Japan’s MMX Probe Launches Monday to Steal a Piece of Mars’ Moon

On Monday afternoon, a new kind of heist begins. Japan’s Martian Moons eXploration spacecraft, known as MMX, is scheduled to lift off from the Tanegashima Space Center at 3:41 p.m. EDT on October 19 aboard an H3 rocket. Its destination is not Mars itself but one of the planet’s two small moons, Phobos, where it will land, collect at least 10 grams of soil and rock, and carry the samples all the way home to Earth. If it succeeds, it will be the first material ever returned from the Martian system.

NASA officials are briefing the media on the mission this afternoon at 4 p.m. EDT on the agency’s YouTube channel, a sign of how deeply the U.S. is invested in a mission led by the Japan Aerospace Exploration Agency. NASA helped develop the MEGANE gamma-ray and neutron spectrometer aboard the spacecraft, which will assist in selecting and collecting the sample on Phobos’ surface. The agency also funded U.S. scientists on the mission’s science team and will provide communications and navigation support through the Deep Space Network, its array of giant radio antennas that talks to spacecraft across the solar system.

The strangest moon in the solar system

Phobos is worth the trouble. The moon is about 22 kilometers wide, lumpy like a potato, and races around Mars three times a day, completing each orbit in just 7.65 hours. Its name means “fear” in Greek. Its sibling, Deimos (“dread”), is drifting ever farther from the planet. Phobos is doing the opposite: Mars’ gravity is dragging it about six feet closer every century, and within roughly 50 million years the moon is expected to either crash into the planet or be torn apart into a ring like Saturn’s.

Nobody fully understands what Phobos is, and that is the point of the mission. Two camps have argued for decades. One holds that Phobos and Deimos are captured asteroids, snagged by Martian gravity. The other argues they formed from debris thrown into orbit by a giant impact with Mars, the same kind of process thought to have formed Earth’s Moon. Telescopes cannot settle it. The answer is written in the isotopic fingerprint of the rock itself, and reading that fingerprint requires a physical sample in a terrestrial laboratory.

Telescopes cannot settle the debate over what Phobos is. The answer is written in the isotopic fingerprint of the rock, and that requires bringing a piece home.

Landing where gravity barely exists

Getting the sample will be one of the most delicate maneuvers in planetary exploration. Phobos’ gravity is roughly one two-thousandth of Earth’s, too weak for a conventional landing. MMX will instead approach in quasi-satellite orbits, trajectories that loop near Phobos without truly orbiting it, then descend slowly on thrusters, touch the surface for about two and a half hours, and collect material with a dual coring and pneumatic sampler.

The spacecraft will also deploy Idefix, a 25-kilogram wheeled rover built by France and Germany, which will become the first vehicle to drive on an ultra-low-gravity world. Idefix will analyze the composition and texture of Phobos’ surface, scouting terrain ahead of the main spacecraft’s touchdown.

The timeline is ambitious but precise. Launch opens a 19-day window through November 7. The 4,480-kilogram spacecraft should arrive at Mars in August 2027, spend years surveying both moons, touch down on Phobos, and depart the Martian system around 2030. The return capsule is scheduled to parachute into Australia in 2031, where the samples will be recovered and shared with mission partners.

Why this sample matters now

MMX is flying in a changed landscape for Mars exploration. NASA’s own Mars Sample Return program, which would have brought back tubes of material collected by the Perseverance rover, was cancelled, leaving those samples stranded on the Martian surface. That makes MMX the only realistic near-term path for any material from the Martian system to reach Earth, and possibly the only one for the rest of the decade.

The scientific stakes go beyond Phobos. Researchers estimate that Phobos’ regolith contains grains blasted off Mars itself, meaning a scoop of the moon’s soil could include fragments of the Martian surface as a bonus. Together with the isotopic measurements that will settle the moon’s origin, the samples could illuminate the early solar system and the conditions that allowed life to emerge on Earth.

There is also a human future in the margins of the mission. As NASA pushes Artemis missions toward the lunar south pole with the goal of sending crews to Mars, Phobos keeps appearing in long-range planning as a potential staging point: close to Mars, low gravity, possibly rich in water. Understanding its geology and resources is, in a practical sense, reconnaissance for the human era.

Japan has done this before at smaller scale. The Hayabusa and Hayabusa2 missions returned asteroid samples from far less ambitious journeys, and MMX builds directly on that heritage at a cost of about 55.3 billion yen. But nothing about this journey is small. A five-year round trip to the Martian moons, a landing in near-zero gravity, and a return capsule dropping into the Australian desert: if the H3 performs on Monday afternoon, the decade of planetary sample-return science begins.