Mars has two moons named Phobos and Deimos, Greek for fear and panic. Phobos is the closer of the two, orbiting Mars 9378 km above the planet’s center. It is very small - the diameter of the moon is only 22 km. It is very odd-shaped, and has a mass of just 1.1E16 kg. It is composed mostly of carbon-rich rock and is heavily cratered. Most astronomers think that Phobos is a captured asteroid.

Phobos orbits Mars very quickly. It usually rises, transverses the Martian sky, and sets twice every Martian day. The moon is also very close to Mars’ surface. Just as an airplane flying over the Earth’s equator cannot be seen above the horizon for an observer in the United States, Phobos is so close to Mars’ surface that it cannot be seen above the horizon from all points on Mars. As it orbits, it slowly spirals in towards the Martian surface. Phobos looses 1.8 meters of altitude per century, and in 50 million years it will either crash into the surface or be destroyed in the atmosphere.

Deimos orbits farther out than Phobos, and it is even smaller, with a diameter of only 12.6 km and a mass of 1.8E15 kg. In fact, Deimos is the smallest known moon in the Solar System. Like Phobos, Deimos is made of mostly cratered carbon-rich rock, is very amorphous, and is thought to be a captured asteroid. Like our own Moon, Deimos orbits far enough away from Mars that it is being slowly pushed farther and farther away from the planet.