Moon of Mars
Phobos
A doomed moon that circles Mars three times a day.
Phobos orbits only about 6,000 km above the Martian surface and laps the planet in about 7.65 hours, faster than Mars itself turns. Because of that, it crosses the Martian sky backward, rising in the west. Tides are pulling it slowly inward.
How it moves
- Phobos completes an orbit every 0.31891 days (about 7.65 hours), roughly three times per Martian day.
- It orbits faster than Mars rotates (24.6 hours), so from the surface it rises in the west and sets in the east.
- Its rotation is synchronous: the same side always faces Mars.
- Phobos is moving closer to Mars by about 1.8 meters per century and should crash or break into a ring in about 50 million years.
Facts
- Mean radius
- 11.08 km
- Mass
- 1.060e+16 kg
- Rotation period
- 7.654 hours
- Orbital period
- 0.319 days
- Average orbital distance
- 9,378 km
- Eccentricity
- 0.0151
- Discovered
- August 17, 1877, by Asaph Hall
Phobos creeps about 1.8 meters closer to Mars every century and may become a ring in 50 million years.
Sources: nssdc.gsfc.nasa.gov, ssd.jpl.nasa.gov, planetarynames.wr.usgs.gov