Thrusters
1Not everything in space flight needs a huge rocket motor, sometimes we have to be far more delicate. Typical jobs involve changing the direction a satellite is pointing in (but not travelling in) or giving it a very gentle nudge when it is docking with another satellite. These low force rocket motors are usually called 'thrusters'.
Cold gas systems
The simplest form of thruster is little more than a container of pressurised gas. It is very like the balloon you saw earlier. When thrust is needed some of the pressurised gas is released through the nozzle. Typical gases used are nitrogen, argon, freon and propane. It is important to make sure that the gas will not damage any components that it might land on such as solar cells, sensors or even an astronaut’s space suit! Because the gas is cold the thrust is very low, typically around 10 mN (around the weight of a one gram mass on Earth) and the specific impulse is only around 50 s at best. Bigger units have been used to allow astronauts to move freely outside the space shuttle and the ISS.
