Deimos Facts

Deimos is the outermost and smaller martian moon. With a radius of 6.2 km, Deimos is one of the smallest moons in the Solar System. It is named after the Greek god of terror who is a son of Ares and Aphrodite and twin of Phobos.

Deimos Profile

Interesting Deimos Facts

  • Deimos is one of the two natural satellites of the planet Mars.
  • It is the smaller and the outermost satellite of the planet Mars.
  • It has been named after the son of Zeus, the Greek God of War and Aphrodite the Greek Goddess of love. It is the personification of panic or terror.
  • Deimos is the twin brother of Phobos.
  • The natural satellite has a diameter of nearly 6 kilometers.
  • Deimos is close to 24,000 km away from Mars. This is much more than the distance between Mars and Phobos which is about 6000 km.
  • It completes one orbit around Mars in nearly 30 hours.
  • Its period of revolution is smaller than Mars’ period of rotation.
  • Deimos appears to rise in the east and set in the west, unlike Phobos.
  • The origin of the two satellites is still a mystery and several theories have been put forward to explain this. However, it is still not confirmed.
  • Phobos and Deimos may be asteroids that have been captured into the Martian orbit.
  • It has not been visited by any spacecraft but has been explored and photographed by many in the past few decades and many future missions to be sent to Mars and its moons have been proposed and planned. These include sample collecting and human space missions as well.
  • Deimos shows frequent transits as seen from the Martian surface that last for a very short duration of time.
  • The writer Jonathan Swift wrote about Mars and its two moons more than a century before they were discovered and even accurately predicted the period of revolution of Phobos.
  • Voltaire also wrote about the two moons of Mars well before they were discovered.

Formation of Deimos

The origin of the two moons of Mars, Phobos, and Deimos, has undergone a lot of speculations and debates. It is still not perfectly clear. It has also been a topic of controversy for many decades. Several theories have been proposed in an attempt to explain this.

The most popular theory suggesting the origin of the two Martian satellites is that they were initially asteroids that were captured by Mars into its orbit. It also has considerably strong scientific evidence. Both of the moons of Mars have a lot in common with Type C asteroids or carbonaceous asteroids.

Based on this fact it is widely speculated and accepted that Phobos and Deimos were initially a part of the main Asteroid belt and were later captured into the orbit of the planet Mars by its gravitational forces, tidal forces or atmospheric drags.

Considering the fact that the asteroid belt is close to the orbit of the planet Mars, it is very much possible that these satellites could have been captured Asteroid belt. In contrast to this theory, the atmosphere of Mars is very thin, and it cannot influence objects as big as Phobos or Deimos and pull them into its orbit.

Another theory says that moons may have also been formed by the collection of gas and dust particles much after the formation of the planet Mars that is much after the formation of the entire solar system. This means that the moons could be second-generation objects of the solar system.

Another theory states that Mars was initially surrounded by many objects like Phobos and Deimos as a result of a collision with a joint celestial object which could have combined together or even exploded or got destroyed. The objects formed after a collision may have drifted away from the planet to long distances due to small sizes.

Deimos is one of the two natural satellites of the planet Mars. It is the outermost satellite. It is the smallest in size among two satellites of Mars.

It was discovered by the American astronomer Asaph Hall in 1877 in the same year as Phobos in the United States Naval Observatory, Washington DC. It was actually discovered 6 days before Phobos.

Deimos gets its name from a figure of Greek mythology. It has been named after the son of the Greek God Ares, the God of War and Aphrodite, the Greek Goddess of Love. Deimos is the personification of panic, dread or terror in Greek mythology. Deimos is the twin brother of Phobos that is the other satellite of Mars. The word Phobos means fears.


Deimos has an average radius of 6.2 km or 3.9 miles. It takes about 30.3 hours to complete one revolution around the planet Mars. It is nearly 23,460 km or 14,580 miles away from the planet Mars. It is much farther away from the planet than its companion moon, Phobos.

Phobos is about 6000 km away from Mars. It has a density of 1.47 grams per centimeter cube. Its orbital inclination is 1.78 °. It has an average surface temperature of minus 40 °C.


Much like the Phobos, Deimos is also an irregularly shaped celestial body. Just like Phobos, Deimos has a lot of characteristics that are common with Type C and type D asteroids. It is mainly composed of rocks that are rich in carbonaceous compounds and chondrites.

Deimos has created structure but is comparatively much smoother than that of its companion moon Phobos.

Martian Moon’s 2 geographical features have been named as Swift or Voltaire. These are named after authors who predicted the existence of the two Martian moons much before their discovery. They are both surface craters.

The impact crater, Voltaire is about 3.1 kilometers or 1.9 miles across. The crater Swift is almost the same size as Voltaire. These craters appear to be a little different from those of the other planets or moons. They are thought to be created by meteorites rather than by cast-off material.

Deimos is known to have a layer of sand and dust particles which can be as thick as 300 meters. Other than this and the presence of a few craters not much is known about the Martian moon. Since it has not been visited by any spacecraft yet the only available information about the Martian moon was gained through photographs by other space missions to Mars and its moons and by the advanced telescopes on earth.

Magnetic field

Deimos does not have an atmosphere. This is because the surface and their size are too small to retain an atmosphere. They also do not have any gravity on their surface. The magnetic field on the moons is also absent.

Orbit & Rotation

The moon Deimos has an orbit that is almost circular in nature. It is close to Mars’ equatorial plane. From the surface of Mars Deimos seems to have an angular diameter of 2.5 minutes. It would only seem like a small star like structure to an observer standing on the Martian surface.

The phases of Deimos can be observed from the surface of Mars with the help of a telescope. The phases of the moon take nearly 1.26 days to complete. At the time of a full moon, Deimos appears to be just as bright as Venus appears from the surface of the earth in the night sky.

Unlike Phobos, Deimos rises in the east and sets in the west. It can not be seen from Mars from latitudes higher than 82.7 °.

The orbit of Deimos is slowly getting larger with time. This is because of its great distance from the planet Mars and also because of tidal acceleration. 

Deimos will soon escape the gravity of Mars and stop orbiting it. This is the total opposite of what would happen to Phobos. Phobos orbit is slowly approaching that of mass at a relatively fast rate and the moon is said to collide with the planet in a time of about a hundred million years.

Distance from Mars

Deimos is close to 24,000 km (14913 miles) away from Mars.

Solar transit

Similar to the Earth’s moon and the companion moon Phobos, Deimos also exhibits solar transits. However, it is too small to conduct a total solar eclipse. Hence, most of the time, during an eclipse, the moon appears to be a small, moving dot on the sun. One transit of the moon does not last for more than 2 minutes because of its rapid period of revolution.

On 4 March 2004, NASA’s Mars rover Opportunity photographed a transit of the Moon. A few days later on 13 March 2004, another photograph of a transit of the Moon was taken by the Mars rover Spirit. Several other spacecraft have taken photographs of Deimos in the past years.

The moon Phobos undergoes solar transits if viewed from the surface of Mars. This could be easily observable as they are regular. The Mars Rover Opportunity has photographed many such solar transits. This phenomenon is similar to the phenomena of solar eclipses as viewed from the Earth.

During the solar transits, the shadow of the moon is casted on the surface of Mars. As Phobos is quite small in size it cannot exhibit a total solar eclipse like the earth’s moon.

Exploration of Deimos

Deimos has been explored by several spacecraft but none of them has visited it exclusively. It has always been observed and explored along with its parent planet and companion moon Phobos. Many spacecraft have also photographed the Moon from up close. No spacecraft has landed on Deimos yet.

Phobos 1 and Phobos 2 were two spacecraft launched by the Soviet Union in 1998 to explore the two moons of Mars. They were both launched successfully. However, the first spacecraft was accidentally shut down and later lost en route to Mars. The second one was able to send some data successfully but fail soon after approaching Phobos.

In 1997 and 1998, a space mission called Aladdin to be sent to the two moons of Mars was almost confirmed. It was planned to conduct slow flybys of both the moons and bring that some samples to the earth after a period of 3 years. However, this mission was later canceled because of the huge expenses that it demanded. Its cost was estimated to be almost 250 million dollars. Instead, the mission MESSENGER was selected to be sent to Mercury.

NASA started planning another mission to be sent to Phobos and Deimos in 2008 which was supposed to also be a sample returning mission.

Another mission called Gulliver has been conceptualized. Its sole aim is to visit and explore Phobos. It also aims to bring 1 kg or 2.2 pounds of sample material from Deimos to the earth which is to be studied.

Yet another mission called the Phobos and Deimos & Mars Environment or PADME has been proposed in 2014. It proposed that an orbiter be sent into the orbit of Mars and study Phobos and Deimos.

It is believed that the exploration of the Martian moons could be a significant event in the process of the exploration and future colonization of Mars. If the Sands of Phobos and Deimos are studied extensively it could be a very important step towards the colonization of the red planet as this material could be extremely helpful for aerobraking.

Potential for Life

No evidence of life has ever been found on Phobos or Deimos and it is not yet certain whether these moons will never be able to sustain life because of the extreme conditions on the surface and in the atmosphere.

As Mars is the only planet other than the Earth which may be able to harbor life, Mars along with its moons is very popular in the Earth’s culture.

Deimos in Pop Culture

The existence of the moons of Mars was first suggested by Johannes Kepler. He predicted that mars could have moons as the Earth and Jupiter was known to have four satellites. However, no signs of the moons were found until Hall began his research. Deimos stayed hidden from astrologers for a long time because of its small size and large distance from the planet Mars.

Jonathan Swift wrote about the two moons of Mars in his book called Gulliver’s Travels more than a century before they were discovered. He also all most accurately predicted the orbital period of the inner satellite of Mars that is Phobos.

Some writings of Voltaire also speculated the existence of the two Martian moons before their discovery.

Many such films, TV shows, books, and video games feature Mars and the moons of Mars as a topic of fascination for people of all age groups.


  • 1877: The two moons of Mars are discovered and named


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