Mars' moons are among the smallest in the solar system. Phobos is a bit larger than Deimos, and orbits only 3,700 miles (6,000 kilometers) above the Martian surface. No known moon orbits closer to its planet.
It whips around Mars three times a day, while the more distant Deimos takes 30 hours for each orbit. Phobos is gradually spiraling inward, drawing about six feet (1.8 meters) closer to the planet each century. Within 50 million years, it will either crash into Mars or break up and form a ring around the planet.
To someone standing on the Mars-facing side of Phobos, Mars would take up a large part of the sky. And people may one day do just that. Scientists have discussed the possibility of using one of the Martian moons as a base from which astronauts could observe the Red Planet and launch robots to its surface, while shielded by miles of rock from cosmic rays and solar radiation for nearly two-thirds of every orbit.
Like Earth's Moon, Phobos and Deimos always present the same face to their planet. Both are lumpy, heavily-cratered and covered in dust and loose rocks. They are among the darker objects in the solar system. The moons appear to be made of carbon-rich rock mixed with ice and may be captured asteroids.
Phobos has only 1/1,000th as much gravitational pull as Earth. A 150-pound (10.7 stones / 68 kilogram) person would weigh two ounces (68 grams) there. Yet NASA's Mars Global Surveyor has shown evidence of landslides, and of boulders and dust that fell back down to the surface after being blasted off the moon by meteorites.