How Do You Land on a Moon With Almost No Gravity?

Japan’s MMX mission aims to sample the Martian moon Phobos Japan's MMX spacecraft will fire its engines to slow down and enter orbit around Mars. Last August, looking upfrom the edge of an ancient river delta on Mars, thePerseverance rover’s electronic eye spied an oblong shadow crossing the distant, glowing Sun.Phobos, the Martian moon named for the Greek god of fear and panic, was transiting the solar disk. Too small to darken the pale, dust-tinged sky, but orbiting close enough to eclipse a significant fraction of the Sun, Phobos passed in less than a minute. NASA’s Perseverance rover captured Phobos passing in front of the Sun from the surface of Mars.ASU/MSSS/SSI/JPL-Caltech/NASA The tiny moon, with its murky origins, is an enigma, and one whose makeup and history could tell us much aboutMarsand the wider solar system. And now the internationalMartian Moons eXploration(MMX) mission, which successfullylaunched on 20 October, seeks to explore that mystery with an unprecedented series of landings on the unknown surface of Phobos. This mission brings unusual challenges—for example, designing an orbit to study a microgravity moon orbiting a much larger planet. Engineers also had to figure out how to assess and land on this low-gravity pile of loosely assembled dirt and rock, how a rover might rove across it, and how the spacecraft might take samples and return them to Earth. “I often say that landing on Phobos is like trying to land a balloon,” saysYasuhiro Kawakatsu, project manager of MMX at the Japan Aerospace Exploration Agency (JAXA). “It’s very difficult to drop a balloon to the ground and prevent it from tilting or bouncing.” Phobos is one of two small Martian moons; the other isDeimos, named after the Greek god of dread and terror. Their cosmic origins are unknown, though there are two prevailing theories. One holds that the moons were once asteroids from the belt between Mars and Jupiter, captured by the smaller planet’s gravity. In the other theory, Phobos and Deimos formed when a large celestial body slammed into Mars, sending debris into orbit. Some models suggest the pair are the last survivors from a cluster of satellites. The enormous Stickney crater, roughly 9 kilometers across, dominates this view of Phobos taken by NASA’s Mars Reconnaissance Orbiter.University of Arizona/JPL-Caltech/NASA Scientifically speaking, Phobos is considered a rubble pile, a loosely consolidated aggregate of rock and dust. Far too small to become spherical under its own mass, it’s shaped like a lumpy potato, roughly 27 kilometers in length, with only 1/1,800thof Earth’s gravity. On Phobos, a typical adult would weigh about as much as a tennis ball does on Earth. More than 10 years in the making, MMX launched on Japan’s flagship H3 rocket from Tanegashima spaceport on 20 October local time. It will arrive at Mars in 2027 carrying scientific payloads from several space agencies, includingNASA, and a new rover calledIdefixdeveloped by the French and German space agencies (CNES andDLR) in collaboration with JAXA. Engineers assembled MMX’s modules at Japan’s Tanegashima Space Center before the spacecraft’s October 2026 launch.JAXA MMX consists of three modules: a propulsion module to power its journey to Mars and apply the brakes to enter orbit around the red planet; an exploration module carrying the rover, sampling gear, and other scientific instruments; and a return module, attached to the exploration module, that harbors the capsule destined for a high-speed reentry into Earth’s atmosphere. After collecting samples on Phobos, the exploration and return modules will lift off together. The exploration module will then be jettisoned while the return module sets a course back home. If all goes according to plan, MMX will deliver a 10-gram sample to Earth in the second half of 2031. But the mission will face a series of daunting challenges to reach that end point, including navigating complex orbits around Phobos and landing on its surface while Mars periodically blocks communications with Earth. The MMX’s modular design includes the propulsion module [left] and the exploration and return modules [right].Kyodo News/Getty Images “It’s the most complex mission I have ever seen,” saysPatrick Michel, director of research at CNRS (the French National Center for Scientific Research) and co-principal investigator on Idefix. An H3 rocket carried MMX into space on 20 October 2026, beginning the mission’s five-year journey to Phobos and back.The Asahi Shimbun/Getty Images Once MMX reached low Earth orbit, its propulsion module’s engine fired, kicking off a nearly year-long deep-space cruise to Mars. On arrival, the spacecraft will need to fire its engines in an extended, carefully timed burn to slow down enough to be captured by Mars gravity. The propulsion module will then be jettisoned, leaving the exploration and return modules to orbit Mars. “If something doesn’t go according to plan, MMX will fly by Mars and get catapulted into the solar system,” saysNicola Baresi, an astrodynamicist who coauthoreda paperon specially designed orbits for MMX. From that initial orbit, MMX will transition into a specially designed quasi-satellite orbit that allows it to keep pace with the small moon but also move around it. Since Phobos is too small to orbit in the conventional sense, the spacecraft will orbit Mars alongside the moon, following a clever egg-shaped path that slings MMX alternately ahead of and behind the moon. “When you’re closer to Mars, you’re accelerating with respect to Phobos, and when you’re farther from Mars, you’re decelerating. So you essentially go back and forth,” explains Baresi. Martian ManeuversTo reach Phobos, MMX will execute three major maneuvers: MOI-1 will put the spacecraft into orbit around Mars, MOI-2 will transfer it toward Phobos, and MOI-3 will place it into its final orbit alongside the moon.Chris Philpot To reach Phobos, MMX will execute three major maneuvers: MOI-1 will put the spacecraft into orbit around Mars, MOI-2 will transfer it toward Phobos, and MOI-3 will place it into its final orbit alongside the moon. The best existing images of Phobos, from spacecraft such as NASA’sMars Reconnaissance Orbiterand ESA’sMars Express, have resolutions of only a few meters per pixel. That’s larger than Idefix, the mission’s rover, itself about the size of a microwave oven. To map the surface in greater detail, MMX will loop in front of and behind Phobos at distances ranging from 200 kilometers to less than 20 km from the surface. During its two-year remote sensing campaign, the exploration module will use several cameras and a laser altimeter to map the surface in high resolution and look for candidate landing sites. When Mars passes behind the Sun as seen from the Earth, causing a roughly two-week communications blackout, MMX will retreat into a larger, stable orbit. After communications resume, MMX will prepare to send its first visitor down to the surface. Before the exploration and connected return modules attempt to land, Kawakatsu says, MMX will send a scout that in Japanese is called “the first penguin,” leaping into unknown waters. For this mission, the first to leap will be the Idefix rover, named for the canine companion to Obelix in the French comics seriesAsterix. The exploration module will release the rover during a low-altitude pass of between 40 and 100 meters above Phobos’s surface in 2028 or early 2029. MMX will use its TENGOO camera [left] andLIDARlaser altimeter [right] to map Phobos’s terrain.JAXA It will be a tense moment that calls back to 2005, during Japan’sHayabusa missionto explore the asteroid Itokawa. In a similarly designed drop, the MINERVA rover was released from an orbiter, but erroneously at 200 meters instead of the intended 70 meters. The rover missed the asteroid and, without propulsion, drifted off into deep space. Idefix will get a slight nudge out of the exploration module toward its negligible-gravity target, fr