At only 17 miles in length, it is the larger of Mars' two moons. Phobos is a tiny moon. It is so close to Mars that it cannot be seen above the horizon from all points on the planet.

Viking 2 view of the Martian satellite Deimos taken from 500 km distance. Close-up image of Mars' satellite Deimos from Viking 2 showing craters and associated streaks. 23,459 km from Mars Mars itself has no rings as it is too close to the Sun.

The tiny moon zips around Mars in only 30 hours and is so close to its parent planet that it was missed for centuries. This proximity means that when an astronomer looks through a telescope at Mars, the light being reflected by Mars overpowers the light being reflected by Phobos and Deimos. It does, however, have two satellites, Phobos and Deimos; Phobos being the larger and closer of the two. At a distance of 5826 miles (9380 km) from Mars, Phobos appears as no more than a tiny, bright dot in the Martian sky. Deimos is about 8 km across at its widest part. DEIMOS (panic) Mean distance from Mars (km) 9,377: 23,436: Orbital period (Mars days) 0.31891: 1.26244: Major axis (km) 26: 16: Minor axis (km) 18: 10: Mass (x 10 15 kg) 10.8: 1.8: Mean density (kg/m 3) 1,900: 1,750: Phobos and Deimos (moons of Mars) Mars has two small moons: Phobos and Deimos. The streaks run from upper right to lower left, and may have been formed by the base surge "wind" of an impacting meteoroid. While our Moon orbits at a healthy distance of 384,000 kilometers from the Earth, Phobos is only 9,378 kilometers distant from Mars and Deimos is a little farther out at 23,459 kilometers.

Deimos' radius is only 3.9 miles.