In Simple Terms
Japan is sending a spacecraft to Mars’s two small moons to find out how they formed. It will explore Phobos, send a small rover to its surface, and bring a sample back to Earth in 2031. The findings may reveal whether the moons are captured asteroids or the remains of a violent event in space.
Two Tiny Worlds, One Big Mystery
Mars has two irregularly shaped moons, discovered in 1877. Spacecraft have flown past and photographed them for decades, but their origins remain a puzzle. Phobos, the larger moon, is a lumpy body about 26 kilometers long, while Deimos measures roughly 16 kilometers. They are small compared with planets, but understanding them could shed important light on the history of Mars and the objects around it.
Japan’s space agency, JAXA, is leading the mission, known as MMX. The journey to Mars is expected to take about one year. Once there, the spacecraft will enter orbit around the planet and make close approaches to Phobos so it can study the moon. It will measure the moon’s shape and map its surface to identify possible landing sites, while also occasionally observing Deimos from farther away.
Finding a safe landing site is no minor detail. Phobos has no smooth, level terrain, and its gravity is extremely weak, so landing and working with its soil will require careful planning. Once engineers on Earth have chosen a suitable location, the spacecraft will release a small rover to explore the surface. The main spacecraft will then land and collect a sample of surface material. The plan is to bring the sample back to Earth in 2031, where it can be studied with advanced instruments that would be impractical to send into space.
A Small Rover in an Unfamiliar Place
The mission carries a rover named Idefix, after a small dog in the French Asterix comic books. It was developed in collaboration between France’s National Centre for Space Studies, CNES, and the German Aerospace Center, DLR. The rover has a mass of about 24 kilograms on Earth—roughly the size of a large shoebox—and stretches a little over 1 meter across when its solar panels are unfolded.
If its deployment succeeds, Idefix will be the first wheeled rover to visit a celestial body this small. The process begins with the spacecraft slowing down and holding position between 40 and 100 meters above Phobos before releasing the rover. The rover will then descend under the moon’s weak gravity, which is only about one-thousandth of Earth’s. Even from this low height, the descent could take several minutes.
The rover is expected to touch down slowly, at less than 1 meter per second—roughly the speed of a person walking. But a gentle landing is not a guaranteed one: engineers cannot fully control how the rover will settle after it reaches the surface. Its mobility system is designed to right it no matter which way it falls, so that it ends up on its wheels and can get moving.
Phobos’s gravity gives a sense of just how strange the rover’s working environment will be. It will weigh only about 15 grams there, or roughly half an ounce, while a sparrow on Earth weighs about twice as much. The rover’s job is about more than taking attractive pictures. It will measure the surface temperature and examine minerals, as well as photograph the tracks left by its wheels. Those tracks could reveal how tightly the soil grains fit together and how large they are, helping scientists understand the material covering the moon.
Powdered Soil That May Preserve a Record of Impacts
The material the spacecraft aims to collect is called regolith: a loose surface layer made up of rocks and grains broken apart by meteorite impacts and temperature changes. At first glance, it may look like little more than dust, but it holds a record of the processes that have shaped Phobos. Measuring the grain sizes and how tightly they pack, then analyzing a sample on Earth, could show how the surface has changed and whether the moon is a solid, coherent body or a collection of rocky pieces.
Phobos is less dense than would be expected if it were a solid mass of rock, so scientists suspect that it is at least partly a rubble pile: many pieces packed close together and held by their mutual gravity. Many small asteroids have a similar structure, but exactly how such bodies form is still under investigation. One possibility is that a powerful impact broke apart a rocky object without scattering all its fragments. The pieces then came back together under gravity.
If Phobos formed this way, measurements of its surface and internal structure will allow scientists to test ideas about how small bodies evolve after impacts. The spacecraft will not just investigate what is on the surface. Its data will also help researchers assess whether the moon partly broke apart and reassembled, and how that process shaped its form and density.
Did Mars Capture Two Asteroids?
Mars lies near the inner edge of the main asteroid belt, prompting one explanation for its moons: Phobos and Deimos may be asteroids that came close to the planet and were caught by its gravity. The idea is supported by observations that their composition resembles that of common asteroid types. But similar materials do not solve the central problem: turning a passing object into a moon orbiting a planet is not easy.
For Mars to keep an asteroid that passes nearby, the object must lose a substantial amount of speed. One proposed scenario has the asteroid follow an unusually favorable path through the upper atmosphere of Mars, slowing down by just the right amount without falling onto the planet or continuing on its way into space. Such precise circumstances are possible, but they make direct capture a difficult explanation.
Capture might be easier if the incoming object were a pair of asteroids orbiting each other. In that case, Mars could capture one while its companion carried away some of the excess energy. But having two moons does not necessarily mean they were captured together: their orbits are clearly different. Phobos orbits about 6000 kilometers above the surface of Mars, while Deimos is a little more than 20,000 kilometers from the surface. So even if this scenario is correct, it would likely require two separate events.
The shape of the moons’ orbits presents another challenge. Both travel in nearly circular orbits above Mars’s equator, whereas a recently captured asteroid would be expected to follow a more elongated, tilted path. In theory, drag from the atmosphere and the tidal forces exerted by Mars could gradually make an orbit more circular. But the process is extremely slow and could take billions of years. That leaves an open question: has Deimos had enough time to settle into its current orbit this way?
An Ancient Impact—or a Moon That Broke Apart
Another possibility links the moons to an event on Mars itself. A collision with a large asteroid could have blasted rocky material into space, where it later gathered to form moons. There is a familiar example close to home: Earth’s Moon is thought to have formed after an object about the size of Mars collided with the young Earth, throwing vast quantities of rock into orbit that eventually came together.
There are indications that the composition of Mars’s two moons has some similarities to that of the Red Planet, lending support to the idea that they share a common origin. But if both formed in the same impact, why are their orbits so different? Perhaps several bodies formed around Mars during the event, and only Phobos and Deimos survived. Other explanations may also be possible, especially since surface images alone do not always reveal how an object formed.
A newer idea suggests a different history: perhaps a single moon formed first, either at the same time as Mars or after a later impact, and was then struck hard enough to break apart. In this scenario, the two moons we see today could be the surviving fragments. The idea could help explain why both moons have circular orbits above the equator, as well as why they orbit at different heights above Mars. But it remains a hypothesis, making direct measurements and samples essential for testing it.
A Mission Informed by Earlier Failures
Attempts to reach Phobos are not new. The Soviet Phobos 1 and Phobos 2 missions failed, and the Russian-Chinese Phobos-Grunt mission also did not reach its destination. Other missions headed for Mars have failed as well, inspiring a joke in the space community about a mythical monster that devours spacecraft before they arrive. The joke does not change the reality: sending a spacecraft to Mars, or to one of its moons, requires a long chain of carefully executed operations.
The destination itself makes the challenge even greater. Phobos is small and has very weak gravity, and approaching its surface, landing, collecting a sample, and launching again are all connected steps. Each one depends on the previous stages, from choosing a landing site to confirming that the rover has settled in a position that lets it operate. That is why MMX combines orbital mapping, exploration by a surface rover, and the return of material to Earth instead of settling for a brief flyby or distant images.
The Bottom Line
MMX aims to turn Phobos and Deimos from mysterious points near Mars into worlds whose histories can be tested with evidence. The spacecraft will approach and map Phobos, deploy the 24-kilogram Idefix rover, and collect a sample intended to reach Earth in 2031; the journey will also allow it to observe Deimos. These measurements matter because several explanations compete to explain the moons: they may be captured asteroids, formed from material thrown off Mars in an impact, or left behind when an older moon broke apart. A small sample may not answer every question, but it will give scientists material they can examine directly and compare with other evidence, opening a broader window onto the history of the moons and other small rocky bodies.