Phobos

What is Phobos?

Moons· Diameter ~22 km. The larger, inner of Mars's two moons — a potato 27 km long racing just 6,000 km above the surface every 7 h 39 min, so it rises in the west and sets in the east. It creeps 1.8 cm closer each year and will one day break into a ring. Compare it with a city on the size ladder.

The orbit of Phobos at this moment: https://nowsky.org/solar/orbit?fw=moon:mars:phobos&sp=0.0004 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How far is Phobos from Earth?

6,000 km above the surface.

Which planet does Phobos orbit?

Mars.

In one line, what is Phobos?

Moons — The larger, inner of Mars's two moons — a potato 27 km long racing just 6,000 km above the surface every 7 h 39 min, so it rises in the west and sets in the east.

What MMX will bring back from Phobos

Japan's MMX mission is planned to launch in 2026, reach the Mars system in 2027, touch down twice on Phobos to collect at least 10 g of material, and return to Earth in 2031. Because Phobos's surface is calculated to contain debris blasted off Mars, the sample could be an indirect Martian sample arriving before any Mars sample return.

The central question is where Phobos came from — a captured asteroid, or the reassembled debris of a giant impact on Mars. Its low density and an orbit lying neatly in Mars's equatorial plane point to the latter; its dark surface colour points to the former. The isotopic composition of the sample will decide. The spacecraft must land where surface gravity is less than a thousandth of Earth's.

The moon in depth

Phobos was found in 1877 by Asaph Hall at the US Naval Observatory, days after he spotted Deimos. A lumpy potato measuring 27 × 22 × 18 km, it orbits only 6,000 km above the Martian surface — closer to its planet than any other moon in the solar system. It completes a lap every 7 h 39 min, three times a day, faster than Mars rotates, so from the ground it rises in the west and sets in the east.

Its largest feature, Stickney crater, is 9 km wide — a third of the moon itself — and the parallel grooves radiating from it remain a puzzle. Because Phobos is so lightweight and porous, scientists have long debated whether it is a captured asteroid or debris from a giant impact on Mars. Japan's MMX mission plans to land and return a sample that should settle the question.

Tides drag Phobos 1.8 cm closer to Mars every year; in roughly 50 million years it will either be torn into a ring or crash onto the surface. Follow Mars in the orbit view to see Phobos racing well inside Deimos's path, and set it beside a city like Seoul on the size ladder.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.