Science Questions

I understand the basic idea that for every action there is a reaction, which is why I can walk across the ground. But when a rocket is in space and they decide that they want to change the direction of the rocket or slow it down, what's the engine firing against? What's it pushing against to slow the rocket down, because we're told that in space, there's nothing there? Adrian in Essex

A rocket works by throwing something out of the back so the rocket goes forward. So the action and reaction are actually inside the spacecraft. The fuel pushes outwards and pushes the spacecraft forwards. So in space what we actually use is a fuel called hydrazine and we burn this out of the back of the spacecraft through little thrusters. This is how we manoeuvre the spacecraft and push it around. There are some other ways we do it as well. We're looking at things such as ion drives. That takes individual atoms, ionises them so that they've got an electric charge, and then uses electric fields to throw these gas molecules out f the back of the spacecraft. We've used that sort of application to send a spacecraft to the moon. It took a long time to get there but was moving very fast by the time it got there. So that could be a possibility for long range spacecraft. The bottom line is that if you want to slow something down you have to apply force in the opposite direction to which you're travelling. So you have these retro-rockets which fire against the direction in which you're travelling. So the rocket has a certain momentum moving in one direction and you want to give it momentum in the other direction so they cancel each other out. So the rocket is pushing against itself.

June 2006