Japan is sending a probe to Mars’ moon Phobos to solve its origin mystery
Across dozens of missions to Mars, humanity has never returned a single rock from either of its moons. That gap is about to close.
Phobos — a dark, potato-shaped body barely 11 kilometers in radius — has puzzled scientists for decades. It looks like an asteroid: carbon-rich, water-bearing, the kind of wandering rock that drifts through the outer solar system. Yet its orbit is suspiciously orderly, tracing a near-perfect circle along Mars’ equatorial plane. That contradiction sits at the heart of a debate no telescope or flyby has been able to settle.
Japan’s MMX mission intends to settle it the only way that counts — by bringing pieces of Phobos back to Earth, according to media such as WIRED Japan.
A first that has never been attempted
The MMX mission, developed by the Japan Aerospace Exploration Agency (JAXA), has one primary goal: retrieve at least 10 grams of material from Phobos and deliver it safely to Earth. That may sound modest. But no mission has ever done it — not a single rock from either of Mars’ moons has ever been studied in a terrestrial laboratory.
The timeline is ambitious. MMX will spend roughly a year traveling to Mars, three years conducting operations in Martian orbit, then another year on the return journey. Scientists expect the samples to arrive on Earth in 2031.
The mission also marks a personal milestone for Japan’s space program — its first Mars probe launch in 28 years, carrying a price tag of approximately $345 million. A significant investment, but one with potentially transformative scientific returns.
The mystery at the heart of the mission
Two competing theories try to explain how Phobos and Deimos came to orbit Mars. The first holds that both moons formed from debris ejected during a massive ancient collision, similar to the leading theory for how Earth’s own moon formed. The second suggests they’re captured asteroids, originally drifting through the outer solar system before Mars’ gravity pulled them in.
The evidence is frustratingly split. Phobos’ dark color and light-reflecting properties closely resemble those of carbon- and water-rich asteroids. Its orbit, though — nearly circular, aligned along Mars’ equatorial plane, moving in the same direction as the planet’s rotation — fits far better with the giant impact hypothesis. Telescopes and flybys haven’t broken the tie.
Physical samples could. By comparing Phobos material to data already collected from the Martian surface, scientists may finally determine which origin story is correct. There’s an added bonus: roughly 0.1% of the collected samples are expected to be actual Martian material — debris blasted off the planet by ancient meteorite impacts and deposited on Phobos over billions of years, a secondary science return no Mars lander has ever offered.
Engineering a landing on a tiny, low-gravity world
At launch, MMX will weigh 4,480 kilograms. More than half of that mass is fuel, divided into reserves for the outbound leg, the exploration phase, and the return trip. As each propulsion module is exhausted, it gets jettisoned — a staged approach designed to keep the spacecraft as light as possible for each successive phase.
Landing on Phobos presents a unique engineering problem. The moon’s gravity is far weaker than Earth’s moon, ruling out descent techniques designed for larger bodies. It’s also roughly 50 times stronger than the gravity on asteroid Ryugu, where JAXA previously landed probes using prolonged hovering — a technique that burns too much fuel in Phobos’ stronger pull.
The solution is a high-precision autonomous navigation system. Radio signals between Earth and the spacecraft can take up to 20 minutes one way, making real-time human control impossible. The probe must independently execute the entire descent, landing, and eventual takeoff sequence. During descent, MMX will match observed terrain against stored topographic maps of Phobos’ craters; below 300 meters altitude, hazard detection scans for unexpected surface elevation changes. If something looks wrong, the probe aborts, ascends, and tries again.
Rovers, drills, and a puff of nitrogen gas
Before the main spacecraft touches down, it will deploy the IDEFIX rover — developed jointly by the French and German space agencies — to conduct roughly 100 days of independent surface exploration. IDEFIX acts as a scout, providing data that helps confirm a safer landing zone for the primary lander.
Once on the surface, MMX collects samples two ways. A robotic arm drills cylindrical tubes directly into the regolith, capturing material from beneath the surface. A separate device, originally developed by NASA, uses a controlled burst of nitrogen gas to gather fine dust particles from the very top layer. The drilled samples offer insight into the moon’s interior composition, while the surface dust is the most likely repository for Martian material deposited by ancient meteorite strikes — a potential window into Mars itself.
A stepping stone toward Mars itself
MMX is a science mission, but it’s also a technology demonstration. JAXA has designed it to validate engineering capabilities that will matter enormously for future exploration: autonomous deep-space navigation, precision landing on low-gravity bodies, and multi-stage jettisoning of propulsion modules across a multi-year round trip.
These aren’t abstract milestones. Any future crewed or robotic mission hoping to travel to Mars and return will need exactly these skills. MMX is, in part, a rehearsal.
The mission is scheduled to launch from Tanegashima Space Center on October 20, 2026. If it succeeds, it won’t just answer a decades-old question about how Phobos formed — it will demonstrate that humanity has the tools to reach deep into the solar system, collect what it finds, and bring it home. That capability, once proven, opens doors well beyond a single potato-shaped moon.
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