Image of Nereid by Voyager 2
|Discovered by||Gerard P. Kuiper|
|Discovery date||May 1, 1949|
|Pronunciation||// or // [a]|
|Periapsis||1,372,000 km (0.00917 AU)|
|Apoapsis||9,655,000 km (0.06454 AU)|
|Semi-major axis||5,513,787 km (0.03685 AU)|
|Orbital period||360.1362 d|
|Average orbital speed||934 m/s|
|Mean radius||170 ± 25 km|
|Mass||3.1×1019 kg (assumed)|
|Mean density||1.5 g/cm³ (assumed)|
|Equatorial surface gravity||~0.072 m/s²[b]|
|Escape velocity||~0.156 km/s[c]|
|Rotation period||0.48 d (11 h 31 min)|
|Temperature||~50 K mean (estimate)|
Discovery and naming 
Nereid was discovered on May 1, 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 5,513,400 km (3,425,900 mi), but its high eccentricity of 0.7507 takes it as close as 1,372,000 km (853,000 mi) and as far as 9,655,000 km (5,999,000 mi) from the planet.
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 the moon's light curve. Later in 2003 another rotation period of about 11.52 ± 0.14 hours was measured. However this determination was later disputed. Other researchers have failed so far to detect any periodic modulation in Nereid's light curve.
Physical characteristics 
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. As of 2010 there is no credible explanation of the variations, but, if they exist, they are likely related to the rotation of Nereid. This moon due to its highly elliptical orbit can be either in the state of forced precession or even chaotic rotation (like Hyperion). 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.
The only spacecraft to visit Nereid is Voyager 2, which passed Nereid at a distance of 4,700,000 km (2,900,000 mi) between April 20 and August 19, 1989. Voyager 2 obtained 83 images of the moon with observation accuracies of 70 km (43 mi) to 800 km (500 mi). Prior to Voyager 2's arrival, observations of Nereid had been limited to ground-based observations that could only establish Nereid's intrinsic brightness and orbital elements. Although the images obtained by the space probe did not have enough resolution to allow surface features to be distinguished, Voyager 2 was able to measure the size of Nereid and did find 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 millenium ago". The protagonist, Louis Wu, speculates that the outsiders evolved on a gas giant moon similar to Nereid. In Niven's short story, "Flatlander" from the book Neutron Star, Beowulf Schaeffer accompanies an earth person nicknamed Elephant, the flatlander of the title, to buy information from the outsiders. Elephant asks for the location of the most interesting unknown planet in the 60 or so light year sphere of local space. He wants to go on an adventure and become famous. The story shows a glimpse of a fascinating powerful alien race and culture.
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