Phobos (moon)
Enhanced-color image of Phobos from the Mars Reconnaissance Orbiter with Stickney crater on the right
|
|
| Discovery | |
|---|---|
| Discovered by | Asaph Hall |
| Discovery date | 17 August 1877 |
| Designations | |
| Mars I | |
| Adjectives | Phobian |
| Orbital characteristics | |
| Epoch J2000 | |
| Periapsis | 9234.42 km |
| Apoapsis | 9517.58 km |
| 9376 km (2.76 Mars radii) | |
| Eccentricity | 0.0151 |
|
0.31891023 d (7 h 39.2 min) |
|
|
Average orbital speed
|
2.138 km/s |
| Inclination | 1.093° (to Mars's equator) 0.046° (to local Laplace plane) 26.04° (to the ecliptic) |
| Satellite of | Mars |
| Physical characteristics | |
| Dimensions | 27 × 22 × 18 km |
|
Mean radius
|
11.2667 km (1.76941 mEarths) |
|
1548.3 km2 (3.03545 µEarths) |
|
| Volume |
5783.61 km3 (5.33933 nEarths) |
| Mass |
1.0659×1016 kg (1.78477 nEarths) |
|
Mean density
|
1.876 g/cm3 |
|
0.0057 m/s2 (581.4 µ g) |
|
| 11.39 m/s (41 km/h) |
|
| Synchronous | |
|
Equatorial rotation velocity
|
11.0 km/h (6.8 mph) (at longest axis) |
| 0° | |
| Albedo | 0.071±0.012 |
| Temperature | ≈ 233 K |
Phobos (systematic designation: Mars I) is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.
Phobos is a small, irregularly shaped object with a mean radius of 11 km (7 mi), and is seven times larger than the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus), and is the personification of horror. The name "Phobos" is pronounced /ˈfoʊbəs/ FOH-bəs or /ˈfoʊbɒs/ FOH-bos, or like the Greek Φόβος.
Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon. It is indeed so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.
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