Nereid

Nereid is one of Neptune’s most unusual and mysterious moons. It was discovered in 1949 by astronomer Gerard Kuiper, long before spacecraft ever reached the outer solar system. Nereid is Neptune’s third-largest moon, but what makes it truly unique is its highly stretched, or “eccentric,” orbit. Instead of circling Neptune in a neat, round path, Nereid moves in an oval-shaped orbit that takes it incredibly far from Neptune at some points and much closer at others. It takes about 360 days—almost a full Earth year—to complete one orbit, and this strange path has made scientists very curious about how it formed.

One popular idea is that Nereid did not originally form around Neptune. Some scientists think it may have once been a wandering object in space that Neptune captured with its gravity. Others believe that when Neptune’s largest moon, Triton, was captured long ago, the event disturbed the orbits of the moons that were already there, pushing Nereid into its current odd path. Because Nereid’s orbit is so extreme, the moon experiences big changes in sunlight and temperature throughout its journey around Neptune.

Nereid is quite small and distant, so we still don’t know very much about its surface. The best images we have are from Voyager 2, which flew past Neptune in 1989 and showed Nereid as a bright, irregularly shaped moon. Scientists think it might be a mixture of rock and ice, similar to many outer-solar-system moons. Even though Nereid isn’t as well-studied as some of Neptune’s other moons, its strange orbit and unknown origins make it one of the most intriguing objects in the Neptunian system. Studying Nereid helps scientists learn more about how planets and moons interact, how orbits can change over time, and what the early solar system might have been like.