Phobos
Moon· orbits Mars
Phobos is the larger, inner moon of Mars, orbiting so low and fast that it crosses the Martian sky twice a day and is slowly spiralling inward toward eventual breakup.
Open in 3D viewSee Phobos in the live, interactive 3D map
Key facts
Mass
1.07 × 10¹⁶ kg
0.000000145 Moon masses
Radius
11.1 km
1% of our Moon's radius
Surface gravity
0.00579 m/s²
0% of Earth's
Day length
7.65 hours
Orbit around Mars
7.6 hours
0.32 Earth days
Science
Physical
- Mass
- 1.07 × 10¹⁶ kg · 0.000000145 Moon masses
- Radius
- 11.1 km · 1% of our Moon's radius
- Density
- 1,871 kg/m³ · 1.9× water
- Surface gravity
- 0.00579 m/s² · 0% of Earth's
- Escape velocity
- 11.3 m/s · 0% of Earth's
Orbit around Mars
- Distance from Mars
- 2,562,054 km
- Orbital period
- 7.65 hours
- Eccentricity
- 0.015
- Inclination
- 1.10 °
Rotation
- Rotation period
- 7.65 hours
- Tidal locking
- Tidally locked to its parent
Temperature
- Mean
- 233 K (-40 °C)
Composition
- Bulk composition
- Dark, carbonaceous-chondrite-like rock with a porous, possibly rubble-pile interior; its origin (captured asteroid vs. re-accreted debris) is still debated.
Radiation & temperature
- Insolation
- 0.443 S⊕ · 44% of the sunlight Earth receives
- Geometric albedo
- 0.07
- Equilibrium temperature
- 223 K (-50 °C)
- Measured mean temperature
- 233 K (-40 °C)
- Greenhouse warming
- +10 K
Discovery & history
- Discovered
- August 18, 1877
- By
- Asaph Hall
- Method
- Visual discovery (telescopic)
- Name
- Phobos ('fear'), a son of the war-god Ares (Mars) in Greek myth.
Asaph Hall found both of Mars's tiny moons over a few nights in August 1877. Phobos, the larger, inner moon, orbits so close that it crosses the Martian sky three times a day and is slowly spiralling inward — in tens of millions of years it will break apart into a ring or strike the planet.
Images
Images from open archives · credit shown per image