Japan’s MMX will land on Mars’s moon Phobos and bring 10 grams back to Earth

Japan is about to try something no space agency has ever pulled off: landing on a moon of Mars, scooping up at least ten grams of its surface and flying that handful of dust back to Earth. The spacecraft is called MMX, short for Martian Moons eXploration, and its target is Phobos, a lumpy, potato-shaped moon less than thirty kilometres long that circles Mars closer than any other known moon circles its planet. If the plan works, laboratories on Earth will hold the first physical sample ever taken from the Mars system.

That matters because nobody knows where Mars got its two moons. Phobos and its smaller sibling Deimos could be asteroids that strayed too close and were captured, or they could be rubble blasted into orbit when something huge slammed into Mars. Each answer tells a different story about the early solar system, and only a sample analysed in a lab can settle it.

The mission is led by the Japan Aerospace Exploration Agency (JAXA), the agency behind the Hayabusa probes that brought back grains of two asteroids. NASA, the French space agency CNES, the German Aerospace Center DLR and the European Space Agency are partners. JAXA calls it the largest international planetary mission Japan has ever led.

“The Martian moon Phobos remains an unexplored frontier for humankind,” MMX project manager Kawakatsu Yasuhiro said in a JAXA statement. “By bringing home samples from Phobos, we hope to explore how the martian moons were formed, how the Mars sphere evolved, and how the water and organic materials found on the terrestrial planets originated.”

How MMX plans to grab a piece of a moon

The spacecraft is about 9 metres long and is built from three stacked modules: a propulsion module to brake into orbit around Mars, an exploration module carrying the landing legs and sampling gear, and a return module that will carry the sample home. It carries 11 science instruments, seven on the main spacecraft and four on a small rover, according to ESA.

Phobos is too small to orbit in the usual way. Its gravity is less than a thousandth of Earth’s, so MMX will fly what engineers call a quasi-satellite orbit: technically it circles Mars, but it moves in step with Phobos and appears to loop around the moon. From there, a telescopic camera called TENGOO can pick out features about 40 centimetres across, while a wide-angle camera called OROCHI, a laser altimeter, a French infrared spectrometer named MIRS and a NASA gamma-ray and neutron instrument called MEGANE map the terrain, minerals and chemical elements.

Before the main spacecraft risks a landing, it will drop a scout. Idefix, a 25-kilogram rover built by CNES and DLR, will fall from 40 to 100 metres above the surface, right itself and roll around for about 100 days. On Phobos it will weigh about as much as 15 grams does on Earth, roughly a few sheets of paper.

Then comes the touchdown. The exploration module will stay on the surface for around two and a half hours and may land a second time at another site. Its main tool is a corer on a robotic arm that bites about 2 centimetres down, deep enough to reach material that sunlight and solar radiation have not chemically altered. A NASA-supplied backup blasts surface grains into a container with a jet of nitrogen gas, the same principle OSIRIS-REx used at the asteroid Bennu, although in Phobos’s feeble gravity that jet carries a small risk of tipping the spacecraft.

Why a moon of Mars, and not Mars itself

The question of where Phobos came from is really a question about how planets get their water. If Phobos is a captured asteroid, it should look like the dark, carbon-rich asteroids that may have delivered water and organic molecules to the young rocky planets. If it formed from debris after a giant impact, it should be made largely of Martian crust.

“The key to this mystery lies in the martian moons, which closely resemble carbonaceous asteroids, the very celestial bodies capable of carrying the ingredients for an atmosphere and water,” said principal investigator Kuramoto Kiyoshi. Phobos does look like a captured asteroid from the outside, but its orbit, nearly circular and close to Mars’s equator, is hard to explain if it was simply snatched from passing traffic.

There is a bonus. Asteroid impacts on Mars have thrown Martian soil into space for billions of years, and Phobos orbits so close that it is thought to sweep up some of that debris. A Phobos sample could therefore contain grains of Mars itself. With the NASA and ESA Mars Sample Return programme effectively cancelled after budget pressures, MMX may deliver the first Martian material to reach Earth by design rather than as a meteorite.

A target that has defeated every visitor so far

The scale of the challenge shows in the history. The Soviet Union lost both of its Phobos probes before either reached the moon, and Russia’s later sample-return attempt, Phobos-Grunt, never escaped Earth orbit. No spacecraft has ever touched Phobos.

Compared with what came before, 10 grams is a large haul. JAXA’s Hayabusa returned only tiny grains from asteroid Itokawa, and Hayabusa2 brought back about 5.4 grams from asteroid Ryugu. MMX would also become the first spacecraft to land on and lift off from a moon of another planet, and the first to return to Earth after a round trip to the Mars system.

The mission carries a few extras too. Two ultra-high-definition cameras built with the Japanese broadcaster NHK will film Mars and its moons in 8K, and on the way home MMX will fly past Deimos, the smaller and even less explored of the pair.

What MMX cannot promise

Sample-return missions are long chains of things that must all go right, and MMX has several weak links. The rocket is the first. Japan’s H3 lost a navigation satellite on a failed flight before returning to service with a successful launch this summer, so a single launch carries the entire mission.

Landing is the second. Nobody has measured how firm or loose Phobos’s surface is, which is exactly why Idefix goes first. The backup gas sampler could tip the spacecraft over, and a mission that lands only once will sample just one spot on a moon whose colour and texture vary across its face.

Finally, even a perfect sample may not end the debate. Grains of Mars mixed into the soil could make Phobos look more Martian than it is, and a moon assembled from impact debris might still contain asteroid material. Scientists expect the answer to come from isotopes and minerals measured over years, not from a single headline result.

Common questions about MMX and Phobos

When does MMX launch?

MMX is scheduled to lift off on an H3 rocket from the Tanegashima Space Center in southern Japan at 04:41 Japan time on 20 October 2026, which is 19:41 UTC and 21:41 CEST on 19 October. Launch dates can slip for weather or technical checks.

How long does MMX take to reach Mars?

About a year. The spacecraft is due to enter orbit around Mars in August 2027 and to begin its quasi-satellite orbit around Phobos the following month.

When will the Phobos sample reach Earth?

The return capsule is planned to land by parachute in Australia in 2031, after which it will be taken to Japan for analysis. The mission is designed to bring back at least 10 grams of material.

Why is Phobos important?

Phobos may be a captured asteroid or a fragment of Mars thrown into space by an ancient impact. Its composition could reveal which, and help explain how water and organic molecules reached Earth and Mars. It may also hold grains of Martian soil.

Has anyone landed on Phobos before?

No. Two Soviet probes were lost in 1988 and 1989 before reaching the moon, and Russia’s Phobos-Grunt failed in Earth orbit in 2011. MMX would be the first spacecraft to touch its surface.

What happens next

If the launch goes ahead, MMX arrives at Mars in August 2027 and reaches its orbit around Phobos in September 2027. Idefix is due to be released in early 2029, the main spacecraft is scheduled to touch down in 2030, and the sample capsule is planned to land in Australia in 2031. ESA’s Mars Express orbiter, confirmed to operate until at least 2029, may support the observations from above.

Tags: astronomy, Mars, JAXA, MMX, Phobos, sample return