How Many Moons Does Mars Have?

A 3D rendering showing Mars with both of its moons.

The Moons of Mars

The moons of Mars are Phobos and Deimos. The two moons were discovered in 1877, however, their origin is still controversial. The appearance of Phobos and Deimos is like that of asteroids. Moreover, the moon that has its orbit closer to Mars than the other is Phobos which is also the larger one. Besides, Mars is the only planet, among the four terrestrial, rocky planets, that has more than one moon.


Early speculation of the two moons showed that their actual orbital distances are 1.4 and 3.5 Martian diameters for Phobos and Deimos respectively. The discoverer of the two moons was Asaph Hall who discovered Deimos and Phobos on August 12, 1877, and August 18, 1877, respectively. Hall was deliberately looking for Martian moons when he discovered the two moons because, on August 10, 1877, he had seen something like the Martian moon which he could not identify because of inclement weather.

In 1959, there was a claim by Walter Scott Houston that the moons of Mars were artificial satellites based on Dr. Arthur Hayall’s report. His assertion brought about the exciting scenario of Mars moon hoax. Walter perpetrated the April Fool’s hoax which gained the attention of the whole world as a celebrated hoax in the Great Plains Observer, April edition.


Although there has been no particular mission to explore the moons of Mars, there are many pictures that spacecrafts have taken during their flybys. Some of the pictures show that Phobos and Deimos were small from afar. Moreover, many other spacecrafts have also made observations from a long distance and have taken videos and more images of the moons.

The following are the descriptions of features of the two moons of Mars.

2. Deimos

Deimos appears like a bright planet or star for a viewer on Mars and is slightly larger than the appearance of Venus viewed from the Earth. The angular radius of Deimos is approximately 1’. Regarding its motion, Demos moves in an east-west direction (the direction of the Earth’s moon), taking 2.7 days to set in the west. Its movement is very slow despite the fact that it has a 30-hour orbit.

1. Phobos

The appearance of a full Phobos viewed from the Martian equator is 1/3 of the Earth’s full moon. However, the moon would appear smaller when the viewer is far away from the planet’s equator and is even invisible from the polar ice caps of Mars. The angular radii of the rising and overhead Phobos are 4’ and 6’ respectively. Therefore, from the angular diameters of the two moons, it is clear that Mars does not have solar eclipses since the angular diameter of the Sun in Mars is approximately 21’, which is larger than the ones for the two moons. Regarding its movement, Phobos typically move in a west-east direction, unlike the Earth’s moon which moves in the east-west direction. Besides, Phobos rises after every eleven hours, one hour less than the rising time of the Earth’s moon. Thus, the nature of motion of the moons of Mars is quite different from that of the Earth’s moon.


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