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Japan is preparing to send a robotic mission to Mars's moons, with the aim of returning material from Phobos to Earth. The Japan Aerospace Exploration Agency (JAXA) has scheduled Martian Moons eXploration (MMX) for launch on an H3 rocket from Tanegashima on 20 October 2026. The mission will examine both Phobos and Deimos, but collect its sample from Phobos. Scientists want to understand whether these moons began as captured asteroids or formed from debris after a collision involving Mars. Bringing grains into laboratories would allow tests beyond what distant images alone can reveal. As of this edition, launch and sample return remain planned steps, not completed achievements. The intended return to Earth is in 2031.
Two small moons with an unresolved history
Phobos and Deimos are not miniature versions of Earth's round Moon. Both are small, irregular bodies with cratered surfaces. Phobos is the larger and closer companion; Deimos travels farther from Mars. Their appearance offers clues about their history, but it does not settle how they came to occupy their present orbits. That uncertainty is the central scientific question behind MMX.
The National Aeronautics and Space Administration (NASA) gives Phobos's unequal dimensions as approximately 27 by 22 by 18 kilometres. It completes roughly three circuits of Mars each day. Its surface bears a particularly large impact feature, Stickney crater, alongside smaller craters and grooves. A description using one exact diameter would conceal its irregular shape rather than make the moon easier to understand.
Deimos measures approximately 15 by 12 by 11 kilometres and takes about 30 hours to orbit Mars. Its surface looks smoother because loose material partly fills its craters. Both moons therefore preserve a history of impacts and surface change. Observing them together can help researchers compare their materials and histories, even though the mission's physical sample will come from only one.
Capture and collision offer different starting points
One explanation is that the moons were once separate asteroids caught by Mars's gravity. Another is that a large impact threw material into orbit around the young planet. That debris could then have gathered into moons. These are competing explanations, not two events that the mission has already proved occurred. Distinguishing them requires evidence about the material itself as well as its present surroundings.
A captured body could preserve information about material from elsewhere in the Solar System. Material assembled after an impact could instead carry a record connected with Mars and the colliding object. The moons may also contain particles thrown from Mars by later impacts. That possibility makes interpretation more demanding: a grain found on Phobos does not automatically represent the material from which the entire moon formed.
Why observations come before a landing
MMX is designed to spend time examining Phobos before attempting collection. JAXA describes an observing path called a quasi-satellite orbit, or QSO. The spacecraft follows a path around Mars that keeps it near Phobos, allowing repeated close observations. It is not simply a conventional circular orbit around an isolated small moon. Mapping the surface will help identify and assess candidate landing areas.
The loose surface layer of dust and broken rock is called regolith. Its behaviour matters to a vehicle touching down in extremely weak gravity. The planned Idefix rover will examine surface conditions before the main spacecraft lands. Developed by the German Aerospace Center and the French space agency, it will study the surface and help prepare the later collection operation. It is a small robot, not a crewed vehicle.
Two collection methods examine different material
The mission aims to return at least ten grams from Phobos. That is a collection objective, not an amount already obtained. One device will take a shallow core, while a complementary pneumatic sampler will use pressurised nitrogen to move loose particles into a container. A core and material lifted from the surface provide different kinds of sample, helping the mission avoid relying on only one collection method.
Measurements made before collection will supply the samples' wider context. NASA's Mars-moon Exploration with Gamma rays and Neutrons instrument, known as MEGANE, will measure radiation emerging from Phobos. Those signals help identify chemical elements in its surface material. This is different from photographing the moon's interior. Combining such measurements with surface images and later laboratory work can connect individual grains with the body from which they came.
A spacecraft designed for more than an outward journey
The MMX spacecraft has propulsion, exploration and return modules with different jobs. Reaching Mars is only the beginning: it must also conduct observations, approach Phobos, gather material and preserve it for the return. Solar panels provide electrical power, while communications equipment connects the spacecraft with ground stations. Separating these functions explains why a sample-return mission is more complex than sending a camera past a target.
The announced sequence anticipates arrival near Mars in 2027, an extended period of investigation, departure in 2030 and delivery in 2031. A capsule will separate for atmospheric entry, with recovery planned in Australia before transfer to Japan. These are mission plans, dependent on successful operations along the way. The 20 October Japanese launch date also corresponds to 19 October in some western time zones; the dates describe the same scheduled launch.
Small samples can answer questions that large pictures cannot
Earth-based laboratories can examine tiny portions of material with instruments that cannot all be carried on one spacecraft. Researchers can study composition, compare minerals and revisit preserved material as methods improve. The European Space Agency is contributing communications support and scientific participation, alongside other partners. The project therefore combines measurements taken near the moons with a longer programme of research after the intended return.
JAXA builds on its experience of asteroid sample return through Hayabusa and Hayabusa2. Extending that work to the Mars system would test a more demanding round trip. The ambition is not evidence that life has been found, or that a human base has been approved. The immediate task is to obtain reliable material and observations that can discriminate between explanations of the moons' origins.
Conclusion
MMX approaches a large question about planetary history through a small physical sample. Its value lies in linking close observations, a carefully understood collection site and detailed laboratory analysis. The forthcoming launch is one stage in that chain, not the scientific answer itself. If the planned return succeeds, material from Phobos could help explain both the moon's own beginnings and the wider history of Mars.