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Phobos

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MOON

Phobos

The larger, doomed moon of Mars — spiraling inward and likely to be shredded into a ring within 100 million years.

Orbits Mars Inner moon of Mars
  • Orbits Mars every 7 h 39 min u2014 faster than Mars rotates, so it rises in the west and sets in the east.
  • Orbit decaying at ~1.8 cm/year toward Mars; will breakup into a ring or impact in ~30-50 Myr.
  • Dominated by the 9-km crater Stickney u2014 nearly half the moon's diameter.
  • Surface covered by deep, regular grooves possibly caused by tidal stress or Stickney-impact ejecta.
  • Target of JAXA's MMX mission (launch 2026) u2014 will return the first sample from a Martian moon.

Physical Properties

11 km
1.0659e16 kg
1.876 g/cm³
0.0057 m/s²
0.0114 km/s
7.66 h
0.071
150 K
300 K

Orbit

Mars
9,376 km
0.0151
1.093°
0.31891 d
2.138 km/s

Missions to Phobos

1 spacecraft tracked on Space Launch Live.

Probe
Mars Express

Sources & Further Reading

Numerical values (radius, mass, orbital elements, temperatures) are drawn from NASA NSSDC Planetary Fact Sheets, JPL Horizons, and the JPL Small-Body Database. Last refreshed: 2026-04-18 18:19:23.

Phobos is the larger and closer of the two moons of Mars, discovered along with Deimos in August 1877 by American astronomer Asaph Hall. It is a small, irregular, heavily cratered body — triaxial dimensions 26 × 23 × 18 km — orbiting Mars just 5,989 km above the Martian surface (closer than any other moon to its primary). Its density and low albedo resemble carbonaceous chondrite asteroids, suggesting either capture or a large-impact formation from Martian material.

Phobos’s orbit is decaying. Tidal interaction with Mars is drawing it inward at roughly 1.8 cm per year; within 30-50 million years it will either be shredded by Mars’s tidal forces into a ring, or impact the Martian surface. Its largest feature, the crater Stickney, is 9 km across — fully half the moon’s width — and the impact that formed it very nearly destroyed the moon.

JAXA’s Martian Moons eXploration (MMX) mission, scheduled to launch in late 2026, will orbit Mars, land briefly on Phobos, collect surface samples, and return them to Earth in 2031 — our first samples from a moon of Mars.