Phobos
The larger of Mars' two tiny moons: a dark, lumpy rock 27 km long, only 6,000 km above the ground.
Phobos orbits Mars faster than Mars turns, so it rises in the west and sets in the east, crossing the sky in about four and a half hours, twice a day.
It looks about a third as wide as our Moon does from Earth. When it passes in front of the Sun it makes a quick, potato-shaped partial eclipse that rovers have filmed.
It is slowly spiralling down: in about 50 million years it will break apart or crash into Mars.
Facts
- Size
- 27 × 22 × 18 km
- One orbit
- 7 h 39 min
- Height above Mars
- about 6,000 km
Did you know?
Phobos is so close to Mars that it can't be seen from the polar regions: it is always below their horizon.