Deimos is the smaller and farther of Mars’ two moons, a 12-km rock that takes 30 hours to circle the planet. From Mars it would look like a bright star that takes 2.7 days to cross the sky.
Escape velocity is ~5.6 m/s - a good sprinter could nearly jump off.
Unlike doomed Phobos, Deimos is slowly spiralling away from Mars.
Its name means "dread"; Phobos means "fear" - the horses of the war god Ares.
Captured or built in place?
Both Martian moons look like carbon-rich asteroids, yet they orbit neatly in Mars’ equatorial plane - hard to explain by capture. They may instead have re-accreted from debris blasted off Mars by a giant impact.
Physical characteristics
Diameter
12.6 km
Dimensions
15 × 12 × 11 km
Mass
1.500e+15 kg
Surface gravity
0 m/s²
Density
1.47 g/cm³
Tidally locked
Yes
Mean temperature
-40 °C
Geometric albedo
0.07
Atmosphere and composition
Atmosphere
None.
Composition
Carbon-rich rock and regolith; smoother than Phobos, its craters softened by dust.
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.