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Mars facts · Moons

How many moons does Mars have?

Mars has two moons, Phobos and Deimos. Both are small and lumpy: Phobos is about 22 km (14 mi) across and Deimos about 12 km (7 mi). Japan’s MMX mission is scheduled to launch on Oct 20, 2026 (Japan time) to bring a piece of Phobos back to Earth.

Updated Sources (19)

Two moons, side by side

Phobos and Deimos compared
FactPhobosDeimos
Size across22 km (14 mi)27 × 22 × 18 km12 km (7 mi)15 × 12 × 11 km
Height above Mars6,000 km (3,700 mi)about 20,000 km (12,000 mi)
Time to circle Mars7 h 39 minabout three laps a day30 h 18 mina little more than a day
In Mars’s skyRises in the west, sets in the eastRises in the east, stays up about 2½ days
Name meansFearDread
FutureSpiraling in, about 1.8 m (6 ft) per centurySlowly drifting away

The two shapes are drawn to scale with each other. Sizes are average widths; both moons are lumpy. For scale, our Moon is 3,475 km (2,160 mi) across, about 160 times wider than Phobos.[1][2][5][6]

Phobos, the moon that’s falling

Phobos circles Mars about three times a day, faster than Mars spins, so from the ground it rises in the west and sets in the east.[13] It flies about 6,000 km (3,700 mi) above the surface, closer to its planet than any other moon in the solar system.[3]

Its orbit is shrinking by about 1.8 m (6 ft) every 100 years. NASA says Phobos will either crash into Mars in about 50 million years or break up into a ring of rubble.[1] A 2015 NASA-led study expects tidal forces to pull it apart in 30 to 50 million years.[3]

Phobos, explained

Deimos, the moon drifting away

Deimos is the smaller, outer moon, about 20,000 km (12,000 mi) up.[4] It takes 30 h 18 min to circle Mars, only a little longer than a Martian day, or SolA sol is a day on Mars — 24 h 39 min 35 s, about 40 minutes longer than ours — and rovers count them from the day they land.More in the glossary, so it creeps across the sky. Astronomy magazine calculates that it stays above the horizon for about 60 hours at a time.[4][8] Unlike Phobos, it is very slowly moving away from Mars.[7]

Where did Mars’s moons come from?

Nobody knows yet, and it’s one of the questions MMX was built to answer. One idea is that both moons are asteroids that Mars’s gravity captured. Another is that they formed from rubble thrown into orbit when something huge hit Mars. Their dark color looks like an asteroid’s, but their nearly circular orbits, close to Mars’s equator, fit the impact idea better.[9][10]

MMX: bringing back a piece of Phobos

Japan’s Martian Moons eXploration mission is scheduled to launch on an H3 rocket from Tanegashima on Oct 20, 2026, at 04:41 Japan time (Oct 19, 19:41 UTC).[11] It should reach Mars in 2027, study both moons, land on Phobos to collect samples and bring them to Earth in Japan’s fiscal 2031, which ends in March 2032.[10]

JAXA calls it humanity’s first attempt to bring samples back from Phobos, a Sample returnA sample return mission collects rock, soil or dust from another world and brings it back to Earth to study in labs.More in the glossary from the Mars system.[10] NASA and ESA’s Mars Sample Return lost its funding when Congress declined to support it in the FY2026 spending agreement, passed in January 2026,[17][18] and China’s Tianwen-3 aims to bring back samples from Mars itself around 2031.[19][14]

Follow MMX

Seen from the rovers

Mars’s RoverA rover is a robot vehicle that drives across Mars, following plans its team on Earth sends ahead of each drive.More in the glossary have watched Phobos and Deimos cross the face of the Sun, which astronomers call a transit and people sometimes call a Martian solar eclipse. On July 2, 2026, Perseverance’s Mastcam-Z camera caught something new: Earth, a bright dot, slipping behind Phobos.[12]

Questions

Does Mars have moons?

Yes, two: Phobos and Deimos, both far smaller than our Moon. Mercury and Venus have none; Earth has one.

What are the names of Mars’s moons?

Phobos and Deimos, which mean fear and dread: the sons of Ares, the Greek god of war. Asaph Hall found both in August 1877.

How big are Phobos and Deimos?

Phobos is about 22 km (14 mi) across; Deimos about 12 km (7 mi). Our Moon is about 160 times wider than Phobos.

Will Phobos crash into Mars?

Eventually, but not for tens of millions of years. Its orbit shrinks by about 1.8 m (6 ft) a century. NASA says it will either crash into Mars in about 50 million years or break up into a ring, and a 2015 NASA-led study expects tidal forces to pull it apart in 30 to 50 million years.

Is a mission going to Mars’s moons?

Yes: Japan’s MMX, scheduled to launch on Oct 20, 2026 (Japan time). It plans to land on Phobos and bring samples to Earth in Japan’s fiscal 2031 (by March 2032).

Sources

  1. NASA Science — PhobosPrimary · checked Oct 2, 2026
  2. NASA Science — DeimosPrimary · checked Oct 2, 2026
  3. NASA — Mars’ Moon Phobos is Slowly Falling ApartPrimary · checked Oct 2, 2026
  4. NASA NSSDCA — Mars Fact SheetPrimary · checked Oct 2, 2026
  5. NASA JPL Solar System Dynamics — Planetary Satellite Physical ParametersPrimary · checked Oct 2, 2026
  6. NASA NSSDCA — Moon Fact SheetPrimary · checked Oct 2, 2026
  7. ESA — Mars and Jupiter moons meetPrimary · checked Oct 2, 2026
  8. Astronomy — Ask Astro: Can Phobos and Deimos be seen from the surface of Mars?Secondary · checked Oct 2, 2026
  9. K. Kuramoto — Origin of Phobos and Deimos Awaiting Direct Exploration (Annual Review of Earth and Planetary Sciences, 2024)Primary · checked Oct 2, 2026
  10. JAXA — MMX: Martian Moons eXplorationPrimary · checked Oct 2, 2026
  11. JAXA — Launch of the MMX spacecraft onboard the H3 Launch Vehicle No. 10Primary · checked Oct 2, 2026
  12. NASA JPL — NASA’s Perseverance Rover Watches Earth Vanish Behind Martian MoonPrimary · checked Oct 2, 2026
  13. NASA Science — Two Moons and the Pleiades from MarsPrimary · checked Oct 2, 2026
  14. Xinhua — China unveils international partners for Tianwen-3 Mars missionSecondary · checked Oct 2, 2026
  15. ESA — PhobosPrimary · checked Oct 2, 2026
  16. NASA NSSDCA — Planetary Fact SheetPrimary · checked Oct 2, 2026
  17. House Appropriations Committee — Division A – Commerce, Justice, Science: Joint Explanatory Statement (Mars Sample Return)Primary · checked Oct 4, 2026
  18. Aerospace America (AIAA) — European Space Agency rethinks Mars plans as it kicks off a mission-dense 2026Secondary · checked Oct 4, 2026
  19. China National Space Administration (CNSA) — China celebrates Space Day with a sneak peek at Tianwen 3's tech (Apr 24, 2026, from China Daily)Primary · checked Oct 6, 2026