Nereid (moon)

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

Nereid was found by the astronomer Gerard Kuiper in 1949, making it 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 the far reaches of space and the objects that orbit our planets.

Discovery and naming

Nereid was discovered on May 1, 1949, by Gerard P. Kuiper using photographs taken with a large telescope at the McDonald Observatory. He suggested the name Nereid when he shared his discovery. 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 and the last one discovered before the Voyager 2 spacecraft arrived, aside from one observation of Larissa in 1981.

Physical characteristics

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

Observations of Nereid’s brightness have shown some changes over time, but scientists are not sure why. Some think it might be because Nereid spins in a unusual way. Recent studies suggest Nereid’s surface is rough and not perfectly round.

Orbit and rotation

Nereid orbits Neptune in the prograde direction at an average distance of 5,504,000 km, but its high eccentricity means it can get as close as 1,381,500 km and as far as 9,626,500 km from Neptune.

Scientists think Nereid might have been captured from space, either as an asteroid or a Kuiper belt object. They also wonder if it used to be a closer moon of Neptune before something changed its path long ago. In different years, scientists measured how long it takes Nereid to spin, getting results like about 13.6 hours, then about 11.5 hours, and finally in 2016 with the Kepler space telescope, they found it spins once every 11.594 hours.

Exploration

The only spacecraft to visit Nereid was Voyager 2. It passed by Nereid in 1989, taking pictures from a distance of 4,700,000 km. These pictures helped scientists learn about Nereid's size and color, showing it is grey and brighter than Neptune's smaller moons.

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