How can a spaceship change direction?

22 November 2011

Question

How can a spaceship change direction in space when – unless I was misinformed - I was told it is a vacuum. So it’s got nothing to thrust against? And the second question is, I'm old enough to remember Sputnik 1 going up, and it didn’t stay up very long, although I was told it was about 500 miles up. But now, I understand that satellites we have got up there now are not as high, but they stay up for longer. How is that?

Dave - Okay, I'll start off with the first question. The way a spaceship changes direction, accelerates, and decelerates in space is by pushing on something. It does push on something but because there's nothing up there, it's going to take the thing it's pushing on with it, and the thing it pushes on is fuel, so it maybe burns hydrogen and oxygen, those two react, get very hot and you get very, very hot water flying very quickly out of the back.

It's a bit like if you've ever fired a gun, the gun fires a bullet out one way, but the gun gets kicked backwards into your shoulder in the other direction with an equal and opposite amount of momentum. So if you imagine a spaceship is a bit like a gun firing a bullet in one direction, so it gets pushed in the other direction.

As for satellites, things like the International Space Station which is flying at about a couple of hundred kilometres up, it does fall downwards relatively quickly but it has a rocket on it which keeps pushing it up occasionally and they'd have to send up more fuel for that quite regularly which is one of the major things they have to do to keep it running as well as taking up food for the astronauts and things for them to do.

The other thing is that just because the orbit was at the maximum 500 miles, it doesn't mean the whole orbit was at 500 miles. It could mean it was quite elliptical. So rather than being exactly circular, if it's very elliptical, the lowest point of the orbit could've been very low, maybe 150 km up, or something like that. At which point, when it came in the lower bit, it would go too close to the atmosphere, it will get slowed down a lot and so, its orbit would decay away quite quickly even though the highest point of its orbit was relatively high.