SafekipediaThe two moons of Mars, Phobos and Deimos, are small and not round. They were found by American astronomer Asaph Hall in August 1877. They are named after characters from Greek mythology, Phobos and Deimos, who followed their father Ares, the counterpart of Mars.
Phobos is a little bigger, with a width of about 22 kilometers. Deimos is smaller, measuring about 12.6 kilometers across. Phobos orbits very close to Mars, going around the planet in just under 8 hours. Deimos takes about 30 hours for one orbit.
Scientists have two main ideas about where these moons came from. One idea is that they were made when a large object hit Mars a long time ago. The other idea is that Mars pulled these moons from space and kept them close. Both ideas match what we know now, and future space missions might help us learn which explanation is right.
History
People have wondered if Mars had moons since the moons of Jupiter were found. The writer Jonathan Swift wrote about two moons of Mars in his story Gulliver's Travels in 1726, long before they were discovered.
The moons were found by Asaph Hall in August 1877 at the US Naval Observatory in Washington, D.C..
Hall discovered Deimos on August 12, 1877, and Phobos on August 18, 1877, using a big telescope. The moons are named after characters from an old story about the god Mars.
Characteristics
If you were on Mars near its equator, the moon Phobos would look about one-third as big as our Moon looks from Earth. Deimos would look more like a bright star or planet, similar to how Venus looks from Earth. Neither moon is big enough to cause a total solar eclipse on Mars, but Phobos can cause total lunar eclipses almost every night.
Phobos moves very quickly across the sky, rising in the west and setting in the east, and it rises again in just eleven hours. Deimos rises more slowly in the east but takes much longer to set in the west. Both moons always show the same face to Mars because they are tidally locked.
| Label | Name and pronunciation | Image | Diameter (km) | Surface area (km2) | Mass (kg) | Semi-major axis (km) | Orbital period (h) | Average moonrise period (h, d) | Eccentricity | Inclination (°) | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| I | Phobos | /ˈfoʊbəs/ FOH-bəs |  | 22.2 km (13.8 mi)(27×21.6×18.8 km) | 1,548 km2 | 1.07×1016 | 9,377 km (5,827 mi) | 7.66 | 11.12 h (0.463 d) | 0.0151 | 1.093 |
| II | Deimos | /ˈdaɪməs/ DY-məss |  | 12.6 km (7.8 mi) (10×12×16 km) | 483 km2 | 1.5×1015 | 23,460 km (14,580 mi) | 30.31 | 131 h (5.44 d) | 0.00033 | 0.93 |
| The relative sizes of and distance between Mars, Phobos, and Deimos, to scale (Load the image in full size to see both Moons of Mars.) | |||||||||||
Origin
The origin of Mars’s moons, Phobos and Deimos, is still a mystery. One idea is that they might be captured asteroids from the asteroid belt. Another idea is that they formed from material thrown into orbit after a huge collision with another space rock. These moons give us clues about the history of Mars and its place in our solar system.
Exploration
Main articles: Phobos (moon) § Exploration, Deimos (moon) § Exploration, and List of missions to Mars § Missions to the moons of Mars
Many probes have looked at the moons of Mars. Only a few were meant to visit them. In 1988, two Soviet probes were launched to study Phobos and Deimos, but they did not land. In 2011, a Russian probe failed to leave Earth and fell back into the Pacific Ocean.
New missions are being planned. Japan plans to launch the Martian Moons eXploration mission in 2026. It will collect samples from Phobos and bring them back to Earth in 2029. The mission will also study Deimos with flybys.
Images
This article is a child-friendly adaptation of the Wikipedia article on Moons of Mars, available under CC BY-SA 4.0.
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