Mars is orbited by two small, potato-shaped satellites, Phobos and Deimos. These tiny Martian satellites are thought to be captured asteroids composed of dark, carbon-rich rock. Each satellite has been heavily cratered by impacting meteoritic debris.
Mean radius: 21 km (13 mi)
Distance from Mars: 9,380 km (5,830 mi) Period of Rotation: 0.3188 days
Mars Global Surveyor View of Phobos
This image of Phobos was taken in August of 1998 by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor spacecraft. The high-resolution image shows new details of the surface of Phobos, the largest of two satellites orbiting the Red Planet. At the top of the image is Phobos' largest crater "Stickney", 10 km (6 miles) in diameter. Features visible in the crater and along its rim offer clues to how the crater was formed and the mixed composition of materials that make up this satellite.
Mean radius: 12 km (8 mi) Distance from Mars: 23,460 km (14,580 mi) Period of rotation: 1.2625 days