The Supermassive Podcast - BONUS - Why aren’t stars closer together?

Episode Date: June 11, 2025

What would happen if planets collided at a snail's pace? How much of Avengers Endgame regarding time is scientifically acceptable? Why aren't stars closer together? And the team discuss a listener's theory to terra-form Mars. Keep sending in your wonderful ponderings. Email them to [e-mail] or via Instagram @SupermassivepodWant to support the show? Join The Supermassive Club for exclusive content, star-gazing forums and ad-free listening.The Supermassive Podcast is a Boffin Media production. The producers are Izzie Clarke and Richard Hollingham.Want to support the show? Join The Supermassive Club for exclusive content, star-gazing forums and ad-free listening... supermassive.supportingcast.fmAnd keep adding to The Supermassive Mailbox with your pictures and questions for the team. Send them to [e-mail] or follow them on Instagram, @SupermassivePod.The Supermassive Podcast is a Boffin Media production for the Royal Astronomical Society. The producers are Izzie Clarke and Richard Hollingham. Hosted on Acast. See acast.com/privacy for more information.

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New episodes drop every Tuesday. Acast helps creators launch, grow and monetize their podcasts everywhere. acast.com. Hello and welcome to another bonus episode of the Supermassive Podcast from the Royal Astronomical Society with me, science journalist Izzy Clark, astrophysicist Dr Becky Smethurst, and the society's deputy director, Dr Robert Massey. Right, let's dive into the Supermassive mailbox. Robert Rustopia has an idea for terraforming Mars. Try this, they say. Hi, I discovered the Supermassive podcast just recently and really enjoy it. You asked

for curveball questions in one of your recent episodes and I have one. Has anyone in the science community ever seriously proposed this method of terraforming Mars? Here goes. One, create little thruster probes which have just enough fuel to get to Saturn's rings. Two, these probes then find a big chunk of ring ice and attach to it. 3. The probe then uses some of the mass of the ice chunk itself as thrust material, steering the chunk to crash into Mars gently if possible. 4. Repeat steps 1-3 sending probes each year over the next century or two. How many probes would we need? 10,000? 100,000? Five. Wait a few decades for everything to settle down into oceans,

thicker atmospheres, etc. Six. Go live on Mars. I hesitate to say I thought of this myself. I could swear I heard this somewhere many, many years ago but cannot find a source. Would it be feasible, like all terraforming? It would take extreme patience and a multi-decade commitment to get off the ground. But wouldn't there be more than enough water and other trace elements out there around Saturn for use in this way? Thanks for your podcast. Can't wait for the next episode, Russ in Canada. Russ, thank you for taking up our challenge of curveball questions.

Joining the numerous listeners who are doing this and hurting our brains every single time, which is what we need. I love it. I think this idea came up in places like Kim Stanley Robinson's Mars trilogy, which is a science fiction staple as one of the ways of doing it. It's probably connected with the fact that we think a lot of the water on Earth might have an extraterrestrial origin, not necessarily all of it, but some of it. It's an interesting question because it sounds quite plausible, but I think the floor is that Saturn's rings don't actually have enough mass. The oceans of the Earth, when I was doing some, again, back-of-the-envelope calculations, it's happy

to be corrected if someone is better than this, but I think the oceans of the earth have about a hundred times as much. So even if you assume shallower oceans, smaller planet Mars and the rest of it, I'm not sure there's enough in the rings. Even if you remove all the rings, which won't make Becky very happy. A Saturn without rings again, depending on how much water you want. So to create oceans and rivers and have a reliable warmer and thicker atmosphere is not going to be easy. And you'd also have to scoop up really many, many billions of icy particles. And that's not going to be very easy. No, I don't think it'll just be a few thousand or even a few tens or even

a hundred thousand probes. It's going to be even more than that. And then bring them to Mars. You probably also don't want to crash them into the surface because that will create other issues. A lot of this stuff might then get ejected into space, for example, it's not gonna be a very efficient way of doing it, all a bit difficult. In theory, you could also, other sources, you could look at say all the ice in the asteroid belt

in the same challenging way, but even that might not quite be enough. And maybe you could go out and scoop out all the comets in the Oort cloud, but the difficulty is there is that some of them are really very, very far away. They're up to a light year from the sun and more, which we've simply never been able to go that far, and it will take a very, very long time to get there. It's interesting that NASA actually looked at whether it was possible to terraform Mars

using our current technology. They looked at a paper in 2018, they published, and they concluded it wasn't. It's still really difficult. We just don't have the means to grab things, the amount of energy we need and all of these things. It makes me a bit sad in a sense. I'm very ambivalent about this. I definitely don't want to trash any indigenous Martian life through terraforming. We have an unhappy history of doing this. It's going to be fragile life. I'm not worried about this stuff getting at us. You should take it seriously, but that's probably not the biggest risk. The biggest risk is going to be we go along, stomp all over the place, and this surviving life deep under the surface of Mars gets wiped out by

us. That's what I worry about with terraforming, with anything. Even if I'd like to imagine we could have these other Earths in the solar system. I think the ethical qualms around that are still very, very strong for me. Bekkah- Okay, I'm Becky. Matt Taylor, and I assume this is a different Matt Taylor of from Comet 67B. Emma- Shout out to Matt Taylor, but... Bekkah- To both Matt Taylor's. So, A Matt Taylor has emailed us a question about stars and distances and says, Thanks for the Q&A section of the podcast. Star sizes for the most part are tiny compared to the distances between them. Likewise for the distances between galaxies. What accounts for these massive distances? Why aren't they closer together? Also, the further we look into the night sky, the further back in time we're seeing when the universe was smaller, yet the further you look the universe appears larger. Hope you can shed some light on these.

Sure. Okay, let's start with why aren't stars closer together? Well, sometimes they are, right? They are in big globular clusters, you know, and in other parts of the galaxy they're further apart. It really just depends on how big and how dense the gas clouds that stars formed out of are. More dense gas clouds tend to form stars closer together, while less dense gas clouds will form stars further apart, just as gravity clumps it together. In terms of why aren't galaxies closer together, well again they used to be, and it depends on the gas clouds that they formed out of in the first place and how gravity sort of pulled everything together, but also the universe is expanding, and that's moving galaxies further apart all the time. And then for the second part

of your question Matt, as we're looking back in time as we do because light takes time to travel to us so as we look at further distances we see galaxies at those distances as they were when the universe was much younger but the universe appears larger as say, but that's all just to do with perspective, right? We have a 360 degree perspective here on Earth so we can look out in any direction and we see the universe sort of spreading out around us in any direction. So yes, I get what you're saying, it appears larger. We look at a larger volume is what we say in astrophysics. When we are looking at more distant objects and we're looking back in time we're not seeing the universe smaller we're seeing it

younger and denser right. Think about it if the universe is infinite which we don't actually know whether the universe is infinite or not because we can't see anything beyond the you know observable universe because of the fact that light takes time to travel to us but if the universe was infinite and it's expanding, it doesn't make sense to say that it was once smaller. If it was infinite, right? It's a common misconception when it comes to like the big bang theory, right? We can't help but picture it like this shrinking balloon that is getting smaller and going back to some point in space. But the entire universe is all of space time, right? You don't have space time if you don't have the universe.

So there's no space time outside of the existence of the universe. There's nowhere for the universe to physically be, you know, if it is like a picturing as a shrinking balloon, there's nothing outside for it to be. And there's also no time for the universe to be either. Harking back to this one's episode a little bit as well. So it's not right to say that the universe was smaller back then. It's right to say the universe was denser and it is now expanding to become less dense.

So as we look at further distances back in time, we're seeing it denser, but not smaller. I hope that makes sense. I hope. Thanks, Becky. And Robert, here's a question on slow collisions from Iliich, which starts with a lovely message that was just to inflate our ego slightly. Hello Supermassive team.

Thank you for a great podcast. I would like to express my admiration to you all for making a complex and mind-boggling subject understandable to a layman like myself complex and mind-boggling subject understandable to a layman like myself and I suppose others like me. Well, good. Thank you. We try. We do. Whether we succeed. Now to my question. Collisions in space are usually very violent events and I imagine the speeds of those collisions are a big part of the violence. What would happen, however, if say an Earth and Mars sized planet were to collide, like with the formation of our moon, but this time colliding at a snail's pace?

My guess is this would not be possible because their gravity getting closer and closer would make them speed up. But let's assume they magically do collide at a snail's pace and normal physics pops back into existence the moment they touch. So convenient. Yes. Would the following merger still be a violent event or would this be a slow process where they would eventually merge into a bigger sphere? I hope this question is interesting. PS, Becky's pronunciation of knocky might be my favourite moment of the podcast. Please don't stop being weird. How can we say exactly? I'll leave Becky to do that one. There's nothing wrong with

them plating our egos, we appreciate that. I was thinking about this one. There are real examples of slow collisions when you get relatively small things and things like the two parts of Arakoth, the Kuiper belt object that was visited by New Horizons in 2019. That's a contact binary where they didn't merge in a violent way. They're sufficiently low in vast. They've got a weak gravitational field between the two, so they collided quite gently, spiralled in slowly. It's really hard to do that with something like the Earth and Mars, because if you started them at zero, you know, they're going to reach a high speed as they come together. I was trying to think about the part of the question about how you could sort of suspend physics and switch it on again, I guess, as

soon as you switch it on again, you know, so you've got some magic thing where you switch off gravity, or you make it very, very weak, or you have some unimaginable force that makes them move slowly together. Once they merge together, the gravitational force is going to take over, you're going to have a very big lumpy object, it's going to have to settle in some way. That is going to be a violent event. I suppose I'm thinking about even the event that formed the Earth-Moon system, which we think was a proto-Earth and something the size of Mars hitting that, that you mentioned. Even if you imagine that those things are merged together, is it going

to be a non-verbal? No, it's still going to be hugely violent. You've got all the settling, you've got all the churning inside. It's just impossible to imagine that it being a sort of quiescent thing. You can't take that much mass, put it together and expect it to be a nice, gentle thing. They're not just going to stick together gently and cuddle up. It just doesn't work like that. Think of like the centrifugal force alone, right? Because they'd be spinning. You'd have two objects touching and spinning. I guess it would go from sort of that to like a really oblate spheroid. Yes.

Where they'd sort of waaah together. Yeah. The force of gravity, I mean, once you switch it on again, you know, we all say in astronomy, gravity is a very, very weak force. But when you add it up on big scales, it matters. And you know, yeah, so, so sadly, there isn't a sort of nice way of sticking planets together. You know, it's not like sort of, I'm trying to, what's a good children's toy where you stick balls together, but something like that, you know, you're not getting like, sort of plastic balls of Velcro on or something, it's always going to be a bit violent and a bit twitchy to make it happen. I mean, on the grander scales, when you look at things like Newton, you know, you're not getting a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort of, you know, a sort but something like that, you're not getting plastic balls of Velcro on or something. It's always going to be a bit violent and a bit twitchy to make it happen. I mean, on the grander scales, when you look at things like neutron star mergers and so on, obviously they're hugely extreme, but this is even in this case, switching physics on again, switching it off, switching it on again, can't see how that's going to end well. Okay. Thank you, Robert. And Becky, Lambert0B10011 has sent in a question following up from our Time episode that I know you're going to love. And they ask, how much of the theory in Avengers Endgame regarding time is scientifically acceptable? Let's cue the rant, shall we?

Oh my god. Oh my God. Um, okay. Well, I would like to preface this with the, I do love Avengers. Those are my ultimate escape films. Like they toe that line perfectly of like more fantasy than sci-fi so I can just turn my brain off. Like I'm one for the book club. I'm reading Project Hail Mary by Andy Weir at the moment. It's fantastic, but I cannot turn my brain off reading that one. There's no suspension of disbelief. Anyway, for the Avengers, there are so many things swept under the scientific rug.

I'll start with the obvious, the Pym particles, the things that they use to shrink themselves down to quantum sized and time travel and whatever. Pym particles aren't a real thing, right? That's completely made up for the film. Although what I would say is that I thought it was really interesting that they went down like the quantum path with Avengers Endgame. Obviously they already had it with like Ant-Man already being there. It was an obvious thing to do.

But like, I feel like the quantum physics sort of aspect of time is so far removed from any of the more plausible scientifically sort of accepted time travel devices, I guess for want of a better word, like for example we have wormholes, they're very much hypothetical still, but like we have the maths to describe them, example, we know that they bend space-time and therefore we can change time with a wormhole. You could punch through from one part of space and pop out in another space with a wormhole, sure. But also, if you can manipulate space-time, you can manipulate time to pop out in a different time as well. So that option was open to them and they didn't go for it, is basically what I was saying. Quantum mechanics though,

it governs the very, very, very, very small, right? And there's a lot we do understand about it and a lot we don't understand about it. And I don't think anyone really fully understands it quite famously. And there's no way right now of linking quantum mechanics with general relativity and our understanding of space and time. So because we don't have that theory and those theoretical physicists all around the world literally devote their entire lives trying to link these two things,

like we can't really quite comment on like the time travel aspect through what they decide to do, which is using like quantum mechanics to do it. There's then the issue that you mess with time and you create paradoxes. They sort of argued that because they've gone down the quantum mechanics, many worlds explanation, which if you remember we covered in an earlier episode of the podcast as well, I remember is that one really did bend your brain.

I love that. I just tried to pull that out of my brain. I was like, we've done it in the past. That's all I've got. You've probably got PTSD and your brain's protected yourself from it. The multiverse episode, yes, we've done it in the past. That's all I've got. You've probably got PTSD in your brains, protected yourself from it. No, the multiverse episode, yes, we did. We covered it then. Yes. Yeah. Yeah. So it's this idea that, like, you know, every single little thing

that could be different, like even a roll of a dice would make a completely different universe that's parallel to our own with different things that have happened. And so, I mean, they really build on that in the multiverse Avengers and Loki and all of that, you know, so they're done. And so what they argue is that through that explanation, there is no paradox created by the fact that they take the Infinity Stones from the like past where they used to be before they were destroyed in the present kind of thing, so they can use them in the present, you know. And so they said it because by taking the stones, all you're doing is creating this new multiverse, almost like a divergent timeline as they describe it as,

that doesn't affect the original master timeline of where they would eventually be, as long as they then return the Infinity Stones back to where they took them from, that then like prunes the divergent timeline. I think it's very convenient that they didn't lose one of the stones while they were at it and they could actually return to the original timeline. But when we're thinking about this in terms of quantum mechanics again, there are some simulations of how photons would behave that,

at least for photons of light, that would be the case. It doesn't matter if you remove them from where they were, but there's still so many questions over the validity of that and it relies so much on this probabilistic nature of quantum mechanics of like, there's a probability that the particle will be here versus over here. And so if you play with that, then it doesn't seem to affect things, but tiny, tiny particles and then even humans shrunk down to a tiny particle size, but still made of all of the cells that make up a human. I'm sure the biologists would probably

have something to say about that as well. So there are a lot of obviously completely different issues, but I think the fact that they at least make an effort to throw in some jargon for the sort of like, you know, people who get a kick out of that is fun, but at the same time, it's, yeah, it's very much swept under the scientific rug. Yeah, I love questions like these. So we said in the main episode, but we are going to do a separate episode at some point on time travel, teleportation, all that other fun stuff. I mean maybe our Christmas special. Maybe, we do do fun things for Christmas.

Yeah, we do the sort of off-the-wall stuff then. When our brains are bushed. Exactly. Do keep sending your questions, pictures, funny little observations to podcast at ras.ac.uk or find us on Instagram at Supermassive Pod or share them in the Supermassive Club. We'll be back in a couple of weeks with an episode all about Pluto, which I'm sure everyone will be again, just annoyed. Yes, rage.

That it's still on our planet. But until then, everybody, happy stargazing. best podcasts. Here's a show that we recommend. and I started to cry and I realized I haven't dealt with anything. My family's motto was I'm not going to be able to I'm not going to be able to handle that. Listen and subscribe to A Love Affair with the Unknown wherever you get your podcasts. New episodes drop every Tuesday. Acast helps creators launch, grow and monetize their podcasts everywhere. Acast.com.