Phobos Facts

Mars has two moons, Deimos and Phobos. Deimos is the smaller of the two, with the larger being Phobos. Deimos was discovered by an American scientist in 1877 and is named after the son of Ares, the Greek God of War. Its surface is comprised of loose, heterogeneous deposits and solid rock.

How was Phobos discovered?

Asaph Hall discovered Phobos in the United States Naval Observatory in Washington DC. It was found six days after its smaller sibling Deimos was found.

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 smaller moons orbiting the planet’s equator.

How did Phobos 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 Phobos, the god of fear in Homer’s ancient poem, “The Iliad”. He is the son of Ares (Mars) and Aphrodite (Venus).

Geological Facts

  • The dimensions of Phobos are 27 km × 22 km × 18 km and just like Deimos, it is not a round shape like Earth’s moon.
  • Phobos does not have an atmosphere due to its low gravity.
  • The moon is one of the least reflective objects in the Solar System.
  • Both Deimos and Phobos are made up of the same material as C-type and D-type asteroids and dawrf planets.
  • The Viking probe has shown that the moon is not made of solid rock but instead has a significant porosity.
  • Phobos is heavily cratered and all
  • Phobos has an impact crater which is 9 km wide, called Stickney. The crater is named after Asptha Hall’s wife, Chloe Angeline Stickney Hall and was discovered in 1973.

Orbital Facts

  • Phobos orbit distance is 9,376 km from Mars; which this makes it the closest satellite to a planet in the Solar System.
  • The moon orbits Mars at 2.138 km/s.
  • Phobos orbit period is 7.7 hours and is below the synchronous orbit radius — which means it moves around Mars faster than the planet moves.
  • The moon orbits the planet so fast that it would rise in the west and set in the east twice per day.
  • It took so long to discover Phobos due to how close it was to Mars.
  • In approximately 50 million years scientists have predicted Phobos will be destroyed due to the tidal forces of the Red Planet. It will slowly enter the atmosphere of the planet and break up, potentially creating a ring.

Origin Facts

  • When it was first discovered, scientists believed that Martians created Phobos.
  • Just like Deimos, scientists believe that Phobos was an asteroid from our Solar System’s asteroid belt.
  • Another theory is that Phobos used to be a ring around the planet, formed by an object violently collied with Mars. The ring was slowly formed into the two moons of Mars, Phobos and Deimos.

Expeditions to Deimos

  • Phobos has been photographed by numerous spacecraft whose mission was to photograph and study Mars.
  • The first spacecraft to explore Phobos 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.
  • In 2011, Russia attempted a mission to Phobos called Phobos-Grunt, which was supposed to bring back a sample of the moon. However, due to a technical malfunction, it crashed into the Pacific Ocean.
  • NASA is considering a Phobos Survey mission which would deploy hedgehog-like probes to the surface of the moon.
  • Japan Aerospace Exploration Agency (JAXA) intends to launch the Mars Moons eXploration (MMX) mission to visit Phobos and Deimos, to collect samples and return in 2029.

Related Questions


Picture of Phobos

Image of Phobos (moon)

Phobos Quick Facts

Mass:1.0659×1016 kg
Equatorial Diameter:22.5 km
Equatorial Circumference:69.7 km
Orbital Distance:9,376 km
Orbit Period:0.3189 Earth days
Mean Surface Temperature:-40.15°C
First Record:August 18, 1877
Recorded By:Asaph Hall