Suppose an object released from rest takes t seconds to reach the ground, then its displacement h equals half g times t squared. Near the Earth's surface, acceleration due to gravity of this object is estimated by measuring the time taken by the object to free-fall through a known height. However, acceleration due to gravity on any other planet can be estimated by measuring its satellite's orbital period. Any satellite of mass m orbiting a planet is always in a free-fall motion. Hence, equating the force on the satellite mg with the centripetal force mω2r, g can be expressed as ω2r. Now, for one complete orbit around the planet, the satellite's angular velocity ω equals 2π divided by its orbital period. Thus, gravitational acceleration on any other planet equals 4π2 times the distance between the satellite and the planet divided by the square of the satellite's orbital period. Subsequently, once the value of the acceleration due to gravity of the planet becomes known, its mass mp can be determined.