MMX Mission to the Moons of Mars: What NASA Adds to Japan's Phobos Sample Return

What NASA is contributing

MMX is led by the Japan Aerospace Exploration Agency (JAXA), with NASA as a partner. NASA's main contribution on board is MEGANE, a gamma-ray and neutron spectrometer. Cosmic rays constantly strike Phobos, and the surface answers with gamma rays and neutrons whose energies depend on which elements are present, so measuring them from orbit reveals the surface's make-up.

NASA also supplied the P-Sampler, a technology demonstrator mounted on one of the landing legs; it uses bursts of high-pressure nitrogen to blow surface material into a container. JAXA's own C-Sampler collects cores from several centimetres below the surface. The Deep Space Network will help with communications and navigation, and about 15% of the returned sample will be curated at NASA's Johnson Space Center in Houston.

Captured asteroids or debris from an impact?

Phobos measures about 27 × 22 × 18 km and orbits about 6,000 km above the Martian surface, circling the planet three times a day. Deimos, about 15 × 12 × 11 km, circles Mars every 30 hours. Both are dark, and the way they reflect light resembles that of carbonaceous asteroids. That resemblance supports the idea that Mars captured them, although NASA notes that some evidence contradicts it.

The rival idea is a giant impact: a large body struck Mars and threw material into orbit, where it coalesced into the moons. JAXA describes the two as competing theories. A third proposal, published in 2024 by researchers at NASA and Durham University, is that Mars's gravity tore apart a passing asteroid, whose fragments collided and ground one another down into a disc from which the moons formed.

Carbonaceous asteroids can carry water and the substances that build atmospheres, so captured moons would be samples of the kind of material that may have delivered water to the inner planets, Earth among them. Moons made from impact debris would instead preserve a piece of early Mars.

What ten grams of Phobos can reveal

MEGANE's map of elements across Phobos will help choose where to land. A laboratory can analyse the chemistry and mineralogy of individual grains. Composition is the key test between a captured asteroid and a moon born in an impact.

Because Phobos orbits so close to Mars, JAXA estimates that it has collected more than a hundred times as much material blasted off the Martian surface as Deimos has. The sample may therefore also contain small pieces of Mars, delivered long after the moon formed, whichever way it formed.

Mission timeline, and how to see the moons yourself

• 19 October 2026, 19:41 UTC (planned): launch on H3 flight No. 10; backup launch dates run until 7 November, Japan date.
• August 2027: arrival at Mars, then a close orbit around Phobos to map it and choose landing sites.
• No earlier than December 2028: sample collection. A small rover lands first, then the main spacecraft.
• 2030: after a flyby to observe Deimos, departure from Mars for a journey of about a year.
• 2031: a capsule carrying the sample will enter Earth's atmosphere and be recovered in Australia.

At the next opposition, in the second half of February 2027, Mars will shine in Leo at magnitude −1.3, Phobos at magnitude 12.1 and Deimos at 13.2. Then Phobos never strays more than about 19 arcseconds from the planet's centre, and Deimos no more than about 48 arcseconds. Without a telescope you will see only Mars: at magnitudes 12 to 13, the moons are far beyond the naked eye and binoculars. Faintness matters less than the glare of a planet more than 13 magnitudes brighter: seeing them takes high magnification, steady air and ideally a way to block it. Mars, about 15° north of the celestial equator, will be well placed for observers in both hemispheres.