What is space debris?

Why is space junk a threat to us? And how come there’s so much of it up there? Kat Arney talks to extraterrestrial rubbish expert Dr Hugh Lewis from the University of Southampton.

Hugh - We’ve been launching satellites into orbit since the late 1950s, so we’ve basically accumulated a lot of junk associated with all those space launches. They can be satellites that have reached the end of their mission. It could be the parts of the rocket that we’ve used to put the satellites there. It could be bits and pieces from those satellites that have somehow come away. But the most numerous objects up there are fragments from explosions and collisions.

Kat - It’s big bits and it’s small bits. But why is it so much of a problem? Doesn't it all eventually just fall back down to Earth?

Hugh - Well actually, many of the objects that are up there will be up there for centuries or even longer and that’s simply because the only mechanism we’ve got for removing the objects from the orbit is the atmosphere at the moment, and the atmosphere is so sparse at those altitudes.

Kat - So thinks aren’t just breaking down and burning up?

Hugh - No, absolutely. So yes, if an object is experiencing enough atmospheric drag it will come down and burn up. But objects that are high enough don't experience that kind of drag and they can be there for… some of the objects - Cath mentioned Envisat in her piece there - and that’s potentially up there for more than a hundred years.

Kat - Now isn’t space massive? We shouldn’t forget that space is enormous so how come this junk poses a problem for other things we want to send up into space?

Hugh - Well I think that the space is big mantra was used and has been used since the beginning of the space age and it’s certainly true. The volume space occupies is enormous. But what we’re doing is sending satellites into quite specific orbits and into orbits that are relatively close to the Earth. And if we keep doing that, as we’ve been doing for the last 50/60 years, then all that junk accumulates relatively close to the Earth in these quite congested orbits.

Kat - The way I like to think about is it’s like a trail of muck going from your front door where you always walk across the same bit of the carpet.

Hugh - Yes, absolutely right. Yes, that’s a very good analogy.

Kat - We saw in the film Gravity, there’s a collision with some space junk and it sets off a massive chain reaction. Is that the worse case scenario that there could be a rocket going up, or a really important communications satellite or something and it gets knocked out?

Hugh - Yes. So, fundamentally, the problem with space debris will be those collisions that could occur. You could lose a satellite from an impact with something in orbit. But that collision cascade that you talk about is, I guess you could say, that’s the absolute worst case that you could expect. That’s something that we’re working very hard to try and avoid.

Kat - There was in the news recently that the Chinese have, apparently, lost control of an abandoned space station that was up there so, presumably, this is now classed as space junk? That’s quite a big thing and what’s going to happen to it up there?

我休——特定的空间站,guess, it’s not as big as the International Space Station. It’s slowly moving down in terms of altitude and will ultimately, we hope, burn up in the atmosphere.

Kat - But what if it doesn’t, is it going to come down? And do these bits of junk actually come down and hit the Earth?

Hugh - It is possible for parts of spacecraft to survive that re-entry process; the heating and the loading that they experience, so some components may survive down to the surface of the Earth. But most of the Earth (three quarters of the Earth) is covered by water so it’s quite likely that we’d never even see these objects as they come in.

Kat - We also heard in Cath’s piece that there’s a whole load of debris from the Chinese trying to blow up something to try and get rid of it, and that’s just made the problem worse. Is it the big bits of debris or the small bits that are the most concerning?

Hugh - I think, in terms of the long term evolution of the space environment, we worry about the big objects because, essentially, all that mass that is locked into that object can be converted into many, many fragments. But in terms of objects that can be lethal, you’re talking about objects that are probably the order of millimetres in size that can actually pose serious risk to operational spacecraft.

Kat - That seems pretty dramatic when you think oh, it’s just going to be something massive and, in fact, it could be almost a fragment of dust something the size of a piece of sand that could bring down your spaceship?

Hugh - Yes, indeed. The space shuttle, when that was flying, they would have to replace the windows on the space shuttle, for example, due to the impacts that they would receive from objects that were very, very small. The craters that were left behind there are just perfect evidence of the risk that space debris poses.