Outer moon of MarsA small, quiet companion

Deimos

Deimos is the smaller and more distant of Mars's two moons, drifting through a slow and much calmer orbit.

Moon of Mars · about 12.4 km mean diameter · 30.3-hour orbit · first seen in 1877

Host: Mars
Type: Small irregular moon
Mean diameter: About 12.4 km
Mean orbital radius: 23,463 km
Orbital period: 30 h 18 min
Rotation: Synchronous
Surface gravity: About 0.003 m/s²
Discovered: 1877 · Asaph Hall

Deimos up close

NASA imagery

Comparison image showing asteroid Gaspra, Mars moon Deimos and Mars moon Phobos at the same scale — **Deimos compared with Phobos** Deimos is the smoother and more distant of Mars's two moons. This NASA comparison places Deimos beside Phobos and asteroid Gaspra at the same scale. Image: NASA/JPL.

A slow martian moonrise

Deimos takes more than a Martian day to complete one orbit. Seen from Mars, it rises in the east and sets in the west, following the same general direction as the Sun.

Its orbit is farther away and much less strongly affected by Mars's tides than Phobos. Deimos is not expected to break apart or impact the planet on the timescale that threatens its sibling.

Why is Deimos so smooth?

Deimos has fewer sharp relief features than Phobos. A blanket of fine regolith, shaken down slopes by small impacts, likely helps fill and soften many craters.

The moon may be a captured asteroid or may have assembled from impact debris around Mars. Its composition and history remain key questions for future martian moon missions.

Worth knowing

Deimos is named for the Greek personification of terror and dread, brother of Phobos.
It was first photographed at close range by Viking 2 in 1977.
The Sun appears only somewhat smaller from Deimos than it does from Mars.

Watch and explore

Research & further reading

Mission imagery and measurements are interpreted alongside orbital data. Follow the source links for the current NASA overview and precise satellite parameters.

Mission reference NASA · Deimos ↗ NASA's overview of Deimos and its exploration. Orbital data NASA/JPL satellite elements ↗ Mean orbital elements and current satellite catalogue data.