Nereid

Nereid

Image of Nereid by Voyager 2
Discovery
Discovered by	Gerard P. Kuiper[1]
Discovery date	May 1, 1949
Orbital characteristics
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)
Eccentricity	0.7507 (Epoch 2000)[2]
Orbital period	360.1362 d
Average orbital speed	934 m/s
Inclination	7.090° to the local Laplace plane (Epoch 2000)[2] 32.55° to Neptune's equator
Satellite of	Neptune
Physical characteristics
Mean radius	170±25 km[3]
Mass	3.1 × 1019 kg (assumed)[3]
Mean density	1.5 g/cm3 (assumed)[3]
Rotation period	0.48 d (11 h 31 min)[4]
Albedo	0.155[3]
Temperature	~50 K mean (estimate)

Nereid (pronounced /ˈnɪəriɪd/; also /ˈnɛriɪd/, as in Greek Νηρηΐδα),^[5] also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard P. Kuiper in 1949.

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.^[1] It was the last moon of Neptune to be discovered before the arrival of Voyager 2.^[6]

Orbit and Rotation

Nereid orbits Neptune in the prograde direction^[7] 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 a captured asteroid or Kuiper belt object, or that it was perturbed during the capture of Neptune's largest moon Triton.

The rotation period of Nereid has been determined by analysis of its light curve to be 11.52±0.14 hours.^[4]

Physical characteristics

Nereid is Neptune's third-largest satellite and has an average radius of about 170 kilometres (110 mi).^[3] It is rather large for an irregular satellite.^[4]

Spectrally Nereid appears grey in colour^[7] and water ice has been detected on its surface.^[8] 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.^[8] The spectrum is markedly different from the outer solar system minor planets Pholus, Chiron and 1997CU26, suggesting that Nereid formed around Neptune rather than being a captured body.^[8]

Halimede may be a fragment of Nereid that was broken off during a collision.^[9]

Exploration

The only spacecraft to visit Nereid is Voyager 2, which passed Nereid at a distance of 4,700,000 km (2,920,000 mi)^[10] between April 20 and August 19, 1989.^[11] Voyager 2 obtained 83 images of the moon with observation accuracies of 70 km (43 mi) to 800 km (500 mi).^[11] 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.^[12] Although the images obtained by the space probe did not have enough resolution to allow surface features to be distinguished, Voyager 2 did find that Nereid is probably grey in colour and had a higher albedo than Neptune's other small satellites.^[7]

References

v • d • e Neptune Major moons Triton · Proteus  · Nereid Characteristics Rings · Great Dark Spot · Trojans · Moons Discovery John Couch Adams · Johann Gottfried Galle · Heinrich d'Arrest · William Lassell · Urbain Le Verrier Exploration Voyager program · Voyager 2 Trojans 2001 QR322 · 2004 UP10 · 2005 TN53 · 2005 TO74 · 2006 RJ103 · 2007 RW10 Related topics Neptune-crosser asteroid · Neptune in fiction