Proteus (moon)

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Proteus
Proteus (Voyager 2).jpg
Discovery
Discovered by Voyager 2
Stephen P. Synnott
Discovery time ~June 16, 1989
Orbit
Reference date 18 August 1989
Shortest distance from what it orbits around 117,584 ± 10 km
Longest distance from what it orbits around 117,709 ± 10 km
Longest distance from the center of its orbital path
("semi-major axis")
117,647 ± 1 km (0.00079 AU)
How egg-shaped its orbit is
("eccentricity")
0.00053 ± 0.00009
How long it takes to complete an orbit 1.12231477 ± 0.00000002 d
Average speed 7.623 km/s
Angle above the reference plane
("inclination")

0.524° (to Neptune equator)

0.026 ± 0.007° (to local Laplace plane)
What it orbits Neptune
Size and Other Qualities
Measures 440×416×404 km (± ~15 km)
Volume inside it 3.4 ± 0.3 ×107km³
Mass ~4.4×1019 kg (estimate)
Average density ~1.3 g/cm³ (estimate)
Gravity at its surface ~0.06 m/s2 (0.006 g)
Slowest speed able to escape into space
("escape velocity")
~0.16 km/s
How long it takes to turn around one time synchronous
Angle at which it turns
(in relation to its orbit)
zero
How much light it reflects 0.096
Avg. surface temp. ~51 K mean (estimate)
Seeming brightness
("apparent magnitude")
20.3

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 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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