Do satellites need constant propulsion?
When a satellite has entered into its orbit around the Earth, it moves so fast that the centrifugal force caused by its motion is equal to the Earth's gravitational pull. In the absence of disturbing influences the satellite would continue along its trajectory forever. However
There are small perturbing forces that slow the satellite down and change its orbit. The satellite's trajectory can for instance be influenced by the radiation pressure of sunlight. If a satellite that has a large surface area is struck by a correspondingly large number of photons - the 'light particles' of solar radiation - enough energy is transferred to change the satellite's motion.
Traces of the Earth's atmosphere, which extend for hundreds of kilometers beyond the surface of the Earth, can have particularly strong perturbing effects. Friction with gas molecules of this residual atmosphere slows down every object that orbits the Earth, gradually bringing it down into a lower orbit. The GOCE satellite, scheduled to be launched this year, will be particularly affected by this problem. It will orbit at a relatively small distance of 260 kilometers (approximately 160 miles) from the surface of the Earth. For this reason, GOCE has a sleek aerodynamic design and comes equipped with special ion thrusters. These thrusters can accelerate the satellite and bring it back into a higher orbit or keep it in its designated orbit - this is called orbit control.
Satellites use rocket propulsion for attitude and orbit control.
In addition to this, satellites also need rocket propulsion systems so that they can be oriented in a certain direction - so for attitude control. This is necessary to ensure that the on-board camera is pointing towards the Earth, or that the solar arrays always face the Sun. For this reason, a satellite is equipped with a large number of small thrusters on its outer surface. These thrusters are fired briefly when required, so that the satellite starts to rotate slowly. As soon as the satellite has reached the desired position, thrusters are activated that fire in the opposite direction in order to stop the rotation.
Credit: German Aerospace Center