Deimos: Mars’s small outer moon

How big is Deimos?

About 12 km (7 mi) across on average.[1] Deimos isn’t round: it measures about 15 by 12 by 11 km (9 by 7 by 6.8 mi). It is smaller than Phobos and less lumpy.[2] Phobos also has more than four times its mass.[3]

Deimos is dark. NASA’s fact sheet gives it an albedo of just 0.08, about the same as Phobos at 0.07.[3] NASA says its surface looks like the dark, C-type material of asteroids in the outer asteroid belt.[2]

Deimos, Phobos and our Moon compared

Sizes are average widths; the small figures are each moon’s longest, widest and shortest dimensions. Heights are each moon’s average distance from its planet’s center minus the planet’s average radius, our calculation from NASA’s fact sheets.[1][2][3][4][5][7]

How does Deimos orbit Mars?

Deimos circles Mars once every 30 h 18 min, about 20,000 km (12,000 mi) above the surface, on a nearly circular path close to Mars’s equator.[3][8] Mars spins once every 24.6 hours, so Deimos moves only a little slower than the ground beneath it.[3] From the surface it rises in the east and sets in the west, like our Moon.[7] But it creeps: Astronomy magazine calculates that it stays above the horizon for about 60 hours at a time.[9]

Deimos turns once per orbit, so it always shows Mars the same face.[3][11] And while Phobos is moving closer to Mars, Deimos is moving away from it, ESA says.[10]

Its orbit is known well, but not perfectly. The first time a rover tried to catch Deimos crossing the Sun, the moon was 40 km (25 mi) from where scientists expected, NASA said in 2019. Each new sighting tightens the numbers.[12] In February 2022, ESA’s Mars Express watched Deimos pass in front of Jupiter and its four largest moons, which pinned down its position more precisely.[10]

Why is Deimos so smooth?

Because a thick blanket of loose dust and broken rock, called regolith, covers it and partly fills some of its craters.[2][13] NASA says this layer may be as deep as 100 m (328 ft). Most of Deimos’s craters are smaller than 2.5 km (1.6 mi) across, and it has none of the grooves and ridges that cross Phobos.[2]

Deimos’s gravity is too weak to hold the dust that impacts throw up. NASA says Mars’s gravity keeps that debris in a ring near Deimos’s orbit, and the moon sweeps it back up over time.[2] Pictures taken in 2009 by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter show the smoothest areas are the reddest. Fresh craters are less red, probably because their material has spent less time exposed to space.[13]

A study published in Nature Astronomy on Aug 18, 2026, offers one explanation for both the dust and Deimos’s biggest feature: a basin about 10 km (6 mi) wide around its south pole. Its simulations show that an asteroid about 320 m (1,050 ft) wide, hitting at a slant, could have carved the basin without breaking Deimos apart. The debris it threw out buried older craters, more than 200 m (660 ft) deep in places.[14][15] The study also finds Deimos highly porous inside, resembling the loose “rubble-pile” asteroids that spacecraft have visited.[15]

Only two features on Deimos have official names: the craters Swift, about 1 km (0.6 mi) across, and Voltaire, about 1.9 km (1.2 mi). The International Astronomical Union adopted both names in 1973.[16][17] Craters on Deimos are named for authors who wrote about Mars’s moons, such as Jonathan Swift, who wrote “Gulliver’s Travels.”[16][18]

Where did Deimos come from?

Nobody knows yet. There are two leading ideas. Mars may have captured a passing asteroid, which would explain why Deimos looks like a dark, carbon-rich asteroid. Or Deimos may have formed from debris thrown into orbit when something huge hit Mars, which better fits its circular orbit over Mars’s equator.[8][19]

Recent results point different ways:

• 2023: The Emirates Mars Mission’s Hope orbiter, run by the United Arab Emirates, flew within about 100 km (60 mi) of Deimos. Its team said the moon’s infrared signature looks more like basaltic Mars than a D-type asteroid, and points to “a planetary origin.”[20]
• 2024: A NASA-led study in the journal Icarus proposed a third route: an asteroid torn apart by Mars’s gravity, with its fragments settling into a disk that could have built both moons. That would put more moon-building material out where Deimos orbits. A giant impact leaves most of its debris close to the planet.[8]
• 2026: The Nature Astronomy impact study found Deimos built like a rubble pile. ESA’s Hera project scientist Michael Kueppers said that “does not necessarily mean the moon is in fact a captured asteroid.”[14]

Who has photographed Deimos up close?

Few spacecraft get close, because Deimos orbits far beyond where most Mars orbitersOrbiterAn orbiter circles Mars without landing, studying the planet from above — and many also relay rover data back to Earth.More in the glossary fly.[14][20]

• 1970s: NASA’s Viking orbiters found Deimos smoother and dustier than heavily cratered Phobos.[14]
• Feb 21, 2009: HiRISE took color-enhanced views showing details as small as about 60 m (197 ft).[13]
• 2023: Hope made repeated flybys, the closest at about 100 km (60 mi), the nearest any spacecraft had come since Viking in 1977. It made the first extreme and far ultraviolet observations of Deimos, and studied parts of its far side whose makeup had never been examined.[20]
• Mar 12, 2025: ESA’s Hera asteroid mission flew past Mars and imaged Deimos from about 1,000 km (620 mi). It saw the side that faces away from Mars, which is rarely seen.[11][21] Its thermal camera, supplied by JAXA, showed Deimos warmer than Mars’s surface, because its pitch-black surface soaks up sunlight.[22]

What does Deimos look like from Mars?

A tiny, bright dot that drifts slowly across the night sky. Astronomy magazine calculates that Deimos appears about 2.5 arcminutes wide, around twice the largest Venus ever looks from Earth. It is usually invisible by day, but at night it outshines every star.[9]

Mars’s roversRoverA rover is a robot vehicle that drives across Mars, following plans its team on Earth sends ahead of each drive.More in the glossary have photographed it many times. Spirit caught both moons crossing the night sky on Aug 30, 2005.[7] Curiosity watched Deimos cross the face of the Sun on Mar 17, 2019. Deimos covers so little of the Sun that scientists call that a transit, not an eclipse. By April 2019, Spirit, Opportunity and Curiosity had recorded eight Deimos transits.[12]

On Mar 1, 2025, at 4:27 a.m. local time, Perseverance’s navigation camera combined 16 long exposures, about 52 seconds in all, to catch Deimos shining before dawn. The stars Regulus and Algieba show up in the same frame.[23]

How did Deimos get its name?

From Greek myth: Deimos means dread. Asaph Hall discovered Deimos on Aug 11, 1877, six days before Phobos, at the U.S. Naval Observatory in Washington, D.C.[2][5][6] He named both moons for the sons of Ares, the Greek counterpart of the Roman war god Mars. NASA translates Deimos as dread; ESA gives terror or panic.[2][6]

MMX: the next visit to Deimos

Japan’s MMX plans to observe Deimos as well as land on Phobos. JAXA is scheduled to launch it on an H3 rocket from Tanegashima on Oct 20, 2026, at 04:41 Japan time (Oct 19, 19:41 UTC).[25] It should reach Mars in 2027 and spend about three years there. It plans to study Phobos, collect samples from its surface and observe Deimos, then bring the samples to Earth in Japan’s fiscal 2031.[24] The 2026 impact study made predictions about Deimos that MMX can test.[14]

Sources

1. NASA JPL Solar System Dynamics — Planetary Satellite Physical ParametersPrimary · checked Oct 6, 2026
2. NASA Science — DeimosPrimary · checked Oct 6, 2026
3. NASA NSSDCA — Mars Fact SheetPrimary · checked Oct 6, 2026
4. NASA NSSDCA — Moon Fact SheetPrimary · checked Oct 6, 2026
5. NASA Science — PhobosPrimary · checked Oct 6, 2026
6. ESA — Martian moons: PhobosPrimary · checked Oct 6, 2026
7. NASA Science — Two Moons and the Pleiades from MarsPrimary · checked Oct 6, 2026
8. NASA — Making Mars’ Moons: Supercomputers Offer ‘Disruptive’ New ExplanationPrimary · checked Oct 6, 2026
9. Astronomy — Ask Astro: Can Phobos and Deimos be seen from the surface of Mars?Secondary · checked Oct 6, 2026
10. ESA — Mars and Jupiter moons meetPrimary · checked Oct 6, 2026
11. ESA — Hera: Target DeimosPrimary · checked Oct 6, 2026
12. NASA JPL — Curiosity Captured Two Solar Eclipses on MarsPrimary · checked Oct 6, 2026
13. NASA Science — Martian Moon Deimos in High Resolution (PIA11826)Primary · checked Oct 6, 2026
14. ESA — Hera’s Mars flyby guided impact study of Deimos moonPrimary · checked Oct 6, 2026
15. S. D. Raducan et al. — Deimos’s shape and geology explained by a subcatastrophic impact (Nature Astronomy, 2026)Primary · checked Oct 6, 2026
16. USGS Gazetteer of Planetary Nomenclature — Swift (crater on Deimos)Primary · checked Oct 6, 2026
17. USGS Gazetteer of Planetary Nomenclature — Voltaire (crater on Deimos)Primary · checked Oct 6, 2026
18. USGS Gazetteer of Planetary Nomenclature — Categories for Naming Features on Planets and SatellitesPrimary · checked Oct 6, 2026
19. K. Kuramoto — Origin of Phobos and Deimos Awaiting Direct Exploration (Annual Review of Earth and Planetary Sciences, 2024)Primary · checked Oct 6, 2026
20. Emirates Mars Mission via European Geosciences Union — Emirates Mars Mission Unveils New Deimos Observations at EGU23, Announces Mission ExtensionPrimary · checked Oct 6, 2026
21. ESA — Mars and Deimos viewed by Hera’s Asteroid Framing CameraPrimary · checked Oct 6, 2026
22. ESA — Mars and Deimos viewed by Hera’s TIRIPrimary · checked Oct 6, 2026
23. NASA Science — Perseverance Views Deimos Before SunrisePrimary · checked Oct 6, 2026
24. JAXA — MMX: Martian Moons eXplorationPrimary · checked Oct 6, 2026
25. JAXA — Launch schedule of the Martian Moons eXploration (MMX) spacecraft aboard H3 Launch Vehicle flight No.10Primary · checked Oct 6, 2026