Proteus (moon)
Proteus (/ˈproʊtiəs/ PROH-tee-əs), also known as Neptune VIII, is the second-largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator.
Voyager 2 image (1989) | |
| Discovery | |
|---|---|
| Discovered by | Voyager 2 Stephen P. Synnott |
| Discovery date | June 16, 1989 |
| Designations | |
Designation | Neptune VIII |
| Pronunciation | /ˈproʊtiəs/ PROH-tee-əs |
Named after | Πρωτεύς or Πρωτέας, Prōteys or Prōteas |
| S/1989 N 1 | |
| Adjectives | Protean (/ˈproʊtiən/ PROH-tee-ən or /proʊˈtiːən/ proh-TEE-ən) |
| Orbital characteristics | |
| Epoch 18 August 1989 | |
| Periapsis | 117584±10 km |
| Apoapsis | 117709±10 km |
| 117647±1 km (4.75 RN) | |
| Eccentricity | 0.00053±0.00009 |
| 1.12231477±0.00000002 d | |
Average orbital speed | 7.623 km/s |
| Inclination | 0.524° (to Neptune's equator) 0.026°±0.007° (to local Laplace plane) |
| Satellite of | Neptune |
| Physical characteristics | |
| Dimensions | 424 × 390 × 396 km |
Mean radius | 210±7 km |
| 554,200 km2 | |
| Volume | (3.4±0.4)×107 km3 |
| Mass | 4.4×1019 kg (7.3672×10−6 Earths) |
Mean density | ≈ 1.3 g/cm3 (estimate) |
| 0.07 m/s2 | |
| 0.17 km/s | |
| synchronous | |
| zero | |
| Albedo | 0.096 |
| Temperature | ≈ 51 K mean (estimate) |
| 19.7 | |
Despite being a predominantly icy body more than 400 km (250 mi) in diameter, Proteus's shape deviates significantly from an ellipsoid. It is shaped more like an irregular polyhedron with several slightly concave facets and relief as high as 20 km (12 mi). Its surface is dark, neutral in color, and heavily cratered. Proteus's largest crater is Pharos, which is more than 230 km (140 mi) in diameter. There are also a number of scarps, grooves, and valleys related to large craters.
Proteus is probably not an original body that formed with Neptune. It could have accreted later from the debris formed when the largest Neptunian satellite, Triton, was captured.