JAXA confirmed that its MMX spacecraft separated from its H3 rocket about 71 minutes after liftoff on October 20, 2026, Japan time, and entered its planned orbit.
What Phobos can reveal
Scientists do not know how Phobos formed. Its dark surface resembles carbon-rich asteroids, supporting the idea that Mars captured it. But Phobos and Deimos move in near-circular orbits aligned with Mars’s equator, which better fits the idea that debris from a large impact formed the moons. A third possibility comes from NASA researchers’ computer simulations: Mars’s gravity may have torn apart a passing asteroid, with some fragments later forming a disk and moons. These explanations remain unconfirmed.
A closer look at the moon
MMX is expected to study Phobos’s shape, surface, composition and gravity, and observe Deimos. Its MEGANE instrument, provided by NASA, will measure gamma rays and neutrons from Phobos to help determine its elemental makeup. JAXA plans to collect at least 10 grams of material. A coring sampler is designed to reach below the surface, while a NASA-contributed pneumatic sampler will use nitrogen bursts to gather loose material.
The sample may also contain rocks blasted from Mars by impacts. JAXA researchers’ simulations suggest this material could be less changed by impacts than Martian meteorites found on Earth. Identifiable Martian grains could offer another record of the planet’s geological past, but their presence is not guaranteed. MMX’s main sample is from Phobos, not a direct landing on Mars.
The challenges of collecting a sample
MMX builds on Japan’s Hayabusa and Hayabusa2 sample-return missions, but operating near Mars’s moons is harder. The project study began in 2015, and the launch was delayed from an earlier 2024 target after the first H3 rocket flight failed in 2023 and JAXA decided the spacecraft needed more ground checks.
Phobos’s gravity is roughly one two-thousandth of Earth’s, making a safe landing difficult: the spacecraft must avoid bouncing or losing contact with the surface. The French-German Idefix rover is intended to examine the terrain and help establish conditions for sampling. It will also test rover movement in the moon’s very weak gravity. ESA describes landing, manoeuvring and sampling there as central engineering challenges.
The long road back to Earth
Phobos’s soil could contain material from different sources, so researchers will compare samples with MMX’s orbital measurements and observations of the landing sites. The results could support one formation theory, reveal a more complicated history or show that the moons’ origins cannot be explained by a single process. They could also help scientists understand how water and organic compounds moved through the early solar system and how Mars evolved.
JAXA plans for MMX to reach Mars in August 2027, survey the moons, deploy Idefix around 2028–29 and collect samples before leaving the Mars system. The return capsule is targeted for 2031, with recovery planned in South Australia. All dates after launch remain plans, and landing and returning the samples are still ahead.