Japan's MMX mission explained: why scientists want a scoop of a Martian moon

What is MMX?

A Japanese spacecraft built to fly to Mars, land on one of its moons and come home.

The Martian Moons eXploration mission, run by the Japan Aerospace Exploration Agency (JAXA), lifted off on the tenth flight of Japan’s H3 rocket from the Tanegashima Space Center in the early hours of Tuesday, Japan time. JAXA confirmed that the spacecraft had separated from the rocket’s upper stage and was heading for Mars.

The probe has three parts: a propulsion module to get it there, an exploration module that will do the work at Mars, and a return module to carry the sample back. It stands 5.3 metres tall and carries 11 instruments. Its cost has been put at about $578 million.

Why go to the moons rather than Mars itself?

Because nobody knows where they came from, and the answer says something about Mars.

Mars has two small, lumpy moons. Phobos, the larger, is about 22 km across; Deimos is smaller still. Scientists have argued for decades over their origin. One idea is that they are asteroids that strayed too close and were captured by Martian gravity. The other is that they formed from debris thrown into orbit when a large body struck Mars, much as Earth’s Moon is thought to have formed.

“Phobos and Deimos may be the remains of a massive impact on Mars,” said Tom Statler, NASA’s programme scientist for the mission. If that is right, Phobos may carry pieces of ancient Mars itself. JAXA describes the mission’s goal as helping to “clarify the formation of the Martian sphere and the Solar System”.

Remote measurements from orbit have not settled the argument. A sample in a laboratory on Earth, where it can be dated and analysed atom by atom, could.

How will it collect the sample?

By landing twice on Phobos and using two different tools.

After reaching Mars in 2027, MMX will settle into what JAXA calls a quasi-satellite orbit, circling Mars while staying close to Phobos. It will map the moon and measure its gravity and make-up before choosing landing sites.

One tool, JAXA’s C-Sampler, is a corer on a robotic arm that will dig material from a few centimetres below the surface. The other, the P-Sampler built by Honeybee Robotics, a Blue Origin company, sits on a landing leg and fires bursts of nitrogen gas to blow loose surface grains into a container. The target is at least 10 grams of regolith, the dust and broken rock that covers the surface.

Before that, MMX will drop IDEFIX, a small four-wheeled rover built by France and Germany, which is expected to roam Phobos for about 100 days.

The MMX round trip

1. 20 Oct 2026Launch on H3 from Tanegashima, Japan
2. 2027Arrival at Mars; mapping of Phobos begins
3. From late 2028IDEFIX rover released; two landings to collect samples
4. 2030Flybys of Deimos, then departure for Earth
5. 2031Sample capsule lands in Australia

What is NASA’s part?

Two instruments, tracking, and a share of the prize. NASA supplied the P-Sampler and MEGANE, a spectrometer from the Johns Hopkins Applied Physics Laboratory that will measure gamma rays and neutrons from Phobos to work out which elements its surface contains. NASA’s Deep Space Network will help track the spacecraft, and NASA will receive about 15% of the returned sample, to be kept at the Johnson Space Center in Houston.

Has Japan done this before?

Yes, with asteroids. JAXA’s Hayabusa brought back grains from the asteroid Itokawa in 2010, and Hayabusa2 returned samples from the asteroid Ryugu in 2020. MMX applies that experience to a much harder target: a moon deep in the gravity of another planet, and a round trip of about five years.

Why does it matter?

It would be a first. No mission has yet brought material back from Mars or its moons, so MMX could deliver the first piece of the Martian system to Earth. “The samples from MMX will bring forth paradigm-shifting science on the Mars system for generations to come,” said Nicky Fox, who heads NASA’s Science Mission Directorate.