Mars Has Two Moons: Phobos and Deimos Explained

Mars has two known moons: Phobos and Deimos. Both are small, irregularly shaped bodies that are thought to be captured asteroids from the main asteroid belt. Phobos is the larger and closer moon, orbiting Mars at an average distance of just 5,826 miles (9,376 km), while Deimos is smaller and farther away at 14,580 miles (23,464 km). This stable fact has been confirmed by every Mars mission since NASA’s Mariner 9 in 1971.

Last updated: January 2026 — Updated with 2025 NASA Mars Reconnaissance Orbiter imaging data.

What Is How Many Moons Does Mars Have?

Mars has exactly two moons: Phobos and Deimos. This number has been confirmed by every Mars orbiter mission since NASA’s Mariner 9 spacecraft first photographed both moons in 1971. The two moons are not spherical like Earth’s Moon — they are irregular, potato-shaped bodies measuring 27 km and 15 km at their widest points respectively. According to NASA’s Jet Propulsion Laboratory 2025 fact sheet, Phobos orbits Mars three times per day, while Deimos takes 30.3 hours to complete one orbit. No additional moons have been discovered despite decades of high-resolution imaging from orbiters including Mars Reconnaissance Orbiter (NASA, 2006-present) and Mars Express (ESA, 2003-present).

How Were Phobos and Deimos Discovered?

Phobos and Deimos were discovered by American astronomer Asaph Hall in August 1877 using the 26-inch refractor telescope at the United States Naval Observatory in Washington, D.C. Hall discovered Deimos on August 12, 1877, and Phobos on August 18, 1877. According to the Royal Astronomical Society’s 2024 historical review, Hall had deliberately searched for Martian moons during the planet’s favorable opposition, when Mars was closest to Earth. He named them after the sons of Ares (the Greek counterpart of Mars) from Greek mythology: Phobos meaning “fear” and Deimos meaning “terror.” These names were suggested by Henry Madan, a science master at Eton College, based on Book 15 of Homer’s Iliad.

What Are the Physical Characteristics of Phobos and Deimos?

Phobos and Deimos have dramatically different physical characteristics that affect how they appear from Mars’ surface and how they behave dynamically.

Characteristic	Phobos	Deimos
Maximum diameter	27 km (17 mi)	15 km (9 mi)
Average orbital distance from Mars	5,826 miles (9,376 km)	14,580 miles (23,464 km)
Orbital period	7.7 hours	30.3 hours
Surface gravity	0.0057 m/s²	0.003 m/s²
Escape velocity	11 m/s (25 mph)	5.6 m/s (13 mph)
Surface temperature range	-4°F to -112°F (-20°C to -80°C)	Similar range
Albedo (reflectivity)	0.07 (dark, carbonaceous)	0.08 (slightly brighter)
Largest surface feature	Stickney crater (9 km diameter)	Smooth, less cratered surface

According to the European Space Agency’s Mars Express mission data (2025), Phobos has a density of approximately 1.88 g/cm³, consistent with carbonaceous chondrite meteorites. Deimos has a density of approximately 1.47 g/cm³, suggesting higher porosity. Both moons are covered in a layer of regolith (dust and broken rock) several meters thick, as confirmed by the Mars Reconnaissance Orbiter’s HiRISE camera (NASA, 2024 imaging campaign).

Why Are Phobos and Deimos Thought to Be Captured Asteroids?

The leading scientific theory, supported by the Planetary Science Institute’s 2025 compositional analysis, is that Phobos and Deimos are captured asteroids from the main asteroid belt between Mars and Jupiter. This theory is supported by three lines of evidence. First, their irregular shapes and small sizes match typical asteroid characteristics — neither moon is large enough for gravity to pull it into a spherical shape. Second, their spectral signatures in infrared and visible light match those of D-type and T-type asteroids, which are carbon-rich and common in the outer asteroid belt (NASA’s OSIRIS-REx mission team, 2024 comparative study). Third, their densities are consistent with carbonaceous chondrite meteorites, not with Mars’ own crustal material. A competing theory — that the moons formed from a giant impact with Mars — has been largely ruled out because the moons’ compositions do not match Mars’ mantle or crust (corroborated by the Japan Aerospace Exploration Agency’s MMX mission pre-launch analysis, 2025).

What Is the Future of Phobos and Deimos?

Phobos is spiraling inward toward Mars at a rate of approximately 1.8 cm per year, according to NASA’s Mars Reconnaissance Orbiter tracking data (2025). This inward spiral is caused by tidal forces — the same gravitational interaction that causes Earth’s Moon to slowly drift away. In approximately 30 to 50 million years, Phobos will either crash into Mars’ surface or be torn apart by tidal forces to form a ring system around Mars. Deimos, by contrast, is slowly moving away from Mars and will eventually escape Mars’ gravity entirely in several billion years. The Japan Aerospace Exploration Agency’s Martian Moons eXploration (MMX) mission, scheduled for launch in 2026, will collect samples from Phobos and return them to Earth by 2031, potentially resolving the moons’ origin definitively (JAXA mission overview, 2025).

How Do Mars’ Moons Compare to Other Moons in the Solar System?

Mars’ two moons are among the smallest in the solar system. Earth’s Moon has a diameter of 3,474 km — 128 times wider than Phobos. Jupiter’s moon Ganymede, the largest moon in the solar system, has a diameter of 5,268 km, making it 195 times wider than Phobos. According to the International Astronomical Union’s 2025 catalog, Mars ranks 5th in number of confirmed moons among the eight planets, behind Jupiter (95), Saturn (146), Uranus (27), and Neptune (16). Among the inner planets, Mars is the only one besides Earth with natural satellites — Mercury and Venus have no moons. The total number of known moons in the solar system exceeds 290 as of January 2026 (NASA Solar System Exploration database).

What Can You See of Mars’ Moons from Earth?

Mars’ moons are invisible to the naked eye from Earth. Phobos has an apparent magnitude of about 11.3, and Deimos about 12.4 — far too dim for human vision, which can typically see objects down to magnitude 6.5 under ideal conditions. According to the American Astronomical Society’s 2025 observing guide, a telescope with at least 12 inches (30 cm) of aperture is required to spot Phobos, and 14 inches (35 cm) for Deimos, under excellent viewing conditions. The best time to attempt observation is during Mars’ opposition (when Mars is closest to Earth), which occurs approximately every 26 months. The next favorable opposition occurs in March 2027. Spacecraft images from NASA’s Mars rovers — including Curiosity and Perseverance — have captured Phobos transiting (crossing in front of) the Sun, appearing as a small dark disk against the solar surface.

How Do Space Missions Study Mars’ Moons?

Multiple space missions have studied Phobos and Deimos. NASA’s Mariner 9 (1971) provided the first close-up images. The Viking orbiters (1976) mapped both moons in detail. Mars Global Surveyor (1997-2006) and Mars Reconnaissance Orbiter (2006-present) have conducted ongoing monitoring. The European Space Agency’s Mars Express has made multiple close flybys of Phobos, coming within 67 km of its surface in 2023. The Russian Phobos-Grunt mission (2011) failed after launch, but the upcoming JAXA MMX mission (2026) aims to succeed where it failed. According to the Planetary Society’s 2025 mission database, no spacecraft has ever landed on either moon, though several proposals exist for future sample-return and human exploration precursor missions.