Japan's MMX Probe Departs for Phobos, Aiming to Return the First Martian-System Samples to Earth

On Tuesday, Japan's Martian Moons eXploration probe, known as MMX, set out on a journey whose outbound leg alone will cover more than 500 million kilometers. At a press conference, Hiroshi Yamakawa, president of the Japan Aerospace Exploration Agency, said the long journey toward humanity's first sample returned from near Mars had begun.

The plan runs about five years in total. Roughly two years are given to the trips to the neighborhood of Mars and back, and about three years to exploration. The target is Phobos, one of Mars's two moons. It orbits at an altitude of about 6,000 kilometers and is small, with a diameter of 20 to 30 kilometers. MMX will land on it, collect samples, and bring them home so they can be analyzed to determine how the moon formed.

Two hypotheses lead the field. In the asteroid capture hypothesis, an asteroid flying past Mars was caught by the planet's gravity and became a moon. In the giant impact hypothesis, a celestial body struck Mars and the scattered debris gathered to form the moon. Kiyoshi Kuramoto, a Hokkaido University professor who oversees MMX's scientific research, said the chemical and mineral compositions implied by the two ideas are completely different, so analyzing the samples will definitively settle the question of origin.

The reading of the samples is fairly direct. If they are rich in water-bearing minerals and organic matter, as the samples Japan's Hayabusa2 brought back from the asteroid Ryugu in 2020 were, the capture hypothesis becomes the leading candidate. Because past missions have identified the main components of Mars's crust, finding the same substances in the Phobos samples would raise the likelihood of the giant impact explanation. Moons like Phobos are believed to have formed in large numbers early in the solar system's history, so its origin could offer a clue to that broader story. Yasuhiro Kawakatsu, MMX project manager at JAXA, said the mission may help scientists understand the mechanisms behind the solar system's formation.

The greatest challenge is the landing, scheduled for around early 2029. Phobos's gravity is only about one-2,000th of Earth's, which creates a risk that the probe could bounce off the surface on impact and tip over. To reduce that risk, the spacecraft carries four landing legs designed to absorb the shock. It also uses image-based navigation, which estimates the probe's position from images of Phobos taken in flight and automatically corrects its course. That technology was tested during the 2024 landing of the SLIM probe on Earth's moon, and JAXA drew on its Ryugu sample-return experience as well.

Precise course control is essential. Seiichiro Watanabe, a Nagoya University professor of planetary science, said the task is extremely difficult even compared with Hayabusa2. Japan's history here is mixed: the Nozomi Mars probe, launched in 1998, failed to reach the planet's orbit.

Scientists are also watching for Martian dust believed to have accumulated on Phobos. Organic molecules and water have already been confirmed on Mars, surface features strongly suggest large quantities of liquid water once existed there, and ice is believed to remain underground and on the surface. If MMX succeeds, humans will for the first time directly examine samples brought back from the Martian environment. Seiji Sugita, a University of Tokyo professor, said the likelihood of finding traces of life is higher than for any other sample. Last year, NASA said rock data sent by the Perseverance rover indicated a substance that can be produced by microbial metabolism.

Junichi Watanabe, a visiting professor at Kyoto Sangyo University, said the project might yield unexpected discoveries. The next test comes around early 2029, when the probe attempts its landing on Phobos.