Last August, NASA’s Perseverance rover recorded a swift solar transit by Phobos, a fleeting reminder of the enigmatic Martian moon that now sits at the center of the MMX mission to Phobos. Launched on 20 October, the international effort aims to conduct close orbits, attempt landings on the moon’s little-known surface, deploy a rover, and return samples to Earth.
The project faces unusual engineering constraints. Mission designers had to craft orbits that track a tiny body moving around Mars, develop techniques to assess and land on an ultralow-gravity rubble pile, enable a rover to traverse it, and collect material for return.
“Landing on Phobos is like trying to land a balloon,” said Yasuhiro Kawakatsu, the MMX project manager at the Japan Aerospace Exploration Agency. “It is very difficult to drop a balloon to the ground and prevent it from tilting or bouncing.”
Why the MMX mission to Phobos targets a unique world
Phobos and its sibling Deimos have uncertain origins. Leading theories say they are either captured asteroids from the main belt or remnants of debris cast into orbit after a large impact on Mars.
Some models suggest they are the last survivors of a once larger group of satellites.
Scientifically, Phobos is a loosely bound collection of rock and dust. Far too small to assume a spherical shape, it stretches roughly 27 kilometers end to end and has about 1/1,800th of Earth’s gravity.
On its surface, a person would weigh about as much as a tennis ball.
More than a decade in development, MMX lifted off from Tanegashima spaceport on Japan’s H3 rocket on 20 October local time. Arrival at Mars is slated for 2027 with instruments from multiple partners, including NASA, and a rover named Idefix developed by CNES and DLR with JAXA.
MMX comprises three main elements. A propulsion module handles the cruise and Mars orbit insertion.
An exploration module carries the rover, sampling tools, and sensors. A return module, attached to the exploration module, houses the sample capsule for Earth reentry.
After sampling, the exploration and return modules will depart Phobos together, then separate so the return module can head home.
If the timeline holds, about 10 grams of material could reach Earth in the second half of 2031. To get there, the team must steer through complex orbits near Phobos and land on a surface while enduring periodic communications blackouts when Mars blocks signals to Earth.
“It is the most complex mission I have ever seen,” said Patrick Michel, a director of research at CNRS and co–principal investigator on Idefix.
How MMX will orbit Phobos
After reaching low Earth orbit, MMX’s propulsion module sets up a nearly year-long cruise to Mars. On arrival, a lengthy engine burn will slow the craft for capture by Martian gravity.
The propulsion module will then separate, leaving the exploration and return modules to orbit Mars.
“If something does not go according to plan, MMX will fly by Mars and get catapulted into the solar system,” said astrodynamicist Nicola Baresi, who coauthored work on the mission’s tailored trajectories.
From initial orbit, the spacecraft will transition into a quasi-satellite orbit that keeps pace with Phobos while looping around it. Because Phobos is too small for conventional orbiting, MMX will orbit Mars in an elongated path that carries it alternately ahead of and behind the moon.
“When you are closer to Mars, you accelerate with respect to Phobos, and when you are farther, you decelerate, so you go back and forth,” Baresi explained.
Current imagery of Phobos offers only meter-scale resolution, coarser than the microwave-oven-sized Idefix rover. MMX will fly passes ranging from roughly 200 kilometers down to under 20 kilometers to map the surface.
Over a two-year remote sensing campaign, cameras and a laser altimeter will refine landing candidates. During roughly two-week communication outages when Mars is behind the Sun, the spacecraft will retreat to a safer, higher orbit before resuming close operations.
Idefix rover: the first penguin
Before attempting to land the main spacecraft, MMX will release the Idefix rover, referred to as the “first penguin” in Japanese for its leap into the unknown. The drop is expected from 40 to 100 meters above the surface in 2028 or early 2029.
The maneuver recalls 2005, when Japan’s Hayabusa mission released its MINERVA rover at the wrong altitude and lost it to deep space. Idefix will receive a gentle push toward Phobos, likely tumbling and touching down at under 1 meter per second.
It will land without airbags or legs, relying on padding on its solar panels and Phobos’s weak gravity for a soft arrival.
“It will bounce probably several times,” said Stephan Ulamec, Idefix’s co–principal investigator at DLR. After settling, four legs will deploy to right the rover automatically, followed by solar panel deployment for power.
Idefix will crawl at up to about 1 millimeter per second using metal wheels with paddle-like blades for traction in microgravity. “If we were to drive our rover faster, then it would actually be more like we were swimming through instead of driving across the surface,” said Cecily Sunday, a doctoral researcher involved with testing the rover’s WheelCams at ISAE-Supaéro in Toulouse.
The two WheelCams will monitor wheel-ground interactions during a planned 100-Earth-day primary mission, helping researchers infer regolith grain size, cohesion, and porosity.
Teams have tested the nine-bladed wheels on Phobos and lunar simulants, glass beads, quartz sand, and clay pebbles to correlate images with material properties. Sunday said careful Earth testing at very slow speeds can approximate the rover’s behavior on Phobos despite the gravity difference.
The rover’s close-up observations will bridge the gap between orbital sensing and laboratory analysis of returned samples. Findings will also guide selection of the landing zone for the main spacecraft.
While NASA’s OSIRIS-REx sank into the surface of asteroid Bennu, Ulamec said Idefix is unlikely to bury itself deeply on Phobos because the two bodies differ significantly.
How MMX will land and collect samples
After Idefix investigates the terrain, the exploration module will descend to roughly 10 meters above the surface and then drop the final distance in free fall for a brief sampling campaign lasting up to two hours. With minimal gravity, even slight sideways motion could cause tipping or a bounce, so an upright landing will be critical.
Drawing on heritage from the Hayabusa missions to Itokawa and Ryugu, MMX will carry two sampling systems for redundancy and scientific breadth. A JAXA-built coring sampler on a robotic arm aims to penetrate at least 2 centimeters below the weathered surface.
“That by itself is a challenge because we have never done it,” Michel said, noting unknowns such as obstructing boulders.
Subsurface material is prized because radiation and micrometeorites have altered the topmost layer. JAXA targets at least 10 grams of sample.
A second device, a NASA-provided pneumatic sampler mounted on a lander leg and built by Honeybee Robotics, will inject nitrogen gas to stir up grains for capture, a method less sensitive to surface hardness.
“If the surface of Phobos proves to be very hard, [the corer] may struggle. [The pneumatic sampler] would not experience this issue,” Kawakatsu said.
Scientists also hope to find a small fraction of material launched off Mars by ancient impacts and later deposited on Phobos. Michel estimated that the return could include a few hundred Martian grains, distinguishable by oxygen isotopes.
Planetary protection rules for direct Mars sample return do not apply here. An expert panel concluded that any microbes ejected from Mars would not survive impact, ejection, and long exposure to radiation en route to Phobos.
Samples gathered by the robotic arm will be transferred to storage, then sealed in a vacuum capsule inside the return module before liftoff.
Following departure, the exploration module will continue observing Phobos and make several flybys of Deimos to study its surface, structure, and composition. In November 2030, the mission is scheduled to begin its return journey.
The return module will separate and carry the sealed samples back to Earth.
In 2031, a 60-centimeter-wide capsule is slated to streak into Earth’s atmosphere at about 12 kilometers per second and land in Australia. JAXA’s Institute of Space and Astronautical Science will curate the samples and share them with partners for analysis.
“We have macroscale measurements from the spacecraft, microscale measurements with the rover, and then sample analysis,” Michel said. “So we can relate the three kinds of scales together because we have all of them.”
Kawakatsu said MMX seeks to explain the Mars system and the processes that shaped the inner solar system. He added that terrestrial planets likely formed dry, with water and organics delivered inward from the outer regions.
Mars, positioned at the boundary between inner and outer solar system zones, makes its moons key witnesses to how potentially habitable worlds evolved.
Insights from returned samples could complement other work on how life and planetary environments change over immense timescales, such as research into dinosaur blood molecules found preserved after 115 million years.