"Japan's MMX Probe: A Mars Moon Sample Is Coming Home in 2031"
On October 19, Japan launched a spacecraft toward a question that has been sitting unanswered for nearly 150 years. The Japan Aerospace Exploration Agency's Martian Moons eXploration mission, MMX, lifted off from Tanegashima Space Center aboard the H3 rocket's tenth flight, and about 71 minutes later the spacecraft separated cleanly onto its planned path to Mars. Its destination is Phobos, the larger and nearer of Mars's two small moons, and its cargo, when it eventually comes home, will be at least 10 grams of that moon's soil. It will be the first sample ever returned from a moon of Mars.
The question is a simple one to state and a hard one to settle: what is Phobos? It is a lumpy, potato-shaped rock about 22 to 27 kilometers across, circling Mars at a distance of only about 6,000 kilometers, so close that it rises and sets twice in a Martian day. One story says it is a captured asteroid, a wanderer that drifted too close to Mars and never left. The other says it is debris from a giant impact on Mars itself, blasted into orbit and reassembled there, which would make it a chip off the Red Planet. From Earth, telescopes cannot tell these stories apart. A handful of soil in a laboratory can, because the two origins leave different chemical fingerprints: the mix of minerals, the water content, the traces of organic compounds from the early Solar System.
Getting that handful is the daring part. Phobos has roughly one-thousandth of Earth's surface gravity, so weak that an ordinary orbit around the moon itself is barely stable; Mars's gravity keeps interfering. MMX will not so much orbit Phobos as hover around it, flying a quasi-satellite orbit that circles Mars while staying close to the moon, stepping down in stages to map the surface in ever finer detail before choosing where to land. In late 2028 or early 2029, a small rover called IDEFIX, built by the French and German space agencies and weighing less than 30 kilograms, will roll onto Phobos first to test how the surface behaves. Then the main spacecraft will touch down twice, in 2029 and 2030, collecting material with two tools: a coring sampler that digs deeper than two centimeters, and a pneumatic sampler that fires pressurized gas to push soil into a container. After flybys of the smaller moon Deimos, the spacecraft will leave Mars in November 2030, and the sample capsule is scheduled to land in Australia in 2031.
I like the cast list of this mission almost as much as the science. Japan flies it, France and Germany built the rover, NASA supplied a neutron and gamma-ray spectrometer, and Europe provides deep-space communications. Nobody's flag is big enough to carry a five-year interplanetary mission alone anymore, and MMX does not pretend otherwise. It is also a mission that has already learned patience: announced in 2015, developed from 2020, delayed from 2024 after the H3 rocket's maiden flight failed. The rocket that finally carried it was flying its tenth flight, and its first interplanetary one.
Ten grams sounds like almost nothing, and that is part of the point. Japan's Hayabusa2 brought back about five grams from the asteroid Ryugu, and those few grams rewrote parts of the story of how water and organic material reached the early Earth. Samples are not souvenirs; they are evidence that can be re-examined for decades, with instruments that do not exist yet, by researchers who were not born when the spacecraft launched. The spacecraft now coasting toward Mars is carrying a question, not an answer, and the answer will arrive in a capsule in the Australian desert, five years from now.
The next checkpoint comes much sooner. If the cruise goes as planned, MMX will arrive at Mars in August 2027 and fire its engines to be captured into Mars orbit. That single maneuver is the moment this long story either continues or ends, and it is worth watching precisely because it will look so undramatic from Earth: a spacecraft, very far away, slowing down by just enough.