On October 20, a Japanese rocket lifted off from Tanegashima Island carrying a very unusual passenger: the MMX probe (Martian Moons eXploration), built by Japan’s space agency JAXA, which will spend nine months traveling to Mars… without ever landing on it. Its real target is Phobos, the larger of the Red Planet’s two moons, and the mission’s name sounds like a spy thriller.
The Jiji Press news agency confirmed the launch, aboard an H3 rocket that notched its third straight success, via nippon.com. If all goes to plan, the spacecraft will reach Mars orbit in 2027, spend a couple of years circling and brushing past Phobos, scoop up at least 10 grams of its surface material, and head home around 2031, with the sample capsule touching down in the Australian desert. It would be the first time humanity has ever brought home a piece of a Martian moon.
Why all the fuss over a 22-kilometer rock?
Because nobody actually knows what Phobos is. Seriously — it has spent decades as one of the solar system’s lingering little mysteries. There are two main competing theories, and they contradict each other in a way that’s deliciously annoying for scientists.
- The capture theory: Phobos and its sibling Deimos would be primitive asteroids that wandered too close to Mars and got snagged by its gravity. This fits the light spectrum reflected by Phobos’s surface, which closely resembles that of dark, D-type asteroids — the same kind found out in the solar system’s outer belt.
- The impact theory: The moons would actually be debris from a colossal collision with Mars billions of years ago, which gradually clumped together in orbit like a cosmic snowball. This theory better explains another key clue: Phobos and Deimos have nearly perfect circular, equatorial orbits — highly unusual for a randomly captured asteroid, which should arrive on a far messier trajectory.
The catch is that each theory explains one clue while stumbling on the other. Moreover, the capture theory carries an extra cosmic engineering problem: for Mars’s gravity to have slowed down and trapped a rock zooming past at speed, the planet would have needed a much denser atmosphere than it has today. Nobody has fully squared that particular equation yet.
What the sample could reveal
This is where fine chemistry comes in. If Phobos is a captured asteroid, its interior should still hold relatively intact volatile compounds (substances that evaporate easily, like certain ices or carbon-based compounds). But if it was born from a violent impact on Mars, that extreme heat would have “cooked off” and expelled much of those volatiles long before the rock ever formed. Analyzing isotopes, minerals and microstructure from an actual sample, back in labs on Earth, can settle something no orbiting camera can resolve from a distance.
The MMX probe isn’t traveling alone: it’s carrying Idefix, a small French-German rover (built by France’s CNES and Germany’s DLR) that will touch down on Phobos to explore its surface before the spacecraft collects material and turns back toward home.
The MMX Probe: A Long-Awaited Gamble
MMX hasn’t had an easy road. Japan originally planned to launch it in 2024, but a failure on the H3 rocket’s inaugural flight forced the whole mission to be pushed back. With three successful launches in a row now, the H3 boasts an 80% success rate, and JAXA confirmed that the probe’s first signals show everything is working normally.
So for the next five years, the MMX probe will orbit a Martian moon without ever setting foot on Mars itself, all to bring us back a handful of dust that might finally settle a mystery older than modern space exploration itself. Meanwhile, other missions keep hunting for similar clues on the Red Planet itself, including the DNA-related molecule the Curiosity rover found on Mars.