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Phobos – one of the two moons of Mars, is pretty interesting. It is actually a doomed satellite, which is slowly but surely moving towards is end with every tick of the clock. Most of the names of the Phobos’ features are given after names of the fictitious places of the novel Gulliver’s Travels. There are many such fascinating Phobos facts. Let us learn about a few of those facts…

Before we jump into our topic, let us learn some quick and short facts about Phobos:

Satellite ofMars
Alternative nameMars I,
Discoverer’s nameAsaph Hall
Discovery date17th August, 1877

Physical characteristics

Closest approach (Periapsis)9234.42 km
Farthest approach (Apoapsis)9517.58 km
Dimensions27 x 22 x 18 km
Mean radius11.2667 km


(1.76941 mEarths)

Surface area1548.3 km2
Volume5783.61 km3
Mass1.0659×1016 kg
Mean Density1.876 g/cm3
Escape Velocity11.39 m/s OR, 41 km/h OR, 25 mph
Time taken for completion of one orbit7.65 hours
Orbital speed (average)2.138 km/s
Mean radius of Phobos11.2667 km

Fascinating Phobos Facts: 1-13

1. Johannes Kelper propounded that Mars may have two moons. How did he do that? He simply made a random guess and thought that since Mars is between Earth, which has one satellite and Jupiter, which has 4 (back then, Jupiter was known to have 4 satellites). So, Mars may have 2 satellites or moons.

2. No study was conducted to prove it. Everyone thought that Mars had no moons. But it was Asaph Hall, an American astronomer who discovered Phobos and Deimos (two moons of Mars) from Washington D.C.’s U.S. Naval Observatory. He hunted for the moons closer to Martian surface as opposed to previous attempts for looking for a moon away from the planet.

3. Asaph Hall was frustrated at first with all his efforts going in vain but he continued trying with his wife’s persuasion (her name was Angelina). On August 12th, 1877, he found Deimos and on 17th August, 1877, he found Phobos.

4. These moons are so close to the surface of Mars that they mostly go unnoticed, because of the glare of Mars.

5. They are roughly of the size of an asteroid and are considered to be two of the smallest of moons in our solar system.

6. Did you know that Phobos is just 3,700 miles away from Mars? Our moon is roughly 384,000 miles apart from our Earth.

7. Phobos is just 7.24 times heavier that Deimos and it is closer to Mars and it is 4 times faster than Deimos.

8. The names are a bit weird. Phobos is named after one of the sons of Rome’s War God – Ares (Mars). Deimos was also named after one of the sons of the War God. Phobos means fear – remember the word phobia? And Deimos means fleeing especially from the battlefield after getting defeated. Ironically enough these two sons accompanied their father for battles (as per Roman Mythology).

9. Though Hall discovered two moons, no further study was conducted to know about them. It nearly took 100 years to start researching about them.

10. Phobos and Deimos are mystery to scientists. They have some qualities of asteroids and they completely show different properties which are not seen in asteroids. Their origin is also a mystery.

11. There are many hypotheses when it comes to the origin of the moons.

  • One such hypothesis is that they were asteroids and were present in asteroid belt present between Mars and Jupiter. The asteroids were pulled by Mars gravitational force and were trapped in the orbits in which they are now. Some aspects which Phobos and Deimos shares with asteroids are irregularity in shape, the material with which they are mostly made of – Type I and II carbonaceous chondrites, the same material that asteroids and dwarf planets are made of! However, there are other aspects which don’t go hand in hand with this hypothesis. Phobos’ orbit is perfectly circular in shape and if it were to be believed as an asteroid then having such a perfect circular orbit is not possible. The two satellites of Mars are less dense in comparison with asteroids which proves that these celestial bodies are not asteroids.
  • The other hypothesis put forward is that Phobos (and Deimos as well) is a part of Mars. When some asteroids collided with Mars, pieces of the planet clubbed together because of gravity and created Phobos. This hypothesis also fails because density wise, Phobos is less dense compare to Mars.
  • The third hypothesis is that a large protoplanet collided with Mars and created a big satellite which got broken down into tiny satellites. These mother and baby satellites were revolving and in this process, all but two got mixed in the Martian rocky surface.

12. Few studies conducted in 2016 approve this theory and NASA, in 2015, has seen some crevices on the Phobos surface, where the crevices indicate that Phobos is disintegrating and NASA assumes that this is the beginning of the process (see #15).

13. We told that Phobos’ revolves fast, right? If you are wondering how fast it revolves, then here is the answer. It revolves three times in a day (Earth day). In terms of Martian day, it revolves twice. It is seen as rising from west and setting in east. It takes only 4 hours 15 minutes for one revolution.

Fascinating Phobos Facts: 14-26

14. It has been observed that not only the Phobos is closer to the surface of Mars than Deimos but also is getting closer as each and every day passes.

15. Scientists are assuming that it may eventually get destructed by the tidal forces of Mars or it may break up to small pieces and some pieces may fall on Mars and others might just form a ring like structure around Mars which may be there for a short period of time. In case you are wondering about how close the satellite is getting to the Mars then here is the answer – Phobos is moving in towards Mars at a speed of 1.8 centimeters every year, which turns into about 6 feet or 1.8 meters every 100 years. With this speed, in about 50 million years, Phobos is destined to destroyed.

16. The Phobos has a dusty layer on its surface which is called as “regolith” and some of the scientists assume that when Phobos rotates and revolves this regolith drifts off from Phobos due to which a faint tail appears (like comets).

17. There is a large 9 km wide crater present on Phobos which is called as “Stickney”. This crater was named after Asaph Hall’s wife, Chloe Angelina Stickney Hall. Did you know that most of its large features are named after the places of the novel, Gulliver Travels?

18. Its temperature varies from -4 degrees Celsius to -12 degrees Celsius. Sometimes it may go till -40 degrees Celsius and beyond like -112 degrees Celsius.

19. NASA sent Mariner 9 spacecraft (first manmade or artificial satellite to revolve around another planet) in 1971. The photos taken by the spacecraft showed that both the satellites have potato like shapes unlike the spherical shape that all other moons have. Like our very own moon, only one side of the Phobos satellite is seen.

20. From 1971, every major space agency of developed countries started exploring more and more about these two moons.

21. NASA’s Viking spacecrafts (1 and 2) and Mars Global Surveyor, Russia’s (then Soviet Union) Phobos 2 and Europe’s Mars Express explored Mars and its satellites. NASA’s MAVEN (Mars Atmosphere and Volatile Evolution mission) took pictures of Phobos to understand the satellite better. Photographs of the satellites were taken from Martian surface as well. Several Rovers (a space vehicle for exploration which is designed in such a way that it moves on the surface of any celestial body) were sent such as Spirit, Curiosity and Opportunity. They gave photos from the ground. The active rovers as of 2009 are Curiosity and Opportunity.

22. Russia tried to send Phobos-Grunt, a space mission which would go to Phobos and take samples and return to Earth, in 2011. However, it couldn’t make its journey and fell back in Pacific Ocean in 2012.

23. NASA is all geared up for Phobos Surveyor Mission, which would send probes to Phobos surface. It is also getting ready for Phobos And Deimos and Mars Environment (PADME), a mission which would try to unravel the mystery of the moons’ origin. Along with this, it would investigate the composition, amount and distribution of dust on the planet etc.

24. Did you know that many of the scientists of 1950’s to 1960’s era thought that the satellite is man-made, a Martian of course? Dwight D. Eisenhower, the then President of America, was quoted saying, “If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made.”

25. The density of the satellite is so low that a 150-pound human would just weigh about 2 ounces there.

26. Phobos and Deimos can be used by NASA as space stations to understand more about Mars. Low gravity would serve our purpose in doing this and there are proposals of sending humans to Phobos before taking the great leap.

Sources: 1, 2, 3, 4

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