Space junk is satellites or vehicles that have been used to launch satellites. Sometimes they fall back into the Earth's atmosphere.
Often, it could be a defunct satellite that's reached the end of its life, and gradually, its orbit decays and it falls into the atmosphere.
Of course, it's travelling at orbital speed - perhaps around 11km a second - and as it runs into the upper part of the atmosphere, the friction causes it to heat up.
Because these aren't solid objects like a rock, they immediately start to disintegrate; bits fall off them and you end up with a spectacular set of streaks in the sky, often in different colours.
The colours are caused by different metals used to make the object, which usually completely burns up, but occasionally bits hit the ground.
Bits of the [Nasa space station] Skylab have landed in Australia, but mostly they just simply disintegrate totally.
Some of the old satellites are deliberately brought out of orbit -- this is modern policy, as we can't leave stuff orbiting the Earth. It's a crowded space up there and they are in danger of colliding with good satellites.
But there's also still a lot of old material orbiting the Earth, tens of thousands of pieces in different shapes and sizes, and over a long period of time their orbit will decay and they will all fall down.
Q: In New Zealand, how often would we get to see them coming down?
A: There have been reports over the years of objects being seen disintegrating over New Zealand.
We only see a small part of the sky, and it's often cloudy, so New Zealand isn't the greatest place to be watching these sorts of events.
But then, we're in a mid-latitude, and some things orbit over the poles, so it's more likely than not that we'd get to see them.
It's just pure chance.
Q: With satellites increasingly being sent into space, is it likely we'll see more objects coming down?
A: It could well be.
There's a move towards the miniaturisation of satellites and then there's the development of technology that can refuel satellites.
At the moment, a lot of the communication satellites that look after things like GPS are very expensive to put up into orbit, and they need to have fuel onboard to adjust their positions in orbit and turn them around.
Eventually, they'll run out of gas and become useless, so you have to launch another billion-dollar satellite to replace them.
What's being developed now is technology that will basically be a refuelling space craft that you can send up, and it moves around, links up with the satellite and refuels it.
So the need for them to come down will be reduced and their lives will be a lot longer.
But there's still a huge reservoir of material up there already that isn't under any control and that will all eventually come down.
Q: Do people need to worry about this stuff at all?
A: Only if the object was very big.
I'm not quite sure how they plan to ultimately de-orbit the International Space Station -- it's not going to happen soon, but one day it'll happen.
That's a big thing - about the size of Eden Park - so I don't think they'd be aiming to bring it all down in one piece.
But they have some control over where these objects land on Earth, once they've decided to send them back into the atmosphere.
Often, though, they're not aero-dynamically designed for re-entry - they are just chunks, like pods - and they'll just bounce around in the atmosphere.
The ability to determine their final resting place is not 100 per cent, but if they expect they are going to be coming down somewhere in the middle of the Pacific or the Atlantic, the chance of hitting a ship is extremely remote.
Although there is always a danger that something will land on a city or a house, I'd say it's a very remote risk.