Earth to gain temporary second moon

Interview with Matt Bothwell, University of Cambridge

You may have read or heard in the wider media that the Earth has allegedly acquired a “new Moon”. Well, ish. We’ve certainly picked a new satellite, of sorts, but it’s a bit of a giant leap to say this lump of rock is going to give our mainstream moon a run for its money. Here's Cambridge space scientist and “public astronomer” Matt Bothwell to clear things up for us…

Matt - So the object is the very imaginatively named 2024PT5 in the long tradition of fantastic, memorable astronomy names. It's a near Earth orbit asteroid. So it's a piece of rock, something about 10 metres wide, which is orbiting the Sun. And on the 29th of September, it's going to get close enough to Earth that it's coming into our gravitational sphere of influence and is going to start sort of orbiting the Earth. And so this is where it hit the news, right? We are going to have something orbiting the Earth. It's gonna be a new mini moon, but it is a little bit more subtle than that. So really to be counted as a temporary satellite, you have to do one orbit around your planet? At least go once round. And this thing's not going to make it all the way around. It's going to do a sort of horseshoe shaped slingshot around the Earth before heading off into deepest space. So we are going to get a temporary fly by companion, but it's a bit of a stretch to call it a temporary moon.

Chris - How far away will it pass?

Matt - It's going to be about five times further away than the regular moon. So the regular moon is 300,000 kilometres. This thing's closest approach is going to be about one and a half million kilometres. So it's not nearby by any stretch of the imagination.

Chris - Visible or not?

Matt - Absolutely not. Unless you have a sort of multi million pound telescope. I think even a very, very big backyard telescope won't be able to pick this thing up. So unless you have access to a professional astronomy telescope, I think your chances are slim.

Chris -
How did it get detected then if it's quite hard to spot? It's quite small. How did scientists spot it in the first place?

Matt - Scientists have programmes to detect near Earth asteroids because there's always a risk that might be dangerous. So we have catalogues of hundreds of thousands of things going all around our neck of the woods in the solar system. And so this was just picked up as part of our regular searching of the sky for anything that might come near us and be potentially hazardous.

Chris - No risk that that might happen.

Matt - No risk whatsoever, no. This thing is going to do a little loop around us at this very safe distance of one and a half million kilometres and then carry on its merry way.

Chris - And if it were on a collision course with the Earth, would it be consequential to something that's that size about 10 metres across, or is that too small to even notice? Would it even make it to the ground?

Matt - Yeah, this is far too small to be a problem, even if it did hit us. In order to be at risk to something like a city, you need to be a couple of hundred metres across. I think something like this would cause a spectacular fireball and probably not make it all the way to the ground. So even if it hit us, we'd be safe, but it's not. So there you go.

Chris - Can we get any clues from its path and trajectory where it might have come from? Where do scientists think its origin was?

Matt - It's a difficult one to say definitively, but I think there's a strong clue that it might be a piece of the Moon, what scientists call lunar ejecta. When we look at its path around the Earth doing it's like a near Earth object thing, it's moving as if it was once ejected from the surface of the Moon. So this could well be the remnants of a meteor strike that hit the moon billions of years ago, threw up shards of rock. And this thing has been orbiting around for billions and billions of years.

Chris - Has this sort of thing happened before or is it a one off?

Matt - It has, it happened semi-regularly. There was something in 2022 and in 2020, that was very similar to this. The last time we actually got a proper temporary moon was in 2006 when a near Earth object came and spent about a year orbiting around the Earth before zipping off. So yeah, every few years something like this does happen.

Chris - How can that happen then? How can something temporarily orbit and then disappear again? Is it on a sort of expanding orbit or something, so it's sort of captured for a while and then eventually gets lost? How can that happen?

Matt - Yeah I think captured for a while and then eventually gets lost is a good way of putting it. Orbits have to be very, very exact to be what we call stable, so that they can exist for thousands or millions or billions of years. In the case of our solar system, you have to have a very, very exactly precisely correct amount of energy to be in a stable orbit. Ever so slightly either side of this, exactly like a bullseye amount of energy, and you can be temporarily captured, but then either fly off into space or fall down onto Earth.