How many Moons does Mars have?

Mars has two small moons, Phobos and Deimos.  These moons were discovered in 1877 by American astronomer Asaph Hall.  Compared to Earth’s moon, both of Mars’ moons are quite small and irregularly shaped.  They are believed to be captured asteroids or objects from the Kuiper Belt and are not round like Earth’s moon.

Characteristics of the Moons

Phobos:

• Size:  About 22.4 km (13.9 miles) in its longest dimension.
• Orbit:  Extremely close to Mars, orbiting at a distance of just about 9,378 kilometers (5,827 miles) from the Martian surface.
• Orbital Period:  It has a very rapid orbital period, taking just about 7.66 hours to complete one orbit around Mars.
• Features:  Phobos has a large, prominent crater named Stickney, which is about 9 km (5.6 miles) in diameter.  The moon is also covered in loose, porous material known as regolith.

Deimos:

• Size:  Even smaller than Phobos, with dimensions of about 12.4 × 12.4 × 10.4 km.
• Orbit:  It orbits at a distance of about 23,460 kilometers (14,580 miles) from Mars.
• Orbital Period:  Deimos takes about 30.3 hours to complete one orbit.
• Features:  Deimos has a smoother appearance compared to Phobos, lacking large features like the Stickney crater. Its surface is also covered in regolith.

Origin Theories

There are several theories about the origin of these moons:

• Captured Asteroids:  One of the most widely discussed theories is that Phobos and Deimos are captured asteroids from the asteroid belt between Mars and Jupiter.

• Mars-Origin Hypothesis:  Another theory proposes that they were formed from debris ejected into Martian orbit following a massive impact on the planet.

• Kuiper Belt Origin:  Some researchers speculate that the moons might be trans-Neptunian objects from the Kuiper Belt, captured by Mars’ gravity.

Future Exploration

Both moons are of great interest for future missions for several reasons:

• Phobos Missions:  Japan’s space agency, JAXA, is planning a mission called MMX (Martian Moons eXploration) aimed to bring back samples from Phobos by the late 2020s.

• Gateway for Mars Exploration:  Due to their close proximity, both moons could serve as initial staging posts for future Mars exploration missions.

• Understanding Mars:  Studying these moons could provide insights into the Martian environment and help to answer questions about their origin, which may also offer clues about the history and formation of the solar system.

• Human Exploration:  NASA has also considered the possibility of sending astronauts to one of the moons as a precursor to a manned Mars mission.

Check out our 3D Mars Learning Center for more information on Mars.   You can also learn more at: NASA Mars Exploration.