What is Japan's MMX mission, and why fetch a piece of Mars's moon Phobos?

Because those two small moons may hold the answer to where they came from. Mars's moons might be captured asteroids, or they might be debris from a giant impact on Mars itself, and both ideas fit the data we have. A sample from Phobos, Mars's largest moon, could tell them apart. That is the goal of Japan's Martian Moons eXploration probe, MMX.

As of 21 October 2026, MMX is on its way. JAXA launched it on an H3 rocket from Tanegashima at 04:41 Japan time on 20 October 2026 (19 October in UTC); the spacecraft separated about 71 minutes after liftoff, and on 21 October JAXA reported it in good health. According to the plan, it enters orbit around Mars in August 2027, lands once or twice on Phobos in 2030, collects at least 10 grams of surface material, and sends a capsule home in 2030 to arrive in 2031.

Phobos was chosen over Deimos for three reasons: its surface looks more varied, it probably carries material thrown off Mars in early impacts, and it has been imaged more. The samples are collected two ways, by a coring sampler for regolith deeper than 2 centimetres and by a pneumatic sampler for samples from the surface.

MMX also carries a small rover called Idefix, named after Obelix's dog in the Asterix comics. It has to show that wheels work on a body with such low gravity. Not everything has gone smoothly in the past: Russia's Fobos-Grunt, launched in 2011 with the same goal, was stranded in low Earth orbit when its Mars burns failed.