What Should We Do With Hubble? | Q&A 434

Tidal locking occurs when gravitational forces between orbiting bodies cause them to synchronize their rotation with their orbital period; objects orbiting slower than their parent body's rotation will migrate outward (like Earth's Moon), while those orbiting faster will migrate inward (like Mars' Phobos), due to conservation of angular momentum and differential gravitational forces creating torque on the orbiting object.