Phobos is a lumpy, cratered rock just 27 km long that circles Mars three times a day - faster than Mars itself rotates. From the Martian surface it rises in the west and sets in the east, twice a day.
It orbits closer to its planet than any other known moon - just 6,000 km above the surface.
Its largest crater, Stickney, is 9 km across - nearly half the moon’s width.
Gravity is so weak you could throw a ball into orbit around it.
A moon on a countdown
Phobos orbits so close that Mars’ tides are dragging it inward by about 1.8 metres per century. In roughly 50 million years it will either crash into Mars or be torn apart into a short-lived ring.
Physical characteristics
Diameter
22.2 km
Dimensions
27 × 22 × 18 km
Mass
1.060e+16 kg
Surface gravity
0.01 m/s²
Density
1.86 g/cm³
Tidally locked
Yes
Mean temperature
-40 °C
Geometric albedo
0.07
Atmosphere and composition
Atmosphere
None.
Composition
Porous carbon-rich rock, similar to a captured asteroid or re-accreted impact debris.
Viking 1 achieved the first fully successful landing on Mars on 20 July 1976 and sent back the first photographs ever taken from the Martian surface. Its lander kept working until November 1982.
Mars ExpressESA · Orbiter · 2003 · active
ESA’s first mission to another planet has been orbiting Mars since December 2003. Its cameras and radar have mapped buried water ice, ancient water-carved terrain, and made the closest-ever flybys of Phobos.