Dunkley: It's fair enough, too. And, uh, hopefully
[22:06] that's solved, uh, Andy's dilemma.
[22:09] Um, he thought it would be twice the
[22:12] speed of light or something to that effect if
[22:13] you got two objects at the speed of light
[22:16] impacting each other head on. But no, can't
[22:19] be done is what you're saying.
[22:22] Professor Fred Watson: Yeah, they're close. I mean, only light
[22:25] can go at the speed of light. So you're
[22:26] talking about things going at nearly the
[22:28] speed of light. Uh, they're not colliding at
[22:31] nearly twice the speed of light. They're
[22:32] colliding at even more nearly the speed of
[22:35] light than they were to start with. But it
[22:37] never exceeds the speed of light.
[22:39] Andrew Dunkley: I get it. There you go, Andy. Uh, solved.
[22:45] Professor Fred Watson: The crew of Artemis 2 now bound for the moon.
[22:48] Humanity's next great voyage begins.
[22:51] Andrew Dunkley: Space Nuts.
[22:52] Andrew Dunkley: And our final question today comes, uh,
[22:55] from. Mark. Hi, Fred Watson, Andrew, uh,
[22:58] and team. It's, uh, Mark again from Sunny,
[23:01] is it Cece.
[23:04] Professor Fred Watson: Yes, it's where Patrick Moore used to live.
[23:07] He used to visit him.
[23:08] Andrew Dunkley: I really have to use a bigger font size with
[23:11] these questions. Sunny, uh, Selsey on the
[23:14] south coast of England. Um, in more
[23:17] than one of your podcasts, you mentioned the
[23:19] growing problem of excess satellites in space
[23:22] and what to do with them. That got me
[23:24] thinking. Would it be possible to use the
[23:26] action reaction principle to place a new
[23:29] satellite in the same place as an old
[23:31] one and move the old one into a higher
[23:34] graveyard orbit? Uh, Then at a later date,
[23:37] collect them to be dismantled safely. The way
[23:40] I look at it, if they want to put more
[23:42] satellites into space, they should also pay
[23:44] to clean the space up. Uh, I know this
[23:47] sounds, uh, a, uh, bit space
[23:50] snook, a bit like space space snooker. Yes,
[23:53] it does. Uh, but would it be possible. By the
[23:55] way, I broke the TV in the Globe Pub as a
[23:58] young man playing snooker, so probably not a
[24:01] good idea to ask me to work out the
[24:03] trajectories for all of this. Keep, uh, up
[24:05] the great work. It means a lot to everyone
[24:07] listening. And those, uh, that don't, well,
[24:09] you just gotta pity them,
[24:12] says Mark. Thanks, Mark, for the question.
[24:15] Uh, I'd love to, I'd love to have been the
[24:17] night he broke the tv.
[24:18] Professor Fred Watson: That would have been spectacular.
[24:20] Generic: Yeah.
[24:20] Andy: Gosh.
[24:22] Andrew Dunkley: Now what I want to know is, was that he's
[24:24] backswing, getting ready for the, the,
[24:27] the move of the queue that hit the screen, or
[24:29] did he actually fire a ball,
[24:32] uh, across the, across the room and hit the
[24:35] tv? Uh, you're gonna have to clarify that
[24:38] one, Mark. Um, look,
[24:41] uh, in, in regard to, um,
[24:43] cleaning up your own mess, there's actually
[24:45] a. Isn't there an international law
[24:48] that requires you to deal with your own
[24:51] stuff up there?
[24:52] Professor Fred Watson: Yes, there is now. Um, I think it was added
[24:55] to the, uh, the
[24:57] approvals given by the International
[24:59] Telecommunications Union, which is a
[25:01] governing body of all this stuff, um,
[25:04] that you. I think this came in
[25:06] probably five, 10 years ago. You have to
[25:09] demonstrate, uh, before they'll give you
[25:12] permission to launch, that you've got a way
[25:15] of removing your spacecraft from
[25:17] orbit. Um, in other words,
[25:20] you've got to be able to clean up your own
[25:22] junk. Uh, now that's
[25:25] fine for new stuff, but there's a lot of
[25:27] stuff up there that didn't
[25:30] qualify for that. And no thought was given to
[25:32] the idea of trashing space that you, you
[25:35] know, your spacecraft would
[25:38] just continue in orbit, um, after
[25:41] its useful life was over. And
[25:44] indeed for many of them, for objects,
[25:47] uh, especially ones with solar panels which
[25:50] are big and act as a drag on the residual
[25:52] atmosphere up there. Uh, even if you're up
[25:55] at, uh, uh, four or five hundred
[25:57] kilometres, there's enough atmosphere that
[26:00] if you do nothing, your spacecraft will,
[26:03] uh, the orbit will decay. It will
[26:06] hit the atmosphere and slow down and that
[26:08] brings it down lower and then it slows down
[26:11] more. And that is how
[26:15] space is kind of almost automatically cleaned
[26:18] up.
[26:19] Andrew Dunkley: And that's what's happening to the Swift.
[26:21] Professor Fred Watson: Uh, yes, that we talked about a couple of
[26:23] episodes ago. Exactly right. That's right.
[26:25] And that one's worth saving, which is why a
[26:28] mission's been mounted to do that, to boost
[26:30] it into a higher orbit. So in a way, what
[26:32] that's doing is actually what Mark is
[26:34] suggesting. You, uh, can go, uh,
[26:37] attach another rocket to it and push it up to
[26:38] a higher orbit to safeguard it. Um,
[26:42] um, so for low Earth
[26:45] orbit, There's below about 5,
[26:48] 600 kilometres. There is this natural
[26:50] sweeping up as things decay
[26:53] unless you do something about it. Many
[26:55] spacecraft have got thrusters that lets you
[26:58] lift its orbit. Um, but if you switch the
[27:01] thrusters off, that means they're going to
[27:03] come back to Earth anyway. And that might be
[27:04] enough to satisfy the international
[27:06] Telecommunications Unit, uh, going higher
[27:09] up, though.
[27:10] Andrew Dunkley: Except.
[27:11] Andrew Dunkley: Yes, one more point. Uh, when these things
[27:13] are burning up, they're putting all those
[27:15] metals into our atmosphere.
[27:17] Professor Fred Watson: Yeah, you're still getting contamination.
[27:19] That's right. We're getting aluminium oxide
[27:21] and all sorts of stuff up there that
[27:23] shouldn't be there. Uh, but,
[27:25] um, yes, for higher orbits,
[27:30] these are the ones, what you might call mid
[27:32] earth orbits above 1,000 kilometres,
[27:35] uh, they're not gonna decay so readily. And
[27:38] so they are an. And then,
[27:41] uh, the, um, geostationary
[27:45] satellites. So the geostationary orbits are
[27:47] very, very specific. Um, in fact,
[27:50] all the satellites are in the same orbit,
[27:52] more or less, um, because it's the one that
[27:55] keeps them over the equator and keeps them
[27:57] going, uh, round once in a day.
[28:00] Um, those geostationary orbits, they're at
[28:03] 36,000 kilometres. They have to have
[28:05] mechanisms to push them into what's called
[28:08] exactly as, uh, Malik mentions, a grave
[28:12] orbit, which just gets them out of the way so
[28:15] that when they become defunct and you can't
[28:16] control them anymore, they're not going to
[28:18] bang into one of the active geostationary
[28:20] satellites. So it is a game of snooker up
[28:23] there, um, in a perhaps more gentle way than
[28:26] knocking one satellite into another, um, and
[28:29] replacing its position in space.
[28:32] All you do, if you do that is
[28:35] you've got another one that's going to decay
[28:37] at the same rate. If it's in low Earth orbit,
[28:38] it.
[28:39] Andrew Dunkley: Yeah, they reckon there's somewhere between
[28:41] three and four and a half thousand inactive
[28:44] or defunct satellites in orbit at the moment.
[28:47] Professor Fred Watson: That's correct, yes. Um, but
[28:49] then on top of that there's a, uh, host
[28:52] of, uh, upper stages, launch,
[28:55] you know, the launch vehicles. Lots of bits
[28:58] and pieces, bits of fairing, bits of junk,
[29:00] debris from previous collisions. It's a
[29:03] fleck of paint, flecks of Paint. That's
[29:05] right. There's even a glove.
[29:08] Andrew Dunkley: And a spanner with a spanner too. Yeah,
[29:10] there's all sorts of stuff floating around.
[29:13] Professor Fred Watson: It's all going at 8 kilometres per second.
[29:15] That's the dangerous bit.
[29:17] Andrew Dunkley: So I think that, uh, was another part to his
[29:20] question. Could you replace a satellite in
[29:23] its exact position, move the
[29:26] defunct one out and put a new one in the
[29:28] exact spot that its predecessor was?
[29:31] Professor Fred Watson: Well, you could, and, uh, indeed that's done.
[29:33] You don't move the other one out. You. Once
[29:36] its orbit's decayed, you, uh, just let it
[29:38] drop. Yeah, you've got that orbit, uh, freed
[29:41] up and you put another
[29:43] spacecraft there. That's what's happening
[29:44] with Starlink. Actually, it's exactly what's
[29:46] happening. The Starlink satellites are all at
[29:49] round about 500 kilometres. They were
[29:51] planning another shell at, uh, 1200
[29:54] kilometres. But, uh, for once, um, SpaceX
[29:57] listened to the astronomy lobby. Because
[29:59] those outer ones can be visible all night in
[30:01] some parts of the world, um, even though
[30:04] they're fainter because they're higher up,
[30:06] uh, it means that they're visible for much
[30:08] longer during twilight.
[30:10] Andrew Dunkley: Yeah, and that's a real problem, isn't
[30:13] it? There you go, Mark. Uh, everything you
[30:16] said, um, is possible. And
[30:19] uh, yes, there is a law requiring people to
[30:22] clean up their messes, but at the moment,
[30:24] letting them burn up in the atmosphere is
[30:26] okay until we all die of some
[30:29] kind of metallic poisoning. Then, um, they'll
[30:32] go, ah, yeah, we should have done, done
[30:33] something about that.
[30:35] Professor Fred Watson: Unintended consequences. Yeah.
[30:37] Andrew Dunkley: Uh, lovely to hear from you, Mark.
[30:38] Um, uh, we've been talking a lot
[30:41] about particle science today, and, uh, Andrea
[30:44] in Western Australia sent, uh, something in
[30:47] a while back and I've kind of been sitting on
[30:49] it, trying to find the appropriate moment.
[30:51] And because of the, the fact that three of
[30:53] our four questions were focused on, on
[30:56] particles, I thought it was appropriate
[30:59] to play, um, uh,
[31:01] Andrea's little voice piece today.
[31:05] Andrea: Hey, you two. The joke for the day.
[31:09] Two neutrinos walked through a bar.
[31:14] Andrew Dunkley: Thanks folks.
[31:15] Andrea: Really enjoy your show and I hope you guys
[31:17] found that really fun.
[31:18] Professor Fred Watson: We did.
[31:21] Andrew Dunkley: Uh, that's a good one.
[31:22] Professor Fred Watson: That is excellent. Yeah, perfect.
[31:24] Andrew Dunkley: Perfect timing. Well, actually, I've been
[31:27] sitting on it for months, but it,
[31:30] um, seemed appropriate. Appropriate today.
[31:32] Professor Fred Watson: Yes, that's the, um, one.
[31:34] Andrew Dunkley: Now a reminder, if you have questions for us,
[31:36] we would love to get them. Uh, you need to go
[31:39] to our website to send them in, uh, which is
[31:41] easy, spacenutspodcast.com or spacenuts
[31:44] IO. Click on the Ask me anything button at
[31:46] the top, it's labelled ama.
[31:49] And that's also the logo for the Australian
[31:51] Medical Association. But don't get confused.
[31:54] Uh, they might answer it too, though. You
[31:56] never know. Uh, but send your questions into
[31:58] us because, um, there's so much stuff that
[32:01] people want to know and if you want to know
[32:02] something, the best way to find out is to ask
[32:05] us and then we'll refer it to somebody else.
[32:07] But, uh, it is, um, uh, text and audio.
[32:10] Don't forget to tell us who you are and where
[32:13] you're from. Thank you so much, Fred Watson.
[32:15] It's been a pleasure.
[32:16] Professor Fred Watson: Always a pleasure, Andrew. Great to talk.
[32:19] Andrew Dunkley: Catch you soon. Professor Fred Watson Watson,
[32:20] astronomer at large. And, uh, thanks to Huw
[32:23] in the studio, who puts everything together
[32:25] with Blu Tack. Couldn't be with us today
[32:27] though, because he drives a proton and it
[32:30] does not do the speed of light. And so he was
[32:33] late. And from me, Andrew Dunkley. Thanks for
[32:35] your company. We'll catch you on the next
[32:37] episode of Space Nuts.
[32:38] Professor Fred Watson: Bye. Bye.
[32:40] Andrew Dunkley: You've been listening to the Space Nuts
[32:42] podcast, available at
[32:45] Apple Podcasts, Spotify,
[32:48] iHeartRadio or your favourite podcast
[32:50] player. You can also stream on
[32:52] demand@bytes.com.
[32:54] Andrew Dunkley: this has been another quality podcast
[32:56] production from bytes.com.

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