Gravity is the force of attraction between masses. It is the centripetal force needed to keep a satellite in orbit. The greater the mass of the satellite, the greater the force involved. A satellite can be a planet orbiting a star, like the Sun; an object orbiting a planet, like the Moon or a manmade satellite orbiting the Earth. Artificial satellites were launched in orbit around the Earth using a rocket. Satellites in lower orbits (nearer the Earth) travel faster.

Satellites in low polar orbits are used for forecasting the weather, spying and observing the Earth's surface. It takes them around 90 minutes to orbit the Earth. Geostationary satellites are in orbit above the equator. It takes them 24 hours to orbit the Earth and they appear to be in the same position. They are used for communications, such as mobile phones, satellite TV and GPS.

Orbits can be highly elliptical (oval), as opposed to circular. Periodic comets have highly elliptical orbits. When they are close to the Sun, the gravitational force is stronger than when they are further out. Their velocity changes as well, being higher close to the Sun and lower further from the Sun.