Various factors perturb a spacecraft’s trajectory and affect the way it behaves in orbit:

• The Earth is not a perfect sphere and its mass is not uniformly distributed. It is flattened at the poles and bulges at the equator, which causes the gravity field to vary by as much as 1/1000 of its main attractive force.
• The vacuum of space is not complete. In the vicinity of Earth, there is a residual atmosphere that increases in density the nearer we get to the planet. Up to an altitude of at least 1,000 km, satellites are subjected to braking due to drag caused by molecules and atoms. For example, a satellite at 800 km above Earth can remain in orbit for half a century, whereas at 300 km it would fall back into the atmosphere after a few months, and at 200 km within days.
• Satellites are also subjected to the attraction of the Moon and Sun.
• The Sun emits photons that, to a lesser extent, can perturb a satellite’s motion.

Other kinds of orbital manoeuvres may also be necessary, for example when two spacecraft need to rendezvous in orbit on crewed missions, to resupply a space station or repair a satellite. Atmospheric re-entry and the return to Earth of astronauts or experimental systems also have to be carefully controlled.