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Proteus or Neptune VIII, is the second biggest moon of Neptune, and its biggest close moon. It is named after Proteus, the shape-changing sea god of Greek mythology.

Proteus
Proteus (Voyager 2).jpg
Discovery
Discovered byVoyager 2
Stephen P. Synnott
Discovery date~June 16, 1989
Orbital characteristics
Epoch 18 August 1989
Periapsis117,584 ± 10 km
Apoapsis117,709 ± 10 km
117,647 ± 1 km (0.00079 AU)
Eccentricity0.00053 ± 0.00009
1.12231477 ± 0.00000002 d
7.623 km/s
Inclination0.524° (to Neptune equator)
0.026 ± 0.007° (to local Laplace plane)
Satellite ofNeptune
Physical characteristics
Dimensions440×416×404 km (± ~15 km)
Volume3.4 ± 0.3 ×107km³
Mass~4.4×1019 kg (estimate)
Mean density
~1.3 g/cm³ (estimate)
~0.06 m/s2 (0.006 g)
~0.16 km/s
synchronous
zero
Albedo0.096
Temperature~51 K mean (estimate)
20.3

Proteus was found from the images taken by Voyager 2 probe during the Neptune flyby in 1989. It got the designation S/1989 N 1. Stephen P. Synnott and Bradford A. Smith said (IAUC 4806) its discovery on July 7, 1989, speaking only of “17 frames taken over 21 days”, which gives a discovery date of sometime before June 16. The name was given on 16 September 1991.

Proteus is more than 400 kilometres in diameter, bigger than Nereid, another moon of Neptune. However, it was not found by Earth-based telescopes because it is so close to the planet that it is lost in the glare of reflected sunlight.

Proteus is one of the darkest objects in the solar system. Like Saturn's moon Phoebe, it reflects only 6 percent of the sunlight that hits it. Proteus is very cratered, showing no sign of any geological changes. It is also not a sphere; scientists believe Proteus is about as big as a body of its density can be without being pulled into a spherical shape by its own gravity. Saturn's moon Mimas has much more spherical shape despite being less massive than Proteus.

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