Japan's Daring Mission to Mars' Moons Could Solve a Decades-Old Mystery

A Bold Leap Into the Unknown

They've been orbiting Mars for billions of years, and scientists still can't explain them. Phobos and Deimos — the two tiny moons of Mars — have baffled researchers for generations. "We can't explain Phobos or Deimos right now. They shouldn't exist, but they do," said Dr. Abigail Fraeman, a research scientist at NASA's Jet Propulsion Laboratory. "They're super weird, they're super small and they're super puzzling." Now, humanity is finally sending a mission specifically designed to crack that mystery wide open.

Japan's Martian Moons eXploration (MMX) mission launched on October 19 from Tanegashima Space Center, riding an H3 rocket that lifted off from Pad-2 at the Yoshinobu Launch Complex at 3:41 p.m. EDT. The JAXA-led mission is bound for Mars orbit, where it will study the planet's two moons, Phobos and Deimos. If everything goes according to plan, MMX will become the first spacecraft to land on and return samples from Phobos for study back on Earth — which scientists think could shed light on a decades-old mystery about the moons' origins and reveal clues about the formation of the early solar system.

A Scientific Mystery Millions of Miles Away

Scientists are split on whether the moons are captured asteroids or the consolidated remnants of material thrown up by a long-ago Martian impact. The orbits of Phobos and Deimos are circular and aligned with Mars's equator — not tilted — which suggests to some researchers that they weren't randomly pulled into orbit, but instead that an impact on Mars might have ejected material that later collected into these moons. The answer to that question has enormous implications for our understanding of how rocky planets across the solar system form and evolve.

Previous Mars missions have observed the peculiar pair whenever convenient. Orbiters have snapped images of the rocky bodies over the years, while rovers including Curiosity and Perseverance have captured "googly eye" eclipses when Phobos moved between Mars and the sun. But even after decades of observations, scientists still don't know how Phobos and Deimos came to be. Actual soil samples are the missing piece — and that's precisely what MMX is going after.

A Global Team, One Tiny Rover, and a Massive Payload of Questions

The risky robotic mission is an international collaboration — including scientific instruments from NASA, the European Space Agency, France's National Center for Space Studies, and the German Aerospace Center — that aims to collect more than 0.4 ounces (10 grams) of material from Phobos and return to Earth in 2031. That's a small amount of material, but space science has proven that even a few grams can rewrite textbooks.

Carrying a small rover named Idefix, the MMX spacecraft — a stack of three modules for propulsion, exploration, and returning to Earth — will spend about a year traveling to Mars and then orbit Phobos for about three years. The 55-pound Idefix rover has two main tasks: providing information to prepare for the landing of the main spacecraft to collect samples, and analyzing the composition and texture of Phobos' surface. The gravity on Phobos is so low that Idefix will only weigh the equivalent of 15 grams on its surface.

More Than Moon Rocks — Possibly a Piece of Mars Itself

JAXA scientists think some returned material could even originate from Mars itself, deposited on Phobos after ancient asteroid impacts blasted rocks off the Red Planet's surface — potentially holding evidence of its geological past. If the samples contain pieces of Mars, MMX could potentially be the first mission to successfully bring back Martian material to scientists on Earth, in a race alongside China. That would be a historic double achievement — samples from a Martian moon and, potentially, from Mars itself.

Once the MMX samples are safely returned to Earth, scientists will study their composition and mineralogy, then compare chemical fingerprints with those of Martian rocks and asteroid material. These analyses, combined with data collected by MMX during its mission, will help nail down the origin of Phobos — and by extension, Deimos. As Dr. Daniel Glavin, senior scientist at NASA's Goddard Space Flight Center, put it: "This is kind of like one of the major outstanding mysteries of solar system formation, the origin of Phobos and Deimos, so we really do need a mission like MMX to figure this out." With five years and millions of miles ahead of it, MMX has only just begun — but the questions it carries are ones humanity has been asking for a very long time.