Question: Q4) How many man made space debris are there orbiting the earth and is there a real risk of these hitting the space station?

Camilla Weiss answered on 25 Jan 2016:

There are more than 20,000 bits of debris larger than 10cm being tracked in orbit around the Earth. We don’t really track objects smaller than this but we can detect objects down to about 3mm using radar and it’s estimated that there are about 500,000 pieces of debris between 1cm-10cm and hundreds of millions more smaller than this. On top of that there are around 1000 active satellites in orbit and another 2500 or so inactive ones so it’s getting pretty crowded up there!

The ISS is very heavily shielded against debris around the 1cm size and it’s unlikely to be hit by objects larger than that (even though there are so many objects space is still really really big). Because a lot of these objects are tracked if something is going to collide with the ISS a warning will be given and the ISS can use its thrusters to move away from it. Because the ISS flies much lower than most satellites there’s not much chance of getting hit by one them unless the satellite is on course to re-enter Earth’s atmosphere in which case the ISS would move out of the way. The main problem the ISS will face is the wearing down of some of its equipment from impacts over time – the solar arrays for example will become pitted and lose some of their energy-storing capability the longer the ISS stays up there.