The other solar planets have orbital velocities that differ from that of Earth. They complete a full revolution around the Sun in either less or more than a year, depending on whether they are closer or farther away from the Sun than the Earth is. Many planets, in turn, have one or more satellites orbiting them. For example, the Earth has one natural satellite , the Moon . It is simply a body that orbits another.
Gravity provides the glue for the orbital motion.
The accurate knowledge of celestial mechanics and the careful study of the motions of natural satellites, like the Moon, have enabled scientists to build artificial satellites and send them into space. Most artificial satellites are orbiting the Earth. A few satellites have been launched to explore other planets of our Solar System. An example of one such satellite is Viking, which explored Mars. Powerful rockets are used to send artificial satellites into space.