Adjective(s) Unknown
Orbital properties
Aphelion/apoapsis 9517.58 km
Perihelion/periapsis 9234.42 km
Semi-major axis 9376 km (2.76 Mars radii)
Orbital eccentricity 0.0151
Orbital period 0.31891023 d(7 h 39.2 min)
Average orbital speed 2.138 km/s
Physical properties
Radius 11.2667 km(1.76941 mEarths)
Mass 1.0659×1016 kg(1.78477 nEarths)
Mean density 1.876 g/cm3
Escape velocity Unknown
Rotation period Synchronous
Average surface temperature ≈ 233 K
Apparent magnitude 11.3
Moons 0
Phobos is Mars's larger moon. Phobos is closer to Mars than Deimos.

Name EtymologyEdit

Phobos was named after the Greek word for fear.

Named Geological FeaturesEdit

Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver Travels.There is one named regio, Laputa Regio, and one named planita, Lagado Planitia; both are named after places in Gulliver's Travels (the fictional Laputa, a flying island, and Lagado, imaginary capital of the fictional nation Balnibarbi).The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Kepler Dorsum. Several craters have been named.


  • Phobos is the closest moon to Mars.
  • It was discovered by Asaph Hall on August 18, 1877.
  • Phobos is most likely an Asteroid that was captured by Mars's gravitational pull.
  • If you stood on the surface of Mars, the moon would appear 1/3rd the size of the Moon from Earth.
  • It takes Phobos 7.66 days to orbit Mars.
Martian Moons
Phobos - Deimos