Phobos: Mars’s larger moon

How big is Phobos?

About 22 km (14 mi) across on average. Phobos isn’t round: it measures about 27 by 22 by 18 km.[1][3] Our Moon is about 160 times wider.[12]

It’s dark, dusty and covered in craters: NASA’s fact sheet gives it an albedo of just 0.07, darker than most of our Moon.[4][12]

How does Phobos move?

Phobos takes 7 h 39 min to circle Mars, faster than Mars spins, so from the ground it rises in the west and sets in the east.[6] It orbits about 6,000 km (3,700 mi) above the surface, closer to its planet than any other moon in the solar system.[2]

What is Stickney crater?

The biggest crater on Phobos, about 9 km (6 mi) wide; NASA and ESA give figures from 9 to 9.7 km.[1][5] It’s named for Chloe Angeline Stickney Hall, a mathematician and the wife of Asaph Hall, the astronomer who discovered both moons in 1877.[1]

Long, shallow grooves run across Phobos. A 2015 NASA study says they are likely early signs of the structural failure that will one day destroy the moon, caused by tidal forces: the pull between Mars and Phobos.[2]

Will Phobos crash into Mars?

Eventually. Tidal forces between Mars and Phobos are dragging it inward: its orbit shrinks by about 1.8 m (6 ft) every 100 years.[1] NASA says it 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.[2]

Where did Phobos come from?

Nobody knows yet. Phobos could be an asteroid that Mars captured, or it could have formed from debris thrown into orbit when something large hit Mars. Its dark color looks like an asteroid’s, but its nearly circular orbit close to Mars’s equator fits the impact idea better.[8][9] Japan’s MMX mission was built to find out.

Seeing Phobos from Mars

Rovers on Mars have filmed Phobos crossing the face of the Sun. Astronomers call that a transit; it’s sometimes described as a Martian solar eclipse. On July 2, 2026, Perseverance’s Mastcam-Z camera went further and caught Earth, a bright dot, slipping behind Phobos.[7]

MMX: a piece of Phobos for Earth

Japan’s MMX is scheduled to launch on Oct 20, 2026, at 04:41 Japan time (Oct 19, 19:41 UTC).[10] It should reach Mars in 2027 and study Phobos from a close orbit. A small French–German rover, IDEFIX, is due to land on Phobos in 2029.[11] MMX itself then plans to land, collect more than 10 grams of Phobos and bring them home in Japan’s fiscal 2031, which ends in March 2032.[9]

Sources

1. NASA Science — PhobosPrimary · checked Oct 2, 2026
2. NASA — Mars’ Moon Phobos is Slowly Falling ApartPrimary · checked Oct 2, 2026
3. NASA JPL Solar System Dynamics — Planetary Satellite Physical ParametersPrimary · checked Oct 2, 2026
4. NASA NSSDCA — Mars Fact SheetPrimary · checked Oct 2, 2026
5. ESA — PhobosPrimary · checked Oct 2, 2026
6. NASA Science — Two Moons and the Pleiades from MarsPrimary · checked Oct 2, 2026
7. NASA JPL — NASA’s Perseverance Rover Watches Earth Vanish Behind Martian MoonPrimary · checked Oct 2, 2026
8. K. Kuramoto — Origin of Phobos and Deimos Awaiting Direct Exploration (Annual Review of Earth and Planetary Sciences, 2024)Primary · checked Oct 2, 2026
9. JAXA — MMX: Martian Moons eXplorationPrimary · checked Oct 2, 2026
10. JAXA — Launch of the MMX spacecraft onboard the H3 Launch Vehicle No. 10Primary · checked Oct 2, 2026
11. DLR — Off to the moons of Mars: MMX mission and Idefix rover ready for launchPrimary · checked Oct 2, 2026
12. NASA NSSDCA — Moon Fact SheetPrimary · checked Oct 2, 2026