In Simple Terms
Japan plans to send a spacecraft to Mars’ two small moons to find out how they formed. The spacecraft will study both moons, send a small robot to Phobos, and collect dust and rock to bring back to Earth in 2031. The sample could reveal clues about Mars and the early history of nearby planets.
Two Tiny Moons, One Big Mystery
Phobos and Deimos are much smaller than Earth’s Moon. Phobos is about 22 kilometers across, while Deimos measures roughly 13 kilometers in diameter. Despite their size, both moons could hold clues to events that took place in the solar system billions of years ago.
Scientists still disagree about where the moons came from. One possibility is that a huge impact blasted material off Mars, and the debris later gathered into two moons. Another is that they began as asteroids, which Mars captured with its gravity. Distant observations and images alone cannot settle the question, making close-up studies and a sample from the surface especially valuable.
The answer could tell scientists more than just the history of Phobos and Deimos. If the moons formed from Martian material—or came from elsewhere—they could help researchers understand the early history of the rocky planets closer to the Sun, when water and organic compounds may have reached worlds such as Earth. NASA is contributing a scientific instrument, a technology experiment, communications support, and funding for American researchers on the mission team.
A Long Journey to Mars
The spacecraft will not reach Mars right after launch. The plan calls for it to arrive in the Martian system in August 2027, then enter orbit around Phobos about a month later. It will spend roughly 18 months observing both moons and gathering information from a distance. This phase will help the team learn about Phobos and prepare for the mission’s most delicate task: approaching its surface.
The launch date reflects another challenge: Earth and Mars line up favorably for interplanetary travel only about once every 26 months. MMX was originally scheduled to launch in 2024, but issues with the H3 rocket prevented it. Once that launch window passed, the mission had to wait for the next opportunity.
A Lightweight Robot on a Low-Gravity World
In early 2029, the spacecraft is expected to deploy a small French-German robot called Idefix onto Phobos. It weighs about 25 kilograms on Earth, but Phobos’s weak gravity means it would weigh only about 15 grams on the moon’s surface. That difference shows how unusual it is to work on a world so small.
Idefix has two related jobs. It will examine the makeup and texture of Phobos’s surface while gathering information to help plan the main spacecraft’s landing and sample collection. Knowing how firm or rough the ground is will help the team prepare to set down on a body with very little gravity.
After studying the moons from orbit and completing the robot’s work, the mission plans to land its main spacecraft on Phobos in 2030. It aims to collect at least 10 grams of surface material. That may sound like very little, but it would be a direct sample from a world that has never before sent material back to Earth—one that researchers can examine in ways that remote observations cannot match.
From Phobos to Earth’s Laboratories
After collecting the sample, the spacecraft will begin its return journey. Under the current schedule, a capsule carrying the material will land on Earth in 2031. Scientists will then search the Phobos sample for clues to the moon’s history and development, and test ideas about how it formed. Comparing it with what is known about Mars and other small bodies could also put Phobos in a broader context.
Returning a sample means researchers do not have to rely only on data gathered by a spacecraft near Mars. They can examine the material repeatedly, and scientists around the world can study it using different methods. But the plan depends on a long chain of steps going right: launch, arrival at Mars, orbital operations, robot deployment, landing, sample collection, and the capsule’s safe return to Earth.
Japan’s Experience Bringing Back Space Samples
Japan has brought samples back from space before. Its Hayabusa mission returned material from the asteroid Itokawa in 2010, and Hayabusa2 brought back samples from the asteroid Ryugu a decade later. That experience is valuable for MMX, although Phobos presents a different challenge: reaching a moon of Mars, working near its surface, and then making the return trip.
The upcoming launch is the start of a multiyear mission, not the end of the story. The spacecraft must first leave Earth safely, complete its long journey, and carry out precise operations around Phobos. Each stage will provide information to guide the next, until the sample reaches Earth and scientists begin analyzing it.
Conclusion
MMX aims to solve a basic mystery: Did Phobos and Deimos form from material blasted off Mars, or are they asteroids captured by the planet’s gravity? The mission is scheduled to launch from Japan, study the moons from orbit, deploy a robot on Phobos, and return a sample to Earth in 2031. If all goes to plan, that material could shed new light on the moons, Mars, and the conditions present as the rocky planets near the Sun took shape.