Nereid (moon)

Nereid, also called Neptune II, is the third-largest moon that goes around the planet Neptune. It has a very unusual path, making its orbit one of the most stretched-out of any known moon in our Solar System. This special path makes it different from most other moons.

Nereid was found by the astronomer Gerard Kuiper in 1949, and it was the second moon of Neptune to be discovered. Scientists have been watching Nereid ever since to learn more about its unique journey and how it fits into the story of our Solar System. Studying Nereid helps us understand more about space and the objects that orbit our planets.

Discovery and naming

Nereid was discovered on May 1, 1949, by Gerard P. Kuiper. He used photographs taken with a big telescope at the McDonald Observatory. He suggested calling the moon Nereid. The moon is named after the Nereids, sea-nymphs from Greek mythology, who served the god Neptune. Nereid was the second moon of Neptune ever found. It was the last one found before the Voyager 2 spacecraft arrived, except for one sighting of Larissa in 1981.

Physical characteristics

Nereid is the third-largest moon of Neptune. It has a mean radius of about 180 kilometres (110 mi), similar to Saturn's moon Mimas. It is the largest known irregular moon. Its surface looks like a mix of water ice and other materials.

Scientists have noticed changes in Nereid’s brightness over time, but they are not sure why. Some think it might spin in an unusual way. Recent studies show that Nereid’s surface is rough and not perfectly round.

Orbit and rotation

Nereid orbits Neptune in the prograde direction. It is usually about 5,504,000 km away, but it can get as close as 1,381,500 km and as far as 9,626,500 km from Neptune.

Scientists think Nereid may have come from space, maybe as an asteroid or a Kuiper belt object. They also wonder if it used to be closer to Neptune before something changed its path. Over the years, they have found that Nereid spins once about every 11.5 hours. In 2016, using the Kepler space telescope, they found it spins once every 11.594 hours.

Exploration

The only spacecraft to visit Nereid was Voyager 2. It flew by Nereid in 1989, taking pictures from far away. These pictures helped scientists learn about Nereid's size and color. They showed that Nereid is grey and brighter than Neptune's smaller moons.

If chosen, a mission called Arcanum would fly by Nereid before orbiting Neptune and studying its moon Triton.