Deimos (“DEE mos”) is the smaller and outermost of Mars’ two moons. It is one of the smallest known moons in the solar system.

How was Deimos discovered?

On August 12th 1877 due to the focused search of Martian moons by Asaph Hall resulted in the discovery of Deimos in the United States Naval Observatory in Washington DC.

The astronomer used a 26-inch refractor to study the region around Mars. Studying closer to the Red Planet than any another scientist, he found the small moons orbiting the planet’s equator. Six days after finding Deimos Asaph Hall also discovered Phobos.

Related: – Why is Mars red?

How did Deimos get its name?

Just like many planets and moons in our Solar System, the small satellite gets its name from Greek mythology. The moon is named after Deimos, the god of terror in Homer’s ancient poem, “The Iliad”. One of the twin sons of Mars (Ares is the equivalent Greek God) and would follow his father into battle.

Geological Facts

  • Deimos is 1027.6x smaller than Earth.
  • Deimos is 56% smaller than its brother Phobos, making it the smaller of the two moons.
  • Unlike Earth’s moon, which is round, Deimos is shaped like a lumpy potato.
  • The moon does not have any atmosphere due to it being so little and no gravity too low maintain one.
  • The surface of the moon is made up of regolith and solid rock.
  • Deimos is made of a similar rock material as C-type asteroids and meteorites found in the Solar System.
  • Deimos has a lot of craters caused by asteroids, but the surface of the moon is smoother than its brother Phobos.
  • Only two geological features have names on Deimos. These are two craters, Swift and Voltaire. They were named after two writers who theorised of the existence of Deimos and Phobos before they were discovered.

Orbital Facts

  • Deimos takes roughly 1.3 days (30 hours) to orbit once around Mars.
  • Deimos is approximately 23,460 km from Mars, which is much further out than Mars’s other moon, Phobos.
  • The moon travels approximately 1.3513 km/s.
  • Deimos has a circular orbit which lies precisely in Mar’s equatorial plane.
  • Deimos’s orbit is getting more significant because of tidal acceleration and will ultimately escape the Red Planet’s gravity and hurtle into space.
  • If you were to stand on Mars and watch an esclispe, Deimos would be too tiny to cause a total eclipse when it frequently passes in front of the Sun. It would only appear as a small black dot moving across the Sun.

Origin Facts

  • Scientists are unsure of the origin of Deimos and there are a number of hypotheses.
  • One theory is that it used to be an asteroid from our Solar System’s asteroid belt.
  • Another theory is that there used to be other small objects like Deimos’ size surrounding Mars and were brought to the Red Planet by a collision of planetesimals.

Expeditions to Deimos

  • Deimos has been photographed by numerous spacecraft who mission is to photograph and study Mars.
  • The first spacecraft to explore Deimos was the Mariner 9 in 1971, which became the first man-made spacecraft to orbit a planet.
  • A number of missions have proposed to study the moons of Mars. One mission is designated Phobos and Deimos and Mars Environment (PADME). Another mission is called OSIRIS-REx 2, a followup to OSIRIS-REx, a NASA asteroid study and sample study currently in development.
  • No landings on Deimos have ever been made.
         orbit:    23,459 km from Mars
         diameter: 12.6 km (15 x 12.2 x 11)
         mass:     1.8e15 kg

In Greek mythology, Deimos is one of the sons of Ares (Mars) and Aphrodite (Venus); “deimos” is Greek for “panic”.

Discovered 1877 August 12 by Hall, photographed by Viking 1 in 1977.

Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. Both are heavily cratered.

Deimos and Phobos are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.

More about Deimos

  • more images

Open Issues

  • There is no completely satisfactory theory as to how Deimos and Phobos came to be in orbit around Mars.