Thanks for asking! And the answer, unsurprisingly, is a lot.
Let me be more specific. Like, millions, or even tens of millions of pieces.
Most of it is small, thinks like flakes of paint and bits of burnt fuel. These aren’t too bad, you can actually shield a spacecraft from them with something called a Whipple shield. This is a thin piece of foil fitted outside the surface of the spacecraft, leaving a gap. Debris hits the shield at such a high velocity that it vapourises into plasma. By the time it hits the skin of the spaceship, it’s dispersed enough that it doesn’t do any damage.
Of course, there are bits you can’t shield, like solar panels or Space Shuttle windshields (on the Endeavour Shuttle in 1994 a piece penetrated halfway through the windshield). To protect against that, they actually fly the Shuttle backwards when in orbit.
Then there are the large pieces, i.e. large enough to track, which is over 10 cm in size. That may not sound like much, but something that size moving at hypervelocity – around 10 kilometres per second – will pretty much destroy your spaceship if it hits you.
Because they’re so big, you can’t shield against them so the best you can do is move your spaceship out of their way. This is fairly routine procedure, even for something as large as the International Space Station. But this is why they track them – and currently about 22,000 of these large pieces of debris are being tracked.
Between the small and large debris – that’s between 1 cm and 10 cm – there’s estimated to be around 500,000 pieces. There’s not much you can do about them, but NASA claims the risk to the International Space Station is “slight”. Their words, not mine.
Where does all this junk come from? Well, a lot of it starts as dead satellites or booster rockets used to get them into orbit. But if these pieces hit each other they break up into more, smaller pieces which then spread out and collide again, etc., etc. This eventually leads to a phenomenon called the Kessler Syndrome, where you essentially get a chain reaction of collisions that completely fills up an orbit with junk, making it too dangerous to enter. There’s reason to believe that some altitudes of Low Earth Orbit – that’s less than 2000 km – are already afflicted with Kessler syndrome.
But some of it is actually sent up there on purpose. In 1963, the United States sent up 480,000,000 tiny copper needles to try and create something from which to reflect radio signals. This caused a lot of complaints from other spacefaring nations, and eventually led to treaty provisions against that sort of thing. Most of those needles have since re-entered the atmosphere, but there are apparently a few clumps still up there.
Then there’s the satellite that China deliberately blew up in January 2007, in a highly controversial missile test. This one event created about 3000 new pieces of space junk, one of which nearly hit the International Space Station in April this year.
By the time they discovered it, it was too late to move the space station out of the way. So instead, the crew prepared to shelter in a Soyuz space capsule, for protection and in case they needed to make a getaway. But ultimately they didn’t need to, as it missed them by 3.5 km, but it was still a close call.
Of course, the other danger is about space junk hitting the Earth. Generally the risk is pretty small, as the Earth is very big and mostly covered by ocean. And even if they hit the bits that aren’t, people make quite a small target when viewed from above, so you’d have to be pretty unlucky to be hit. To date, only one person is known to have been hit by space debris: in 1997, Lottie Williams was hit on the shoulder by a piece of lightweight metallic material, believed to have come from an old Delta rocket.
Still, there have been a few notable events. In 1979, the US space station Skylab re-entered the atmosphere, crashing into Western Australia. And in 1978 and 1983 a couple of nuclear-powered Soviet spy satellites crashed to Earth, causing worries about radioactive contamination. Fortunately they ended up in remote areas, of Canada and the Indian Ocean respectively.
So, what can we do about space junk? One of the simple rules is to move all disused satellites and booster rockets out of the way, into safe orbits where they won’t hit anything. But since what’s already up there can still lead to a Kessler syndrome, NASA researchers have suggested using a laser to slow down debris pieces so they fall out of orbit.
But that sort of “laser broom” is still a long way off. Till then, try to keep out of the more overpopulated orbits, and maybe wear a helmet.