Deimos Facts

Deimos Facts

Deimos is the smaller of the two moons of Mars and it is much further away from Mars than the other moon Phobos.

Deimos moon

Just like Phobos, Deimos has a dusty covered surface and is made of rocky material, and it’s a really, really strange shape. Read on for more facts about this interesting moon of Mars.

Deimos Overview

Diameter: The diameter of Deimos is 7.7 miles (12.4km)

Distance from Mars: The distance from Deimos to Mars is 14,574 miles (23,455km)

Deimos and Phobos: As Deimos is a lot further from Mars than Phobos, it often looks like a star in the sky. From some places on Mars, you wouldn’t even be able to see it!

Where did Deimos Come From?

  • As Deimos and Phobos are made from the same materials, which are very similar to asteroids, some scientists think that they used to be asteroids in the asteroid belt. Others believe that Mars was once surrounded by many rocks that are the same shape and size as Deimos that have disappeared over time.
  • Deimos was discovered by American astronomer Asaph Hall on August 12, 1877. He discovered Phobos 6 days later in his search for Martian moons.

A Star Moon

We’ve already mentioned that Deimos would look a similar size to star when viewed from the surface of Mars, but when it eclipses the sun it shows as a small dot moving across the surface.

At full moon, Deimos shines just as brightly as Venus, which is the brightest object in our solar system.

Deimos makes its Orbit

  • Deimos takes 30.3 hours to orbit around Mars
  • Its orbit is almost circular and it travels around the equatorial plan of the planet.
  • This orbit is nearly 6 hours longer than a day on Mars.

Deimos orbiting Mars

Where did Deimos get its Name from?

The moon is names after the son of the Greek god of War, Ares. This means terror! Wonder why the moon is associated with terror. Deimos often joined his father in battle along with his twin brother Phobos, which is of course the name of the first Mars moon.

Craters on Deimos

  • Deimos has fewer impact craters than Phobos, but it still has two big craters.
  • They are called Swift and Voltaire.
  • Swift has a diameter of 0.6 miles (1km) and Voltaire has a diameter of 1.2 miles (1.9km).
  • Swift was named after Jonathan Swift, the author of Gulliver’s Travels who wrote about the two moons on Mars in 1725 – that was 151 years before they were discovered. Wow!
  • Voltaire was named after Francois-Marie Arouet, who was also a writer but was commonly known as Voltaire.

deimos map voltaire swift

The Rising and Setting of Deimos

  • Deimos rises in the east and sets in the west every 2.7 days.
  • As the orbit of the moon (30.3 hours) is longer than a Martian day (24.7 hours) the moon only rises and sets every 2.7 days which you would see if you were on the surface of Mars.

The Moving Deimos

Unlike Phobos, which scientists believe will collide with Mars in about 50 to 100 million years, the orbit of Deimos is actually taking it further and further away from Mars.

Eventually the moon will go hurtling into space when it gets too far from the gravity of Mars to remain in orbit.

What are the Average Temperatures on Deimos?

The temperatures are similar to that of Phobos.

The temperatures range from around 25°F (−4°C) on the sunlit side of the moon, down to around −170°F (−112°C) on the shadowed side. It’s fairly cold up there. 

Expeditions to Deimos and Phobos

  • The first craft to explore Deimos and Phobos was the Mariner 9 in 1971, which became the first spacecraft to orbit a planet.
  • The moon has also been visited by the Viking Orbiters, the Soviet Phobos 2, NASA’s Mars Global Surveyor, the European Mars Express and the ground rovers Spirit, Opportunity and Curiosity.


Deimos sure seems like an interesting moon of Mars, with a really odd shape, but not sure it’s the place to visit with temperatures like it has. Do you know any other Deimos facts that we’ve left out?

More Resources