Japan launches MMX probe, aiming for Phobos samples by 2031

On Monday, October 19, at 19:41 UTC, an H3-24L rocket lifted off from pad LA-Y2 at the Tanegashima Space Center, carrying the Martian Moons eXploration probe, or MMX. The 63-metre rocket, the most powerful member of the Japanese H3 family, burns liquid hydrogen and liquid oxygen in two LE-9 engines and carries four solid boosters. About ten minutes and forty seconds after launch, the second stage and the spacecraft reached a parking orbit around Earth. At 1 hour 11 minutes into the flight, MMX separated from the rocket and began its trip toward Mars. JAXA had timed the launch for an ideal trajectory, one available in this year's window, which lasted until November 9.

The probe is heading for the two small moons of Mars. Phobos, the larger of them, measures 27 by 22 by 18 km, and it was chosen as the main target because its terrain is more varied than that of Deimos, which measures 15 by 12 by 11 km. Both moons were discovered in 1877, and earlier missions, including Europe's Mars Express, NASA's orbiters and the Emirati Hope probe, have already imaged them from a distance. What none of them has done is land on a moon and bring material back. The mission carries 13 science instruments, including a French infrared spectrometer and a NASA gamma-ray and neutron instrument built at Johns Hopkins University's Applied Physics Laboratory. Its cameras are also meant to show Mars in 4K and 8K resolution, a capability that has not yet been achieved at Mars.

Landing is the difficult part. Phobos has only 1/1700th of Earth's gravity, which is too weak to keep a spacecraft in a normal circular orbit. MMX will instead settle into a quasi-satellite orbit, moving so that it appears to circle the moon, which is slowly spiralling toward Mars. The IDEFIX rover, named after the dog in the French comic Asterix, will be released onto the surface. Interesting Engineering reports that it may bounce before it settles, and that soft soil or rocks could leave it stuck, though it can right itself after an upside-down landing. A corer that drills more than two centimetres into the ground and a pneumatic sampler, built by NASA and Honeybee Robotics, will collect material. Two touchdowns are planned for late 2028 or 2029.

What changes if it works? For planetary scientists, the answer is a set of samples that can be studied in laboratories on Earth, which could help test whether Phobos was made from debris or captured from elsewhere. The same material could also show how other moons formed. The timetable is long: MMX reaches Mars in 2027, spends roughly three years studying the moons, and is set to leave in November 2030, with the capsule expected in Australia in 2031. Japan is not new to this kind of return, since its earlier asteroid missions already brought material home. Phobos is harder, because its weak gravity leaves little to hold a lander down, so the 2028 and 2029 landings are where the plan will be tested.