Deimos

Deimos is the smaller and more distant of Mars's two moons, a dark, lumpy body whose longest axis runs to about 15 kilometres but which averages only around 12 kilometres across and roughly 6 kilometres in mean radius. Where our own Moon is a world you can map with the naked eye, Deimos is closer in size to a modest asteroid — an object you could walk the long way around in an afternoon, if its feeble gravity let you walk at all rather than drift.

It circles Mars once every 1.26 days at an average distance of roughly 23,500 kilometres, on a near-circular orbit tilted less than a degree to the planet's equator. Smooth-looking, muted, and geologically quiet, Deimos has none of the drama of Mars below it — and yet its origin remains one of the more stubborn puzzles in the inner solar system.

Discovered in a week of two moons

Deimos was found on 12 August 1877 by the American astronomer Asaph Hall, working with the 26-inch refractor at the U.S. Naval Observatory in Washington during a favourable close approach of Mars. It was the first of the pair he caught: six nights later, on 18 August, he spotted the brighter, faster inner moon, Phobos. Hall had nearly given up the search before his wife, Angeline Stickney, urged him to keep looking — a debt later paid by naming Phobos's largest crater Stickney.

The names come from Greek mythology, where Deimos ('dread' or 'panic') and Phobos ('fear') are the sons and attendants of Ares, the war god the Romans called Mars. Only two features on Deimos carry official names, both craters: Swift and Voltaire, honouring Jonathan Swift and Voltaire, who each wrote of two Martian moons more than a century before anyone actually saw them.

A small, smoothed-over world

Deimos is a triaxial ellipsoid, measured at roughly 15 by 12 by 11 kilometres, which averages out to the mean radius of about 6.2 km listed here. That is small enough that its gravity is only a few thousandths of Earth's — its escape velocity is a mere 5.6 metres per second, slow enough that a hard leap could carry an astronaut off the moon entirely. Its surface is dark, reflecting only around 7 percent of the sunlight that hits it, and its low density points to a porous, primitive, carbon-rich composition resembling the C-type asteroids of the outer main belt.

The moon is heavily cratered but looks notably smoother than Phobos. Its largest impact crater is only about 2.3 km across, and a blanket of loose regolith perhaps 100 metres deep has partly buried and softened older craters. Crucially, Deimos lacks the deep parallel grooves that scar Phobos. Because its gravity is too weak to hold onto the debris thrown up by impacts, much of that material escapes into a faint ring around Mars before slowly raining back down, gradually mantling and smoothing the surface.

The origin puzzle

Deimos's asteroid-like colour and composition long suggested the simplest story: a passing body captured by Mars's gravity. But capture is hard to arrange, and both moons sit on tidy, nearly circular, low-inclination orbits in Mars's equatorial plane — exactly what you would expect from moons born in a disk of debris, not snatched from solar orbit. The leading rival idea is that a large impact on early Mars flung up a ring of material that coalesced into Phobos and Deimos.

Neither picture fully fits, and resolving it matters for understanding how Mars itself formed. It is also the central question driving the next visitor: only a returned sample can settle whether these moons are captured asteroids or reassembled pieces of Mars.

Missions past and future

Our close-up views of Deimos come mainly from the Mariner 9 orbiter in 1971 and the two Viking orbiters in the mid-to-late 1970s, which resolved fine detail and fixed the moon's shape, dark surface, and synchronous rotation — the same hemisphere always faces Mars, just as our Moon always shows one face to Earth. Europe's Mars Express has imaged Deimos on close approaches since the mid-2000s, and NASA's Perseverance rover photographed it as a bright point in the pre-dawn Martian sky in 2025.

The next chapter belongs to Japan's MMX (Martian Moons eXploration) mission, led by JAXA and slated to launch in 2026. MMX will make dedicated flybys of Deimos to study its composition, then focus on Phobos — deploying the small Idefix rover and collecting a surface sample to be returned to Earth around 2031, the first material ever brought back from a moon of Mars.

Seen from the Martian surface

Because Deimos orbits more slowly than Mars turns, an observer standing on the planet would watch it rise in the east and drift lazily westward across the sky over about two and a half sols before setting — the mirror image of quick, west-rising Phobos. It is far too small to show a disc to the naked eye; at its brightest it would look like a bright star, roughly matching Venus as seen from Earth, and would run through its full cycle of phases about every 1.26 days.

For an Earth-based skywatcher, Deimos is a genuinely difficult target. At around 12th magnitude it is technically within reach of a large amateur telescope, but it hides in the glare of Mars just a few tens of arcseconds away and is realistically visible only near a close opposition, using a Mars-blocking occulting bar. For nearly everyone, Deimos remains a moon known through spacecraft rather than the eyepiece.