Martian moon · moon of Mars
Phobos
The larger of Mars's two small moons, a lumpy, grooved body spiralling slowly inward toward the planet.
- Distance from Earth today
- 210.6 million km1.408 AU · light: 11 min 43 s
- Distance from the Sun today
- 239.0 million km1.597 AU
- As of
- 4 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 22.2 km
- Mass
- 0.0106 × 10¹⁸ kg
- Density
- 1,872 kg/m³
- Mean distance from the planet
- 9,375 km
- Orbital period
- 7.6 h
- Orbital eccentricity
- 0.015
- Orbital inclination
- 1.1°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1877 – Asaph Hall
Description
Phobos is the inner and larger of Mars's two moons, discovered by Asaph Hall in 1877 and named after the Greek personification of fear. It is a dark, irregular body dominated by the large crater Stickney and crossed by long parallel grooves. Phobos orbits closer to its planet than any other known moon does and circles Mars faster than Mars rotates, so from the surface it rises in the west. Tidal forces are slowly drawing it inward, and in the distant future it will either break apart into a ring or crash into Mars. It has been imaged by many Mars orbiters, starting with the Vikings, and the rovers Curiosity and Perseverance have watched it pass in front of the Sun.