Japan's new H3-24L rocket launches MMX probe to Mars' moons

On Monday 19 October, at 19:41 UTC, a rocket lifted off from pad LA-Y2 at the Tanegashima Space Center in Japan, carrying the Martian Moons eXploration probe, MMX, toward Mars. It was the maiden flight of a new configuration, the H3-24L, of Japan's H3 rocket.

The H3 is a two-stage rocket built by Mitsubishi Heavy Industries, burning liquid hydrogen and liquid oxygen in two cryogenic stages. Its first stage uses two or three LE-9 engines and can add zero, two or four SRB-3 solid boosters. The H3-24L's designation describes its setup: two first-stage engines, four boosters and a long payload fairing, the nose cone that protects the probe during ascent. Monday's flight was the third H3 mission of 2026.

After the probe separated from the rocket, MMX will spend about a year in transit before reaching Mars. It will then enter a quasi-stationary orbit around Phobos, a path that lets it stay close to the moon for an extended period because the moons' masses are so low. JAXA will study Phobos's topography, structure, composition and gravity, and MMX carries instruments including a gamma-ray and neutron spectrometer and a laser ranging system, known as LIDAR. The French-German rover IDEFIX, just under 23.5 kg, will be released 40 to 100 m above the surface. After landing, the probe is designed to collect about 10 g of soil, a sample that the mission will bring back to Earth.

The project is led by JAXA, with NASA, France's CNES, Germany's DLR and the European Space Agency, ESA, as partners. After a stay at Phobos, MMX will depart and observe Deimos, the second moon. The spacecraft is due to leave Mars in 2030 for a roughly yearlong return, and the samples are scheduled to land in Australia in 2031.

For researchers, the change is concrete: the soil will be studied in laboratories on Earth rather than only read from orbit and the surface by instruments, which is a step beyond remote observation. For JAXA, the next H3 launch is expected in the first half of 2027. The return of the samples depends on the journey from Mars in 2030, so the scientific results will arrive in 2031 at the earliest, once the capsule lands.