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Moon of Mars
Deimos
The small, dusty outer moon of Mars that looks like a bright star from the ground
Key facts
- Diameter
- About 15 × 12 × 11 km (mean radius about 3.9 miles / 6.2 km)
- Distance from Mars
- About 14,600 miles / 23,460 km from the centre of Mars, roughly 7 Mars radii
- One orbit
- About 30.3 hours (1.26 Earth days)
- Rotation
- Synchronous: one turn per orbit, same face always towards Mars
- Mass and density
- About 1.5 × 10¹⁵ kg; density about 1.47 g/cm³
- Surface gravity
- About 0.003 m/s², with an escape velocity of about 20 km/h
- Discovered
- 11 August 1877 by Asaph Hall, United States Naval Observatory
- Named after
- Deimos, meaning dread, a son of Ares in Greek mythology
- Surface
- Dark, reddish and dusty, with regolith perhaps 100 m deep; largest crater about 1.4 miles / 2.3 km
- Visited by
- Mariner 9, Viking 1 and 2, Mars Reconnaissance Orbiter, Hope (2023 flyby); MMX due to launch October 2026
About Deimos
Deimos is a tiny moon of Mars. It is shaped like a lumpy potato. It is not round like our Moon. Deimos is very small. It is about the size of a town. Its surface is dark and dusty. The dust is very deep. Deimos goes round Mars every thirty hours. From Mars it looks like a bright star. You would be very light there. A jump would take you high. Its name means dread. Its brother moon is called Phobos. Both moons were found in 1877. A man called Asaph Hall found them.
Deimos is the smaller and more distant of the two moons of Mars. It is only about 15 by 12 by 11 kilometres across. That is far too small for gravity to squeeze it into a sphere. Instead, Deimos is a lumpy, potato-shaped rock with a dark, reddish surface.
It has a smooth look because deep dust has partly filled its craters. This loose dust may be as deep as 100 metres in places. Deimos travels around Mars in an orbit about 23,000 kilometres from the planet. One orbit takes about 30 hours, a little longer than a Martian day.
Because it rotates once for every orbit, the same side always faces Mars. From Mars, Deimos looks like a bright star crossing the sky slowly. Its gravity is so weak that a fast run could launch you into space. The American astronomer Asaph Hall discovered Deimos in August 1877.
He named it after a son of Ares, the Greek god of war. The name Deimos means dread, and its brother moon Phobos means fear. Scientists still do not know whether Deimos is a captured asteroid. The Hope orbiter from the United Arab Emirates flew close by it in 2023.
Japan's MMX mission will study both moons after it launches in October 2026.
Deimos is the outer and smaller of the two moons of Mars. NASA gives its dimensions as about 15 by 12 by 11 km, with a mean radius of roughly 6.2 km according to JPL Solar System Dynamics. It is far too small for its own gravity to pull it into a sphere, and its low mean density of about 1.47 g/cm³ suggests a porous, rubble-like interior. Surface gravity is around 0.003 m/s², and escape velocity is only about 20 km/h, so a thrown ball would never come down.
Deimos orbits about 23,460 km from the centre of Mars, close to seven Martian radii out, completing one circuit every 30.3 hours in a nearly circular path tilted less than one degree to the Martian equator. It rotates synchronously, keeping one face towards the planet. Because its orbital period is only slightly longer than a Martian day, Deimos rises in the east and crawls westward across the sky over more than two days, appearing star-like rather than as a disc. Unlike Phobos, which is spiralling inward, Deimos is very slowly drifting outward.
The surface is dark, with an albedo of about 0.07, and spectrally resembles C-type asteroids from the outer asteroid belt. Deimos appears smoother than Phobos because a regolith perhaps 100 metres deep has partly filled its craters. The two named craters, Swift (about 1 km) and Voltaire (about 1.9 km), honour writers who imagined Martian moons long before they were found. The largest crater is about 2.3 km across.
Asaph Hall discovered Deimos at the United States Naval Observatory on 11 August 1877, six days before he found Phobos. Henry Madan suggested the names from the sons of Ares in Homer's Iliad; Deimos means dread.
Mariner 9 and the Viking orbiters imaged Deimos in the 1970s, with Viking 2 making close passes in October 1977. Mars Reconnaissance Orbiter captured colour views in 2009, and NASA's rovers have photographed it from the ground, including transits of the Sun. In March 2023 the Emirates Mars Mission's Hope orbiter made a close flyby and returned the first close-up global images. Its origin, captured asteroid or debris from a giant impact on Mars, remains unresolved; JAXA's MMX mission, due to launch in October 2026 with NASA's MEGANE instrument aboard, is designed to answer that question.
Did you know?
- Deimos's gravity is so weak that its escape velocity is only about 20 km/h, so a good sprinter could, in theory, run straight off it.
- Deimos was discovered on 11 August 1877, six days before its brother Phobos, even though Phobos is the bigger and brighter moon.
- From the surface of Mars, Deimos looks like a bright star and takes more than two days to cross the sky from east to west.
Neighbours around Mars
Questions children ask
Why is Deimos not round like our Moon?
Deimos is only about 15 kilometres across. A body needs to be much bigger for its own gravity to pull it into a sphere. Deimos is too small and too weak, so it keeps its lumpy, potato-like shape.
Where did Deimos come from?
Nobody is sure yet. Its dark surface looks like the C-type asteroids in the outer asteroid belt, so it may be a captured asteroid. Another idea is that it formed from debris thrown out by a giant impact on Mars. Japan's MMX mission aims to settle the question.
Could a person visit Deimos?
No one has yet, but it would be a strange place. The gravity is so weak that you would feel almost weightless, and jumping too hard could send you into orbit. Robots have flown past it, and MMX will study it up close.
How is Deimos different from Phobos?
Deimos is smaller, further from Mars and takes about 30 hours to orbit, while Phobos whizzes round in under eight hours. Deimos looks smoother because deep dust fills its craters, and it is slowly drifting away from Mars while Phobos is slowly falling in.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- How long does Deimos take to orbit Mars once?
- What does the name Deimos mean?
- Who discovered Deimos in 1877?
Phobos
The Moon
Mars