Tidal Locking occurs when a satellite's rotation period synchronizes with its orbital period through tidal dissipation. The differential gravitational force across the body creates a tidal bulge; if the spin and orbit are not synchronized, this bulge is dragged out of alignment, creating a torque that spins the body toward synchrony.
dΩ/dt = −(3k₂GM²R⁵)/(2Qa⁶) · (Ω/n − 1) | τ_lock ∝ Qa⁶/(M²R⁵)
The Moon is tidally locked to Earth (always shows the same face). Mercury is in a 3:2 spin-orbit resonance due to its orbital eccentricity. Many exoplanets in the habitable zone are expected to be tidally locked.