Interesting Facts About Phobos and Deimos, Friends of Mars in the Solar System

Interesting Facts About Phobos and Deimos, Friends of Mars in the Solar System
Interesting Facts About Phobos and Deimos, Friends of Mars in the Solar System


If we are on Mars, not only one moon appears in the sky but two giant moons like potatoes. Phobos and Deimos.

Just as Mars in Greek mythology is the God of War, Phobos and Deimos are his two twin brothers who take part in wars to cause chaos. Phobos is the representation of panic and fear while his twin brother Deimos is the cause of terror.

In the Solar System, the twins also always accompany Mars, the Red planet, when orbiting the Sun. But, don't imagine that these two objects are round like the Moon.

Phobos and Deimos actually look like two low-density, potato-shaped blobs orbiting in a circular orbit around Mars. Astronomers suspect that the two satellites are a combination of rock debris held together by gravity.

Fear and Terror

Phobos and Deimos began to be suspected of its existence after Galileo discovered the existence of two bulges on Saturn and the discovery of 4 Galilean satellites of Jupiter. This discovery in 1610 led Johannes Kepler to misinterpret an anagram of Galileo's report of a bulge on Saturn that was mistaken for a distant planet, as a pair of Mars twins.

In 1726, Jonathan Swift also described how the fictional astronomer Laputa discovered a pair of moons on Mars in Gulliver's Travel. Apparently, Swift was inspired by the arguments that developed at the time. If Mercury and Venus have no satellites, Earth has one, and Jupiter has four. If so, Mars may have two satellites.

More than a century later, in 1877, astronomer Asaph Hall conducted a search for the moon on Mars with the 66 cm telescope at the United States Naval Observatory. The telescope was the best and largest telescope at that time.

On August 12, Asaph Hall discovered the presence of a faint light near Mars. Observations for several days showed this object was not a star but was moving across the sky with Mars. That is, this object is a companion to Mars.

Six days after the first discovery, on August 18, 1877, Asaph Hall discovered a second object near Mars. This object has a closer distance to Mars.

This discovery not only shows that Mars has two satellites. Asaph Hall managed to determine the mass of Mars from the calculation of the distance and orbital period of the two satellites!

In Hall's calculations, the two Mars companions are less than 10,000 km from Mars and can complete their orbits in less than 8 hours!

Characteristics of Phobos and Deimos

Mars' two companions in the sky are named after the twins Mars and Aphrodite (Venus). The big one is called Phobos (fear) and the smaller one is called Deimos (terror).

What's interesting about these two objects is their irregular shape. Like potatoes. They are not spherical because their mass is very small. Small mass has little impact on gravity. As a result, Phobos and Deimos' gravity wasn't strong enough to make them both round.

The giant Phobos measures 27 x 22 x 18 km and orbits from a distance of 9377 km with an orbital period of about 8 hours. That means, Phobos orbits Mars three times in one day. The implication is, if we are on Mars, in one day (sol), observers can see Phobos rising 3 times from the west and setting in the east.

The smaller Deimos, orbits at a greater distance. About 2.5 times the distance from Phobos to Mars, or 23,436 km. Deimos is half the size of Phobos with dimensions of 15x12x11 km, and orbits Mars every 30 hours. One sol or one day on Mars is 24 hours 37 minutes. So Deimos will appear to move slowly in the sky and take 2.5 days to rise and set.

The two small moons are carbon-rich rocks such as type C asteroids and carbon chondrite meteorites or carbon chondrites. The low density of Phobos is also an indication that the constituent rock is not solid rock. In addition, astronomers suspect there is an ice reservoir beneath the Phobos regolith layer.

If there are many craters in Phobos due to collisions, the surface of Deimos is even smoother. It appears that the impact at Deimos caused material to be ejected to form a crater, but the material either fell back onto the surface of nearby Deimos or filled the newly formed crater.

Origin and Future

From their irregular shape, astronomers suspect Phobos and Deimos are asteroids captured by Mars.

However, there is an oddity. Phobos and Deimos orbit at the Martian equator in circular orbits. Characters like this are usually owned by objects that formed in the orbit of Mars, not captured.

If Phobos and Deimos were asteroids captured by Mars, their orbits would have to be oval, not circular. The captured asteroids certainly have different orbits and speeds from Mars.

As a result, once trapped by Mars' gravity, the two asteroids will have very elliptical orbits. Gravitational interactions for a very long time can make the orbits of the asteroids captured by Mars slowly form a circle. But, the span of time required for it is very long.

Thus, the orbits of these two satellites must remain oval in shape. In fact, observations show that the orbits of the two are nearly circular.

Apparently, Phobos and Deimos formed from debris or rings of matter around Mars. There are allegations that when Mars was young, this planet had a fairly large moon.

However, the gravitational interaction between the two causes the Moon to crash into the Martian surface leaving debris around the young Mars. This debris combines to form a satellite.

From observations, the orbital period or the time it takes Phobos to circle Mars is getting shorter day by day. About 2 meters per century.

It may seem small, but in 50-100 million years, Phobos will be very close to Mars. As a result, Phobos could be torn apart by Mars' gravity and end up as a ring of debris around Mars.

Deimos still survives about 3.5 billion years after Phobos was destroyed. After that, the outermost satellite of Mars will separate from the orbit of Mars.

Space Mission

Phobos and Deimos have been photographed by various space probes whose primary mission is to photograph Mars. The first is the Mariner 7 mission, followed by Mariner 9, Viking 1, Phobos 2, Mars Global Surveyor, Mars Express, Mars Reconnaissance Orbiter, the robot explorer Spirit, and Mangalyaan.

There were two Phobos missions launched by the Soviet Union in July 1988. Both were successful launches, but failed to carry out their missions. Phobos 1 experienced a system failure and disappeared on the way to Phobos. Meanwhile, Phobos 2 arrived in the Martian region in January 1989 but before studying the surface of Phobos, the spacecraft also experienced a software failure.

In 2011, Russia launched the planned Fobos-Grunt mission to sample Phobos rock. In addition to Fobos-Grunt, there is Yinghuo-1, a Chinese survey satellite that will be launched into Mars orbit. However, upon reaching Earth orbit, Fobos-Grunt failed to initiate combustion to Mars and eventually crashed to Earth in 2012.


Several other missions were considered and planned but not selected to be carried out. Another mission targeting Phobos is the Exploration of the Moon for Mars by the Japan Space Exploration Agency. Exploration of the Moon for Mars is planned for launch in 2024 with the aim of bringing home samples of material from Phobos, as well as flying over Deimos and monitoring the Martian climate.

Russia also plans to repeat the Fobos-Grunt mission as well as the Phootprint mission to bring home material by the ESA. Both are scheduled to launch in 2024.

Note: Don't expect to survive on Phobos. The temperature is very cold between -4º to -112º C! Meanwhile in Deimos, the average temperature is around -40.5º C!
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