Nereid (moon)

Nereid is the third-largest moon of Neptune. It has a highly eccentric orbit. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.

Discovery and naming
Nereid was discovered on 1 May 1949 by Gerard P. Kuiper on photographic plates taken with the 82-inch telescope at the McDonald Observatory. He proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune. It was the second and last moon of Neptune to be discovered before the arrival of Voyager 2 (not counting a single observation of an occultation by Larissa in 1981).

Orbit and rotation
Nereid orbits Neptune in the prograde direction at an average distance of 5513400 km, but its high eccentricity of 0.7507 takes it as close as 1372000 km and as far as 9655000 km.

The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.

In 1991, a rotation period of Nereid of about 13.6 hours was determined by an analysis of its light curve. In 2003, another rotation period of about 11.52 ± 0.14 hours was measured. However, this determination was later disputed. Other researchers have so far failed to detect any periodic modulation in Nereid's light curve.

Physical characteristics
Nereid is Neptune's third-largest satellite and has an average radius of about 170 km. It is rather large for an irregular satellite. The shape of Nereid is not known.

Since 1987 some photometric observations of Nereid have detected large (by ~1 of magnitude) variations of it brightness, which can happen over years and months, but sometimes even over a few days. They persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations. This means that they may be quite chaotic. To date there is no credible explanation of the variations, but, if they exist, they are likely related to the rotation of Nereid. Nereid's rotation can be either in the state of forced precession or even chaotic rotation (like Hyperion) due to its highly elliptical orbit. In any case its rotation should be rather irregular.

Spectrally Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material. The spectrum is markedly different from the outer-Solar-System minor planets, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body.

Halimede, which has similar colours, may be a fragment of Nereid that was broken off during a collision.

Exploration
The only spacecraft to visit Nereid is Voyager 2, which passed it at a distance of 4700000 km between 20 April and 19 August 1989. Voyager 2 obtained 83 images with observation accuracies of 70 km to 800 km. Prior to Voyager 2's arrival, observations of Nereid had been limited to ground-based observations that could only establish its intrinsic brightness and orbital elements. Although the images obtained by Voyager 2 do not have a high enough resolution to allow surface features to be distinguished, Voyager 2 was able to measure the size of Nereid and found that it was grey in colour and had a higher albedo than Neptune's other small satellites.

In fiction
In the Larry Niven book Ringworld, Nereid is described as having been leased by the outsiders "half a millennium ago". The protagonist, Louis Wu, speculates that the outsiders evolved on a gas giant moon similar to Nereid.