A question has intrigued astronomers for decades: are the moons of Mars, Phobos and Deimos, captured asteroids or fragments of the planet torn off by an impact? The Japanese mission MMX (Martian Moons eXploration) will soon depart to solve this mystery, with an ambitious goal: to bring back to Earth a sample of Phobos.
To do this, the spacecraft arrived at the end of March on the island of Tanegashima, Japan, to be prepared for a launch scheduled between November and December 2026. This will be a new launch window to Mars, after an initial delay in 2024 due to problems with the H3 launcher. A new failure of this rocket in December 2025 raised fears of another delay, but the anomaly was quickly identified and corrected.
Once in Martian orbit in 2027, MMX will begin mapping Phobos and Deimos to choose a landing site. Landing on Phobos is planned for 2029, with the mission to collect about 10 grams (0.35 oz) of regolith. After its departure from Mars in 2030, the sample should arrive on Earth in 2031 to be studied by scientists.
MMX is not alone: it carries the IDEFIX rover, developed by the German and French space agencies. Weighing 25 kg (55 lb) and equipped with four wheels, this autonomous robot will land on Phobos before the mothership. Its role: to explore the very low gravity environment to ensure the safety of the main mission.
This project is part of the Japanese tradition of sample returns. After Hayabusa, which brought back dust from asteroid Itokawa, and Hayabusa2, which delivered fragments of Ryugu in 2020, MMX could offer a new scientific treasure. The collected data will also help better understand the formation of Mars and the inner Solar System.
The JAXA MMX spacecraft arrives at Tanegashima on March 31, 2026. The launch is expected in November or December of this year. Credit: JAXA The origin of the Martian moons
Two main theories compete to explain the presence of Phobos and Deimos. The first proposes that they are asteroids captured by Mars' gravity. The second proposes that they are the result of a giant impact, similar to the one that formed our Moon. Analyzing the composition of Phobos will allow this debate to be settled.
If the moons are asteroids, their composition should resemble that of primitive meteorites. On the other hand, if they come from an impact, they would contain materials similar to the Martian crust. The samples brought back by MMX will therefore be compared to these two hypotheses.
The IDEFIX rover: a miniature explorer
IDEFIX is a 25 kg (55 lb) rover developed by France and Germany. It will land on Phobos before the main spacecraft to analyze the soil and test the environment. In a gravity of only 1/1000th of Earth's, moving around is a major technical challenge.
This robot uses special wheels and an autonomous navigation system to avoid obstacles. It will measure temperature and regolith composition, and send this information to MMX. This data is necessary to prepare the mothership's landing.
IDEFIX is designed to operate for only a few days, but its role is essential. It paves the way for future robotic missions on celestial bodies with very low gravity.