A body in the absence of a force acting on it, continues to remain in a state of rest or motion with a constant velocity. If a body is required to move along a circular path, a force known as centripetal force has to be applied. In the motion of satellites around the Earth, the centripetal force is provided by the force of gravitational attraction between the satellite and the Earth. This force is one that is present between any two objects that have a mass is given by the formula: `F = (G*m_1*m_2)/r^2` . As seen from the formula the force is directly proportional to the product of the mass of the two objects and inversely proportional to the distance between them.
A satellite can orbit around the Earth, along a path with a particular radius that is dependent on the angular velocity of the satellite and its frequency of rotation around the Earth. This cannot be set arbitrarily. If the frequency of rotation of the satellite or the radius of the satellite's orbit has to be changed an external source of power has to be used. Usually satellites have a small jet engine that helps in maintaining the required characteristics of the orbit.