What are Phobos and Deimos?
Two tiny moons, 22 and 12 km across — probably captured asteroids or debris from an ancient impact.
Phobos orbits so close and so fast that it rises in the west and sets in the east twice a day. It is slowly spiralling inwards and will break up or crash into Mars in about 50 million years.
Deimos is farther out and moves so slowly that it stays up for about two and a half days at a time.