Is falling space junk dangerous?
Dear Urban Astronomer,
Is it true that space junk is constantly falling down to earth?
Should I be worried that I’ll get hit?
Should I be worried that I’ll get hit?
Regards
Cathrine
Dear Cathrine
Your letter included a link to this Washington Post article, which talks about UARS (Upper Atmosphere Research Satellite) which will be making an uncontrolled re-entry. At the time I write this, NASA’s best estimate is that it will re-enter the Earth’s atmosphere “Sometime in the afternoon” on Friday 23 September, New York time. This seems surprisingly vague for an agency famous for counting down the seconds before launching rockets, and aiming robotic space probes to precisely hit targets billions of kilometers away, but large satellites (UARS is the size of a municipal bus, and weighs almost six tonnes) can behave in surprisingly unpredictable ways. NASA are quick to reassure us that there is nothing to worry about, that most of it will land in the ocean, and that so far no piece of orbital debris has ever hit a person, or caused property damage. And you know what? I believe them. Even though the expected debris track (the area in which pieces of the satellite will be scattered) is something like 800km long, only 23 pieces of satellite are made of materials hardy enough to survive the heat of re-entry. That’s 23 chunks of metal and carbon fibre scattered across a very large area. In fact, NASA have calculated a 1 in 3200 chance of anybody being hit by any part of the satellite. If you consider that there are just about 7 billion people on Earth, that works out to a 1 in 22,400,000,000,000 chance of you personally getting in the way. You’ve got a better chance of being struck by lightning on the same day that you win the lottery.
So to answer your question, Cathrine, no you don’t need to worry. UARS is unusual in that it’s pretty huge, so a lot of material will make it to the ground (and a lot will bounce back into space – the attached video simulation shows a lot of detail about what’s expected to happen). The stuff that does make it will have slowed to terminal velocity (still pretty fast by human standards, but we’re not talking the sort of cosmic rifle bullets we saw in films like Armageddon punching holes through skyscrapers!) and probably not even leave a crater in the ground. NASA have issued a warning to anybody finding debris that they should not touch it, but this is mainly for boring Health and Safety reasons – they’re worried that you might cut yourself (or worse, keep it – it remains US government property and they don’t like people stealing their stuff!).
It is actually true that stuff from space is constantly falling down to Earth, but this is nothing to do with the human space activity – about 100 tonnes of dust and meteoroids fall to Earth every single day. By comparison, the occasional bits of satellite are so rare as to be newsworthy!
Comments? Questions? Why not mail me at [email protected]