NASA’s Astronomy Picture of the Day for October 9, 2026 shows one of the strangest landscapes in the inner Solar System: Stickney Crater, the largest crater on Phobos, the bigger of Mars’s two moons. It is a tiny world with a gigantic wound, and the story around it involves a doomed orbit, mysterious grooves, a pioneering woman, and a spacecraft that left Earth just two weeks after this picture was published.

A crater almost too big for its moon

Stickney spans a little over 9 kilometers, which is nearly half the width of Phobos itself. According to the APOD explanation, the impact that dug it out likely came close to shattering the moon. Phobos is a small, lumpy body only about 22 kilometers across on average, so the scale is hard to picture: imagine a crater almost half as wide as the entire object that was hit.

The image was taken by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter in March 2008, as the spacecraft passed within roughly 6,000 kilometers of Phobos. The colors are enhanced, so they exaggerate real differences in the surface rather than showing what your eyes would see. Even so, the picture holds real clues. Streaks down the inner crater walls suggest that loose material has slid downhill over time, which is notable because the surface gravity of Phobos is less than one thousandth of Earth’s. The light bluish areas near the rim could be relatively fresh, recently exposed material.

The woman behind the name

Stickney is named for Chloe Angeline Stickney Hall, a mathematician and the wife of astronomer Asaph Hall, who discovered both Martian moons in August 1877. She had been his mathematics teacher in college. Accounts of the discovery say Hall grew discouraged by poor weather and the slim odds of finding anything, and that he might have given up the search without her encouragement. Hall himself acknowledged this. It is a rare case of a crater name that marks a real contribution to the discovery itself.

Things about Phobos that sound made up, but are not

It rises in the west and sets in the east. Phobos orbits closer to its planet than any other moon in the Solar System, about 6,000 kilometers above the Martian surface, and completes an orbit in just 7 hours and 39 minutes. That is faster than Mars rotates, so from the surface of Mars it would appear to move backward across the sky, crossing it several times in a single Martian day.

It is slowly falling toward Mars. Tidal forces are pulling Phobos closer at roughly 1.8 centimeters per year, or about 1.8 meters per century. Most researchers expect it will be torn apart by Mars’s gravity before it ever hits the surface, leaving a temporary ring of debris. Published estimates of when this might happen range widely, from about 10 million to around 100 million years, depending on assumptions about the moon’s strength and internal structure.

Its grooves are still a puzzle. Long, parallel grooves run across the surface. NASA says their origin is mysterious and may be linked either to tidal stresses on the close-orbiting moon or to the Stickney impact itself. A modeling study reported in 2020 found that tidal strain on a rubble-pile interior covered by a more cohesive outer layer could produce evenly spaced fissures, which is one hypothesis, not a settled answer.

Nobody is sure where Phobos came from. One idea is that Phobos and its smaller sibling Deimos are captured asteroids. Another is that they formed from debris thrown into orbit by a giant impact on early Mars. JAXA notes that in the second case the moons would preserve fragments of the early Martian environment.

Gravity there is almost nothing. ESA points out that the 25 kilogram rover Idefix, carried by the new mission described below, will weigh the equivalent of only about 15 grams on the surface of Phobos.

A mission is on its way

On October 20, 2026 (Japan time), JAXA launched the Martian Moons eXploration (MMX) spacecraft on an H3 rocket from Tanegashima Space Center, and reported that it separated from the rocket and was operating normally. MMX is due to reach the Mars system around August 2027. According to ESA, it will be the first spacecraft to orbit Phobos, the first to collect samples from the moon’s surface, and the first to return to Earth from the Mars system. The goal is to bring back at least 10 grams of material in 2031, which could help settle the question of where the Martian moons came from. A landing site has not been announced in the sources, so whether MMX will touch down near Stickney is not known.

Sources

Image credit: HiRISE, MRO, LPL (U. Arizona), NASA. Enhanced-color image of Stickney Crater on Phobos, as featured in NASA’s Astronomy Picture of the Day, October 9, 2026.