Write an equation for the potential energy of a satellite. Why is the potential energy of a satellite negative?

(N/A) The gravitational potential energy $U$ of a satellite of mass $m$ orbiting a planet of mass $M$ at a distance $r$ from the center of the planet is given by the equation:
$U = -\frac{GMm}{r}$
where $G$ is the universal gravitational constant.
The potential energy is negative because the gravitational force is attractive in nature. By convention,the potential energy of a system is defined as zero at an infinite distance $(r = \infty)$ from the source mass. Since the gravitational force acts to pull the satellite towards the planet,work must be done by an external agent to move the satellite from its position $r$ to infinity. Because the system is in a bound state,the energy required to reach the zero-potential state at infinity is positive,implying that the energy at any finite distance $r$ must be less than zero.