Phobos Mean Distance from Mars 9,378 km (5,827 miles) Orbital Period around Mars 0.319 Earth days Mean Diameter 22.2 km (13.8 miles) Year Discovered 1877 Deimos Mean Distance from Mars 23,459 km (14,577 miles) Orbital Period around Mars 1.26244 Earth days Mean Diameter 12.6 km (7.83 miles) Year Discovered 1877 Original images located on various NASA websites including: http://photojournal.jpl.nasa.gov/index.html and http://nssdc.gsfc.nasa.gov/photo_gallery/