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Why does Triton always show the same face toward Neptune?
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Triton
Image of Triton, Neptune's largest satellite, captured by a camera aboard Voyager 2, 1989.
Triton always shows the same face toward Neptune because it is in a state of synchronous rotation.1
This means that the time it takes for Triton to rotate once on its axis (5.877 days) is exactly the same as the time it takes to complete one revolution around the planet. In this state, the moon’s rotation and orbital periods are matched, causing it to keep the same hemisphere facing the planet at all times.12
This condition is the result of tidal interactions between the two bodies. Over time, these gravitational forces—which involve cyclic deformations—exerted a braking effect on Triton’s spin until its rotation became synchronized with its orbital motion.12