Answer: Neptune

Neptune’s moon Triton is curious in all manner of ways: it’s the only large moon in the solar system with a retrograde orbit, it’s in synchronous rotation and always faces Neptune the same way, and its equator is almost exactly aligned with its orbital plane.

All of those facts pale in comparison to this fact about the fate of Triton, however. Thanks to a slowly decaying orbital pattern, Triton will eventually be consumed by Neptune. Scientists estimate that in 3.6 billion years the orbit will have degraded to the point that Triton will be close enough to Neptune to either be drawn right to the planet and crash into the surface or to be ripped apart by gravitational forces and turned into a ring of debris around the planet.