Deimos
What is Deimos?
Moons· Diameter ~12 km. The smaller, outer moon of Mars, a rock about 12 km across. It takes 30 hours per orbit, so from the Martian surface it looks like a bright star that stays up for more than two days while drifting slowly west.
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Orbit and size
| Size | 15 × 12 × 11 km |
|---|---|
| Distance from Mars | 23,460 km |
| Orbit | 30.3 h |
| Note | looks like a bright star from Mars |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Captured asteroid or piece of Mars: where Deimos came from
For a long time Deimos was thought to be an asteroid captured by Mars's gravity, because its dark surface and low density resemble carbon-rich asteroids. But a captured object would be expected to have a tilted, stretched orbit. Deimos's orbit is almost a perfect circle and lies over Mars's equator, which capture struggles to explain.
In 2023 the United Arab Emirates' Hope orbiter came within about 100 km of Deimos and studied its surface. The results showed a makeup closer to Martian rock than to an asteroid. This supports the idea that a giant impact on Mars long ago threw out debris that gathered into moons. The Phobos sample from MMX will serve as a benchmark for checking whether the two moons were born from the same material.
The two named craters on Deimos honour writers who imagined Martian moons before any were found. In Gulliver's Travels (1726) Jonathan Swift wrote that the astronomers of the flying island of Laputa knew of two small moons of Mars. Voltaire also mentioned two Martian moons in his 1752 story Micromégas. Both came more than a century before the real discovery.
The moon in depth
Asaph Hall of the United States discovered Deimos, the outer moon of Mars, on 12 August 1877. It is named after a son of Ares, the Greek god of war, and its name means 'dread'. It measures 15 × 12 × 11 km, about half the size of Phobos, and circles 23,460 km from the centre of Mars once every 30.3 hours.
Deimos orbits a little more slowly than Mars spins (24 hours 37 minutes). So from the Martian surface it rises in the east, drifts very slowly west, and stays in the sky for more than two days at a time. It is too small to look like a disk and appears as a bright star. At its brightest it shines about as brightly as Venus does from Earth.
Deimos looks smoother than Phobos. It has craters, but a thick layer of dust softens their outlines. Only two craters have names, Swift and Voltaire, and they are small, just 1 to 2 km wide. Following Mars in the orbit view shows Deimos moving lazily outside the fast inner path of Phobos.
Frequently asked
- How far is Deimos from Earth?
- The distance from mars of Deimos is 23,460 km.
- What is notable about Deimos?
- The note of Deimos is looks like a bright star from Mars.
- Which planet does Deimos orbit?
- Deimos orbits Mars.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.