Astronomy Study Guide

For an object to maintain a stable orbit around a larger body, such as a satellite around Earth, there must be a precise balance between the gravitational force pulling the object inwards and the tendency of the object to move in a straight line (inertia). If the orbital speed of a satellite changes, this balance is disrupted. If the orbital speed increases, the satellite's inertia effectively becomes stronger relative to the gravitational pull. To re-establish a stable orbit, the satellite would need to move to a larger radius, where the gravitational force is weaker, to match its increased speed. Conversely, if the orbital speed decreases, the gravitational pull becomes stronger relative to the satellite's inertia. To restore a stable orbit, the satellite would need to move to a smaller radius, where the gravitational force is stronger, to compensate for its reduced speed. In essence, a higher speed requires a larger orbit, and a lower speed requires a smaller orbit to maintain stability.

Objects in space, such as moons, planets, comets, and artificial satellites, move in predictable paths around larger, more massive bodies. This motion is called an orbit. An orbit is essentially a continuous fall towards the central body, but with enough tangential speed to continuously miss it. The gravitational force between the two bodies provides the centripetal force required to maintain the orbit. For planets orbiting a star, or moons orbiting a planet, these paths are typically elliptical, although for simplicity they are often approximated as circular. The speed of an orbiting object changes depending on its distance from the central body; it moves faster when closer and slower when further away.