Mars has two tiny moons, Phobos and Deimos. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this picture of Phobos you can see a huge crater. Phobos is about 17 miles across and complets an orbit around Mars in less than 8 hours. Phobos is doomed. It orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it could crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

Pictured beside is Deimos, the smallest moon of Mars. In fact, Deimos is the smallest known moon in the Solar System measuring only 9 miles across. The existence of two Martian moons was predicted around 1610 by Johannes Kepler, the astronomer who derived the laws of planetary motion.