Phobos Facts

Phobos is a small, heavily cratered, non-spherical and irregularly shaped natural satellite of Mars with a mean radius of 11 km. It is the innermost and bigger of the other moon of Mars, Deimos. It revolves around Mars very quickly. It orbits Mars 3 times a day.

Phobos Profile

Interesting Phobos Facts

  • Phobos is one of the two natural satellites of the planet Mars.
  • It is the larger and innermost satellite of the planet Mars.
  • It has been named after the son of Zeus, the Greek God of War who is a personification of fear or phobia.
  • The natural satellite has a diameter of nearly 11 kilometers.
  • Phobos is close to 6000 km away from Mars. This is the least distance between any planet and its natural satellite and the entire solar system.
  • It completes one Orbit around Mars in only 7 hours and 39 minutes.
  • Its period of revolution is smaller than Mars’ period of rotation.
  • Phobos appears to rise and set twice from the surface of Mars.
  • 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 been visited by several spacecraft in the past few decades and many future missions to be sent to Mars and its moons have been proposed and planned. These include human space missions.
  • Phobos is thought to be hollow or porous in nature.
  • Phobos shows phases, transits, and eclipses just like the earth’s moon.
  • 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.
  • Phobos is getting closer to Mars at a rate of 1.8 meters every 100 years.

Origin or History of Martian Moons

How the two moons of Mars, Phobos, and Deimos, originated has undergone a lot of speculations and debates. It is still not clear and has been a topic of controversy for many years. The most popular theory of the origin of the two Martian satellites is that they were asteroids that were believed to be captured into their orbit.

It also has considerably strong scientific evidence. Both the moons of Mars have a lot and common with Type C Asteroid 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 which were later captured into the orbit of the planet Mars by 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 quite possible that these satellites may have been captured from it. In contradiction to this theory, the atmosphere field of Mars is very thin, and it could not have Phobos captured by atmospheric braking.

The moon may have also been formed by the accumulation of gas and dust particles much after the formation of the planet Mars with the entire solar system. This means that could be a second-generation object 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 another celestial object. These chunks must have either drifted away or pulled by Mars and made its moons.

Phobos is a natural satellite of the planet Mars. It is one of the two satellites of the planet, along with Deimos. It is the larger and the innermost of the two satellites. It was discovered by the American astronomer Asaph Hall in 1877 along with Deimos in the United States Naval Observatory, Washington DC.

Phobos gets its name from a figure from Greek mythology. Phobos has been named after the Greek God Phobos who was the son of Ares, the God of War and Aphrodite, the Goddess of Love. Phobos is the personification of fear in Greek mythology. Deimos is the brother of Phobos. Deimos means panic.


 Images taken by Viking 1 during one of its flybys of Phobos

If a person was standing on the surface of Phobos they would view the planet Mars to be more than 6400 times bigger and 2500 times brighter than Earth’s Moon.


Phobos is an irregularly shaped celestial body. This is because it has two little matter bound or rounded up by its own gravitational forces. It is very small with an average radius of around 11 kilometers or 7 miles. It is more than 7 times larger than Deimos, which is the other moon of Mars.


The natural satellite is also heavily cratered. The Stickney crater is a prominent crater of Phobos. It is a large impact crater which has a diameter of about 9 kilometers or 5.6 miles. It was named after the wife of Asaph Hall, the discoverer of the natural satellite.

This crater takes up much of the moon’s surface area. There are also many grooves and streaks on the surface of this moon. These grooves can be about 30 meters or 98 feet in depth, 100 to 200 meters or 330 to 360 feet in width and as much as 20 kilometers or 12 miles in length.

It was predicted that the two of the Martian satellites produce rings of dust and gas. However, they have never been observed as of today and this possibility is bleak. It is believed that Phobos has a surface that is covered with nearly a hundred meters of rocky sands and dust.

This could be because of the impacts that it has had from other celestial bodies. However, it is still a mystery how an object like the natural satellite which does not have any gravity can hold so much of sand intact.

Most geological features on the natural satellite have been named after Jonathan Swift’s Gulliver’s Travels and astronomers who have studied the Martian moon.


Phobos has a composition which is similar to that of the surface of the planet Mars despite infrared spectra showing that it is composed of carbonaceous chondrites. It has a low density and hence is not rocky but somewhat porous in nature. Because of this, some scientists believe that Phobos may contain a reservoir of water ice!

Magnetic field

Phobos is not known to have a strong magnetic field.


Phobos does not have any atmosphere. Many of its properties are similar to that of asteroids.

Phobos has unusual orbital characteristics. In 1958 a metal sheet model was suggested for the Martian moon. Considering the Martian atmospheric density, it was found out that Phobos shows a very big braking effect. For this reason, Martian moons are believed to be light and hollow.

Orbit & Rotation

The orbital motion of Phobos is the best-studied orbital motion of any natural satellite in the solar system. Its period of revolution is much shorter than masses period of rotation and hence the moon of Mars appears to rise and set twice. The moon would appear to rise in the west and set in the east to the person standing on the surface of Mars.

Phobos is very close to Mars. It also moves in an equatorial orbit. This is the reason why the moon cannot be seen from latitudes that are at an angle of less than 70.4°. As observed from Mars the phases of the moon take place every 0.319 days.

It orbits around Mars and just about 7 hours and 49 minutes.

This time is less than the time taken for Mars to complete one rotation about its own axis. This is because of its small distance from the planet. From the surface of Mars, it appears to rise in the west and set in the East. This phenomenon takes place twice due to the rotation of mass and the revolution of the natural satellite.

Distance from Mars

Phobos is about 6000 km or 3700 miles away from the surface planet Mars or 9,376 km from the core of Mars. The distance between Mars and its natural satellite is the smallest in all of the planets and the natural satellite of the solar system.


The surface temperatures on Phobos can range from -4 °C or 25 °F to -112 °C or -170 °F. The higher range of temperature is found on the left side where the sunlight falls while the low temperatures are found on the shadowed side. Phobos is also one of the least reflective bodies in the Solar system.

Solar Transit

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 Phobos

Phobos has been visited by manmade spacecraft’s. Many of these spacecraft’s have photographed Phobos from a close distance and provided great details about its characteristics. In 1969, NASA’s Mariner 7 became the first man made spacecraft to visit the Martian moon.

In 1977, another spacecraft by NASA called the Viking 1 visited Phobos. The Mars Global Surveyor visited Phobos between 1998 and 2003. The Mars Express visited the moon in 2004, 2008 and 2010.

Another mission to visit the Martian moon was the Mars Reconnaissance Orbiter in 2007. The Spirit Rover took several photographs of the night sky from the surface of Mars on 25th of August 2005. Both the moons of Mars are clearly visible in the photographs. All of the spacecraft’s were launched by NASA.

In 1988 the Soviet Union launched to space probes called Phobos 1 and Phobos 2. Phobos 1 was accidentally shut down and lost while still in route to the moon. Phobos 2 successfully landed in the Martian system in January 1989 and conducted detailed examination of the moon of Mars.

In November 2011 a sample return spacecraft was launched by the Russian Space Agency. The spacecraft was called the Phobos Grunt. However, after reaching the earth’s orbit the mission could not survive and despite several attempts to save it, failed and crashed onto the surface of the earth and never sent to Mars.

In 1997 and 1998, spacecraft called Aladdin was selected to be launched which was later canceled because of its high budget, and the mission MESSENGER to Mercury was chosen over it.

Many more missions like these have been launched to observe Mars and its moons and the entire Martian system. Several future missions have been proposed as well by space agencies all over the world.

Phobos is also being considered one of the first targets for a human space mission. Looking at the current progress in Astronomy human flight to Mars and its moons could not be that many years away. This would be a great help in discovering more about the planet Mars which has been a part of human curiosity since ancient times.

Potential for Life

No evidence of life has ever been found on Phobos or Deimos and it is not yet certain whether these moons would ever be able to sustain life because of the extreme conditions on the surface and in the atmosphere.

Future of Phobos

It is believed that the radius of Phobos is decreasing by two meters every 100 years. This could be because of tidal deceleration. It is estimated that Phobos will be completely destroyed in 30 to 50 million years.

Phobos is coming close to the planet Mars at the rate of nearly 6 feet or 1.8 meters every hundred years. If this continues, the Martian moon will either crash into Mars or break up into a ring in a period of nearly 50 million years.

Phobos in Pop Culture

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.

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 almost accurately predicted the orbital period of the inner satellite of Mars that is Phobos.

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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