. Ce le sti al ca le nd ar . Satellites of Mars . (Orbital, physical and photometric data) . Name . Orbital Period . Semimajor Axis 1 . Mass . Radius . Magnitude 2 . d . ? 10 3 km . 1/Planet . km . m . I Phobos . 0.32 . 9.38 . 1.65 ? 10 -8 . 13.4 ? 11.2 ? 9.2 . 11.4 . II Deimos . 1.26 . 23.46 . 3.71 ? 10 -9 . 7.5 ? 6.1 ? 5.2 . 12.5 . 1 Half the length of the major axis of the ellipse; a standard element used to describe an elliptical orbit. 2 This is the mean opposition magnitude of the satellite.