- There is also an asteroid called 9313 Protea.
 | |
| Discovery | |
|---|---|
| Discovered by | Voyager 2 Stephen P. Synnott |
| Discovery date | ~June 16, 1989 |
| Epoch 18 August 1989 | |
| Periapsis | 117,584 ± 10 km |
| Apoapsis | 117,709 ± 10 km |
| Semi-major axis | 117,647 ± 1 km (0.00079 AU) |
| Eccentricity | 0.00053 ± 0.00009 |
| Orbital period | 1.12231477 ± 0.00000002 d |
| Average orbital speed | 7.623 km/s |
| Inclination | 0.524° (to Neptune equator) 0.026 ± 0.007° (to local Laplace plane) |
| Satellite of | Neptune |
| Physical characteristics | |
| Dimensions | 440×416×404 km (± ~15 km)[2][3] |
| Mean radius | 210 ± 7 km[4] |
| Volume | 3.4 ± 0.4 × 107km³[5] |
| Mass | ~5.0 × 1019 kg (estimate) |
| Mean density | ~1.3 g/cm³ (estimate)[4] |
| Equatorial surface gravity | ~0.06 m/s2 (0.006 g) |
| Escape velocity | ~0.16 km/s |
| Rotation period | synchronous |
| Axial tilt | zero |
| Albedo | 0.096[3][4][2] |
| Temperature | ~51 K mean (estimate) |
| Apparent magnitude | 19.7[4] |
| | |
Proteus (pronounced /ˈproʊtiəs/,[6] or as in Greek Πρωτεύς), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. It is the largest known non-spherical moon in the solar system. It is named after Proteus, the shape-changing sea god of Greek mythology.
Discovery
Proteus was discovered from the images taken by Voyager 2 probe during the Neptune flyby in 1989. It received the temporary designation S/1989 N 1. Stephen P. Synnott and Bradford A. Smith announced (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.[7]
Physical characteristics
Proteus is more than 400 kilometres in diameter, larger than Nereid, another moon of Neptune. However, it was not discovered by Earth-based telescopes because it is so close to the planet that it is lost in the glare of reflected sunlight.
Proteus is very cratered, showing no sign of any geological modification. It is irregularly shaped; scientists believe Proteus is about as large as a body of its density can be without being pulled into a spherical shape by its own gravity.[8] Saturn's moon Mimas has a more spherical shape due to a lower density, despite being less massive than Proteus.
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