How is the mass of a planet measured? In Chapter 4 you learned how a planet’s gravitational attraction affects the motion of a nearby small body, such as one of its moons or a passing spacecraft. The motion of the small body can be accurately predicted from the planet’s mass, Newton’s law of gravitation, and Kepler’s third law. This procedure can be used in reverse to calculate the planet’s mass by observing the motion of a small body. Prior to the space age, a planet’s mass was measured by observing the motions of its moons. Planetary spacecraft have now made it possible to measure the masses of planets with unparalleled accuracy. As a spacecraft flies by, the planet’s gravity tugs on it and alters its trajectory. Tracking and comparing the spacecraft’s radio signals reveals minute changes in the spacecraft’s trajectory, giving a highly accurate measure of the planet’s mass.