Phobos (moon)

Phobos (or Mars I) is one of Mars' moons. The other is Deimos.

Phobos

Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23 2008.
Discovery
Discovered by	Asaph Hall
Discovery date	August 18, 1877
Orbital characteristics
Epoch J2000
Periapsis	9235.6 km
Apoapsis	9518.8 km
Semi-major axis	9377.2 km[1]
Eccentricity	0.0151
Orbital period	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	26.8 × 22.4 × 18.4 km[2]
Mean radius	11.1 km[3] (0.0021 Earths)
Surface area	~6100 km² (11.9 µEarths)
Volume	5680 km³[4] (5.0 nEarths)
Mass	1.072×1016 kg[5] (1.8 nEarths)
Mean density	1.887 g/cm³[4]
Surface gravity	0.0084–0.0019 m/s² (8.4-1.9 mm/s²) (860-190 µg)
Escape velocity	11.3 m/s (40 km/h)[5]
Rotation period	synchronous
Equatorial rotation velocity	11.0 km/h (at longest axis' tips)
Axial tilt	0°
Albedo	0.071[3]
Temperature	~233 K
Apparent magnitude	11.3[6]

Orbits of Phobos and Deimos (to scale), as seen from above Mars

Phobos is the larger of the two moons, and is only 27 kilometers in diameter. This is about as far as a car can travel on the highway in 15 minutes. It is covered with craters, as Earth's moon is.^[7]

It is named after the god Phobos in Greek mythology. Its name means "fear".

Phobos is trapped in tidal drag, with its orbit lowering roughly 1.8 meters per century. In about 50 million years, Phobos will reach the Roche limit, where it is likely to be torn apart. Some fragments will fall on Mars and some will form a planetary ring or rings around Mars.

The other moon, Deimos, is the smaller of the two.

Spacecraft

The Soviet Union sent at least two space craft to this moon, Phobos 1 and Phobos 2. Both failed or lost contact with Earth, but Phobos 2 managed to take some pictures of the moon in 1989 before dying.

Features

There is one large crater on Phobos called Stickney. It is ${\displaystyle 1/3}$ the size of the moon itself.