Data on Phobos:Primary: Mars Semimajor axis (mean orbital distance): 9,377 km Orbital period: 0.31891023 days (= 0.000873 years) Orbital eccentricity: 0.0151 Orbital velocity: 2.138 km/second Orbital inclination: 1.08° to Mars's equator
Diameter:
22 km (mean)
27 km (along axis pointed toward Mars)
22 km (along axis pointed along orbit)
19 km (along polar axis)
Mass: 1.1x10^{16} kg, or 0.000000002 times the Earth's mass Average density: 1,870 kg/m^{3} Gravitational acceleration at surface: Escape velocity:Geometric albedo: Amount of sunlight at surface relative to Earth: 0.43 Visual magnitude (as seen from Earth): Equilibrium surface temperature (average over one day-night cycle): Apparent size of Sun as seen from Phobos: 0.3° Apparent size of Mars as seen from Phobos: 42°
Discovery: 11 August 1877 by A. Hall Space probes which have observed Phobos, with closest approach date and distance: