JAXA's MMX Probe Launches to Mars to Bring Back Samples from Phobos in 2031

At 4:41:03 a.m. on October 20, 2026, H3 Launch Vehicle No. 10, carrying the Martian Moons eXploration (MMX) probe, lifted off from the Tanegashima Space Center in Kagoshima Prefecture. According to an announcement from the Japan Aerospace Exploration Agency (JAXA), the rocket flew as planned, and about 1 hour, 11 minutes and 13 seconds after liftoff it separated MMX normally and placed it on its intended trajectory. The probe will now spend about 10 months traveling to Mars, with the aim of returning samples collected from Phobos, the larger of Mars's two moons, to Earth in 2031.

How Mars's two moons, Phobos and Deimos, were born is an unresolved question in solar system science that has been debated for more than half a century. Phobos, MMX's main target, is an irregularly shaped small body only about 27 km across at its widest. Its surface is covered with countless craters, and it looks very different from Earth's Moon. Researchers have regarded two hypotheses as leading candidates: that an asteroid from the outer solar system was captured by Mars, or that the moons formed from material thrown out by a giant impact on Mars. Observations to date have not settled which is correct.

The successful launch is an important first step toward answering that question, but MMX's challenge has only just begun. Over the next five years or so, the mission faces difficult stages: interplanetary cruise, insertion into Mars orbit, landing and sampling in a microgravity environment, and the return to Earth. Only when all of these are completed will researchers be able to test the origin of Phobos directly with physical samples.

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From Tanegashima to Mars: How the H3 Launch Unfolded

The MMX launch used the H3 rocket in its "H3-24L" configuration. The first stage carries two LE-9 engines burning liquid hydrogen and liquid oxygen, with four solid rocket boosters (SRB-3) arranged around the body and a large long fairing at the tip. According to the specifications published by JAXA, each LE-9 engine produces 1,471 kN of thrust in vacuum, for a combined total of about 2,942 kN from the two engines.

After liftoff, the rocket climbed according to the planned flight sequence. Within about two minutes, the four solid rocket boosters were jettisoned two at a time, and the fairing that had protected the payload was then separated. The first-stage engines finished burning about five minutes after liftoff and handed propulsion over to the second-stage LE-5B-3 engine. By this point the rocket had passed through the dense part of the atmosphere and gained the speed needed to enter Earth orbit.

After the first burn of the second-stage engine, the vehicle entered a parking orbit around Earth. About 1 hour and 4 minutes after launch, a second burn to inject the probe into a Mars transfer orbit began, continuing to accelerate the vehicle for about 6 minutes and 25 seconds. During this process the upper stage reached a speed of about 38,858 km/h. About 71 minutes after launch, MMX separated from the second stage and was placed on a trajectory toward Mars.

JAXA announced after the launch that it had confirmed the probe separated normally and was placed on its planned trajectory. However, the success confirmed so far covers only the rocket's launch and orbital injection of the probe. For MMX to cruise through interplanetary space, the probe's own functions must still be verified, including attitude control after separation, deployment of its solar array paddles, and establishing communications with Earth. During the roughly 10 months before arrival at Mars, it will also repeat navigation observations and trajectory corrections.

Arrival at Mars in August 2027, With Sample Return Targeted for 2031

The MMX mission spans about five years from launch to Earth return. After roughly 10 months of interplanetary cruise, the probe is scheduled to reach the vicinity of Mars in August 2027. On arrival it will fire the engines of its propulsion module to decelerate and enter an orbit captured by Mars's gravity. It will then move to an orbit suited to detailed observation of Phobos and proceed with selecting candidate landing sites.

The orbit used to observe Phobos is a special one called a Quasi-Satellite Orbit (QSO). Here the probe does not directly circle Phobos itself; instead it orbits Mars while appearing, from Phobos's point of view, to circle around it. Because Phobos has very little mass and weak surface gravity, Mars's gravity has a large influence nearby. This makes it difficult to keep a probe stably orbiting right next to Phobos as an ordinary satellite would, so a QSO is needed.

A 2021 paper on MMX's observation operations plan published in Earth, Planets and Space describes a scheme that uses five types of orbit: QSO-H, QSO-M, QSO-LA, QSO-LB and QSO-LC. After grasping the overall picture from a high-altitude orbit relatively far from Phobos, the probe will lower its altitude step by step and carry out high-resolution imaging and spectroscopic observations. These will be used to study terrain features, surface mineral composition and hazards for landing, and to choose landing sites on both safety and scientific value.

【MMXの主要マイルストーン】

2026年10月
種子島から打ち上げ、火星遷移軌道へ投入（実施済み）

2027年8月
火星到着、周回軌道投入
フォボス近傍の擬衛星軌道へ移行

2028年末〜2029年初め
小型ローバーIDEFIXをフォボス表面へ投入

2029〜2030年
フォボスの詳細観測と、探査機本体による
2回の着陸・試料採取を計画

2030年
ダイモスの複数回フライバイ観測
11月ごろに火星圏を離脱する計画

2031年
地球帰還カプセルを分離
オーストラリアで試料回収予定

※打ち上げ以外の時期は計画段階であり、
今後の運用状況によって変更される可能性がある。

Before the main probe lands, the small rover "IDEFIX," jointly developed by France's National Centre for Space Studies (CNES) and the German Aerospace Center (DLR), will be deployed onto Phobos's surface between late 2028 and early 2029. According to DLR's announcement dated October 19, 2026, IDEFIX will be released from an altitude of roughly 40 to 100 m and reach the surface in free fall. It is expected to bounce on touchdown, then right itself and deploy its solar panels once it comes to rest. For about 100 Earth days afterward, it will move across the surface, investigating the terrain and the properties of the regolith (the layer of sand and fragmented rock covering the surface).

Two landings by the MMX main body are planned, each aiming to collect samples at a different site. JAXA assumes a short stay on the surface of about two to three hours per landing, after which the probe will lift off again and return to a standby orbit. After finishing its observations and sampling at Phobos, it plans to approach Deimos, the other moon orbiting farther out, and conduct multiple flyby observations.

Under the mission plan JAXA currently presents, the probe will leave the Mars system in 2030 and take about a year to return to Earth. In 2031 it will release a capsule containing the samples, which is planned to be recovered in Australia. The launch was originally scheduled for 2024 but was postponed to 2026 in light of the H3 rocket's development status and other factors. Because the positions of Mars and Earth limit the windows suitable for interplanetary travel, a change in launch date also affects the schedule of the whole mission. The specific timing of landings and sampling may change depending on future operations, and none of it is a confirmed result at this point.

Captured Asteroid or Giant Impact? Two Hypotheses for the Origin of Mars's Moons

The main reason MMX is headed for Phobos is to determine how Mars's moons formed. Two hypotheses have mainly been debated for the origin of Phobos and Deimos. One is the "captured asteroid hypothesis," in which primitive small bodies formed in the outer solar system were captured by Mars's gravity. The other is the "giant impact hypothesis," in which a huge body struck early Mars and the scattered debris accreted around the planet to form the moons.

What makes the contest between the two hypotheses difficult is that the moons' material characteristics and orbital characteristics suggest different origins. The surfaces of Phobos and Deimos are dark and have reflectance spectra similar to carbonaceous asteroids, so in terms of material composition the capture hypothesis looks stronger. On the other hand, both moons travel in nearly circular orbits close to Mars's equatorial plane. Explaining these orbits is more natural if the moons were born from a disk formed around Mars, which favors the giant impact hypothesis.

The main evidence for and challenges facing each hypothesis can be summarized as follows.

The paper by Kei Kuramoto and colleagues, "Martian moons exploration MMX: sample return mission to Phobos elucidating formation processes of habitable planets" (Earth, Planets and Space, 2022), also lays out the observations and sample analyses needed to test these hypotheses. What matters is to examine not only the optical properties of the surface obtained through remote observation, but also the actual mineral composition, elemental ratios and isotope ratios.

One piece of evidence supporting capture is the reflectance spectra of Phobos and Deimos. Both moons have a low reflectance of about 0.07 and show almost no distinct absorption bands from visible to near-infrared light. They also have a characteristic red slope, with reflectance rising as wavelength increases, and resemble asteroids classified as D-type and T-type. These asteroids are thought to be primitive bodies, rich in carbon and volatile substances, that formed in the relatively outer parts of the solar system.

Phobos's low average density of about 1.86 g/cm³ is another feature consistent with the capture hypothesis. It may have many voids inside, and it may also contain relatively light materials such as ice. However, low density alone cannot settle its origin, and the internal structure and rock density must also be considered.

The major challenge for the capture hypothesis lies in orbital mechanics. If a small body that traveled from the outer solar system were captured by Mars, it would normally be expected to end up in a highly eccentric elliptical orbit or one tilted from Mars's equatorial plane. Yet Phobos and Deimos both have nearly circular orbits close to the equatorial plane. Although their orbits may have changed after capture through atmospheric drag, tidal effects and similar processes, it is difficult to fully explain how both moons arrived at their present orbits.

The giant impact hypothesis, by contrast, explains the moons' orbits relatively naturally. If a large body struck early Mars and the ejected material formed a disk around the planet, moons growing from that disk would tend to have orbits close to the equatorial plane. This has points in common with the formation of Earth's Moon, and numerical simulations have been used to study the accretion of impact debris and the orbital evolution of the moons.

However, uncertainty remains about the composition of the disk formed by a giant impact. For example, a 2017 study by Hyodo and colleagues estimated that about 50% of the material came from the impactor, while a 2018 study by Canup and Salmon put the figure at about 20%, with the remaining roughly 80% derived from Mars. This difference means the composition of the resulting disk changes depending on the impact conditions and the computational model. In addition, if material was exposed to high temperatures during the impact, volatile components might have been lost and the mineral composition altered. Even so, why Phobos today shows optical properties resembling carbonaceous asteroids has not been fully explained.

These mixing ratios and heating processes are predictions from numerical simulations, not measured values from Phobos. Moreover, the moons' surfaces have undergone long-term space weathering, which limits how accurately remote observation can estimate the composition of their interiors. To judge which of the capture and giant impact hypotheses is closer to the actual formation process, material unique to Phobos must be brought back to Earth and examined in detail.

The Goal: At Least 10 g of Samples, Including Material From Deeper Than 2 cm

MMX's sampling has clear scientific requirements set to resolve the origin of the moons. Under the requirements presented in the paper by Kuramoto and colleagues, the mission aims to collect a total of at least 10 g of samples from two different sites, including material from deeper than 2 cm below the surface, and bring them back to Earth. The key is to secure material less affected by the condition of the surface layer, rather than simply collecting surface sand.

Material from deeper than 2 cm is sought in order to minimize the effects of space weathering. The surface of Phobos, which has no atmosphere, is constantly exposed to charged particles in the solar wind and to micrometeoroids. These processes can alter the surface structure of minerals and sometimes form nanometer-sized metallic iron particles. As a result, the reflectance spectrum changes, making it harder to read the characteristics the original rock had.

Because alteration by the solar wind proceeds mainly in the very shallow layer, collecting subsurface material raises the likelihood of examining minerals that are relatively unaffected by weathering. However, a depth of 2 cm does not completely block all space weathering processes. In sample analysis, in addition to differences between the surface and interior, mixing by micrometeoroids and contamination by foreign material must also be taken into account.

To meet this requirement, MMX carries two types of sampling devices using different methods. One is the Coring Sampler (C-sampler) developed by JAXA, in which a cylindrical sampler attached to the tip of a robotic arm is pressed into the ground to collect subsurface regolith. The other is the Pneumatic Sampler (P-sampler), developed by Honeybee Robotics and provided by NASA. It is built into the landing legs and collects particles kicked up by blowing nitrogen gas onto the surface. Combining different methods is intended to cope with the unknown surface environment while also securing samples from different depths.

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In analyzing the returned samples, the ratio of silicon to iron (Si/Fe ratio), the presence of hydrated minerals and isotope ratios will be important indicators. The Si/Fe ratio in particular offers a clue as to whether Phobos's constituent material is closer to primitive asteroids or contains a large share of Mars-derived rock. In primitive chondrites, including carbonaceous chondrites, the Si/Fe molar ratio generally lies in the range of 1.1 to 2.0. In Martian mantle material, by contrast, the ratio is higher, at roughly 2.9 to 3.7, so if a giant impact mixed in a lot of Mars-derived material, the effect could show up in the elemental composition.

If the captured asteroid hypothesis is correct, the samples are expected to contain minerals and chemical compositions resembling primitive carbonaceous chondrites. Confirming phyllosilicates (clay minerals) containing hydroxyl groups or water of crystallization, or organic matter, would support the capture hypothesis. If signs that ice remains in the interior were also found, the likelihood of an origin in a volatile-rich small body would increase. However, the presence of such materials alone would not confirm the capture hypothesis, and a comprehensive evaluation together with elemental composition and isotope ratios is needed.

If the giant impact hypothesis is correct, the samples are predicted to contain a mixture of silicate material from Mars's crust and mantle and material from the impactor. Confirming igneous textures, a history of high-temperature heating or depletion of volatile elements would serve as evidence supporting formation by a giant impact. Radiometric dating of the rocks to determine when they formed and how they were heated might also constrain the age of the impact event that led to the moons' formation. However, because the composition of the mixture and the heating conditions differ among impact models, it is difficult to determine the origin from any single indicator.

The returned samples are also expected to have another kind of scientific value: investigating the history of Mars itself. Some rock ejected into space when meteorites struck Mars may have reached the surface of Phobos. If Mars-derived particles can be identified, Phobos might provide information on the ancient Martian crust and surface environment. Depending on the minerals and chemical components they contain, they could also offer clues to past water environments and atmospheric conditions.

Research institutions in the United States and Europe, as well as Japan, will take part in sample analysis. Under an agreement signed in March 2023, at least 15% of the returned samples are to be provided to NASA. NASA plans to store and analyze its share at the Astromaterials Research and Exploration Science facility at Johnson Space Center in Houston. For the remaining samples, research institutions in Japan and abroad will use advanced analytical instruments to study mineral composition, trace elements, isotope ratios and more.

Instruments That Examine Phobos From Both Orbit and the Surface

The samples brought back to Earth will come from only a very limited number of locations on Phobos. Examining the samples alone therefore cannot reveal whether they represent the average material of the whole moon or exist only in specific places. To address this problem, MMX plans to study the terrain and elemental composition of the whole moon from orbit and compare them with the geological characteristics of the sampling sites. Combining remote observation with sample analysis is important for accurately inferring the moon's origin.

One of the main instruments is MEGANE, a gamma-ray and neutron spectrometer developed by the Johns Hopkins University Applied Physics Laboratory (APL) with NASA's support. The instrument consists of a gamma-ray spectrometer using a high-purity germanium (HPGe) detector and a neutron detector using helium-3 (³He). It measures gamma rays and neutrons produced when cosmic rays interact with Phobos's surface material, and estimates the abundance of elements in material from the surface layer down to a depth of several tens of centimeters.

The main measurement targets are hydrogen (H), oxygen (O), magnesium (Mg), silicon (Si), potassium (K), calcium (Ca), iron (Fe) and thorium (Th). The silicon-to-iron ratio in particular is an important indicator for distinguishing between the capture and giant impact hypotheses. MEGANE is designed to measure elemental composition while suppressing background noise by observing from low altitudes of about 11 km or less, which corresponds to Phobos's mean radius. By comparing with the analysis results of the returned samples, it will be possible to evaluate how well the composition at the sampling sites represents the moon as a whole.

For optical observation, the telescopic camera "TENGOO," the wide-angle multiband camera "OROCHI" and the near-infrared spectrometer "MIRS," developed by France's CNES, play important roles. TENGOO acquires high-resolution terrain images to study craters, boulders and slopes in detail. OROCHI and MIRS measure reflectance properties from visible to near-infrared light to estimate the distribution of hydrated minerals and other minerals. Combining these observations will narrow down flat terrain suitable for landing and locations where scientifically valuable samples can be obtained.

In spectroscopic observation, the mission aims to survey the whole of Phobos at a spatial resolution of about 20 m, and to observe candidate landing sites in detail at a high resolution of about 1 m. It also plans to examine in detail an area of about 50 m square around the landing site, evaluating not only surface terrain but also differences in mineral composition and the presence of freshly exposed bedrock. In particular, fresh rock believed to be little affected by space weathering would be a prime observation target for estimating the moon's original material composition.

The small rover IDEFIX will also gather surface information that cannot be obtained from orbit. It carries the Raman spectrometer "RAX," the infrared radiometer "miniRAD," stereo navigation cameras and other instruments. RAX irradiates surface material with laser light and identifies mineral types from the scattered light. miniRAD observes surface temperature and thermal properties, while the cameras are used to understand the terrain and analyze interactions between the wheels and the regolith. These observations are planned to investigate in detail the surface properties needed for the main probe's landing and sampling.

If IDEFIX drives as planned, it will be the first exploration to move on the surface of a Martian moon by wheel. On a body with extremely weak gravity like Phobos, the reaction force from turning the wheels strongly affects the vehicle's attitude and movement, so driving control different from that on Earth or Mars is required. Observing the surface grain-size distribution, hardness and how deeply the wheels sink will also be useful for future small-body exploration.

These instruments are not merely auxiliary to sample return. Even if sampling or the return to Earth were not achieved, elemental analysis and spectroscopic observation from orbit and surface exploration by IDEFIX could still yield important scientific findings about Phobos's origin. MMX plans to examine the formation of Mars's moons from multiple angles by combining three approaches: remote observation, direct observation on the surface, and precise analysis of samples brought back to Earth.

Past Failures and Microgravity Show How Hard the Mission Is

Although the H3 launch succeeded, many technical challenges remain for MMX. Exploration aimed at Mars's moons is a difficult field in which several missions have failed in the past. In particular, no mission has yet accomplished operations near Phobos, landing on its surface, sample collection and return to Earth.

Phobos 1, launched by the former Soviet Union in 1988, lost communications on the way to Mars after a problem caused by a command from the ground. Phobos 2 succeeded in reaching the vicinity of Mars and made some observations, but became uncontrollable before it could carry out a full approach to Phobos and surface exploration. Russia's sample-return probe Phobos-Grunt, launched in 2011, also failed to fire its propulsion to leave Earth orbit for Mars and reentered Earth's atmosphere in 2012.

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There are also issues to be confirmed regarding the performance of MMX's instruments. An abstract from the MEGANE development team, presented at the 2025 European Planetary Science Congress (EPSC-DPS2025), reported that the gamma-ray spectrometer's high-purity germanium detector showed a larger leakage current than expected. Because leakage current can affect detector noise and energy resolution, the development team has been considering measures to limit its effect. At the same time, the abstract also reports that functional tests of the whole instrument met the required performance, so the leakage current does not immediately mean observation is impossible. How much performance is actually achieved in the space environment will have to be evaluated through future instrument checks and calibration.

An even bigger challenge is the extremely weak gravity on Phobos's surface. The surface gravity of Phobos is less than one-thousandth of Earth's, and its escape velocity is only about 11 m/s, or roughly 40 km/h. Even a slight recoil at the surface could send a spacecraft bouncing high, so the landing and mobility techniques generally used on Earth and Mars cannot be applied as they are. For the main probe's landing, it must keep its attitude under microgravity while touching the surface, collect samples, and then lift off safely again.

IDEFIX's landing also has its own difficulties. With a free-fall deployment from above, how it bounces after hitting the surface and the attitude in which it finally comes to rest depend on the regolith's hardness, grain size, friction properties and other factors. The surface conditions of Phobos are not well understood from remote observation alone, and uncertainty remains about how closely environments simulated on the ground match the actual surface. For this reason, IDEFIX is equipped with a mechanism that lets it right itself regardless of the orientation in which it comes to rest after landing.

Even after successful sampling, the probe faces a long voyage and complex operations. Everything must proceed as planned, from the maneuver to return from the Mars system to Earth, separation of the recovery capsule as it approaches Earth, atmospheric reentry, and recovery in Australia. In particular, whether a sufficient quantity of samples has been secured from Phobos will not be known until the actual sampling results are confirmed.

What matters in judging MMX's scientific results is not simply choosing one of the captured asteroid and giant impact hypotheses. The questions are what the mineral composition and isotope ratios of the returned samples show about the relationship to Mars and to primitive asteroids, and how far they agree with observations from orbit. If the sample return planned for 2031 is achieved, the origin of Mars's moons, which long-running observation and theoretical study alone could not resolve, can be tested against actual material. The results would also provide important clues for understanding not only Phobos but also the formation of Deimos, from which no samples will be collected directly, and the early planetary evolution of the solar system.