compiled by Wm. Robert Johnston
last updated 12 September 2004

Banner artwork is imaginary view from 85 km above Phobos, showing Mars in the background. Image © 2004 by Wm. Robert Johnston (see notes below).

Phobos is one of the two natural satellites of Mars. Both Phobos and Deimos (the other) are small irregular rocky bodies, probably with origins in the asteroid belt.

The image below depicts six views of Phobos from various angles. It is based on models from WWW sources. The views are as follows:

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: Mass: 1.1x1016 kg, or 0.000000002 times the Earth's mass
Average density: 1,870 kg/m3
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:


Banner artwork: © 2004 by Wm. Robert Johnston. Image is a 3-D graphic based on a NASA/MSSS map of Mars (Mars Global Surveyor images) and a map and shape model made available by Phillip Stooke.

© 2001-2002, 2004 by Wm. Robert Johnston.
Last updated 12 September 2004.
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