2031: The Year a Japanese Probe Could Bring Mars's Moon Home

Japan's space agency is set to launch a spacecraft on October 20 that will visit both moons of Mars, land on one and bring a piece of it back to Earth, in an attempt to settle where they came from.

NASA says the Martian Moons eXploration mission, known as MMX, is targeting liftoff at 4:41 a.m. Japan time from the Tanegashima Space Center, JAXA's launch site in southern Japan ^[1]. The date is a target, not a done deal. The launch has not yet happened.

The prize is an answer to a question nearly as old as the moons themselves. Phobos and Deimos were found by the American astronomer Asaph Hall during a six-day span in 1877 ^[4]. Nobody has been able to say for certain how Mars got them.

Two stories, neither clean

There are two leading explanations. In one, the moons are asteroids that wandered close to Mars and were caught by its gravity. That idea dates to the 1960s ^[4]. In the other, a large impact on Mars threw material into orbit, and that debris formed moons that changed over time, with Phobos and Deimos the survivors ^[1][4].

The impact story has a famous cousin. A collision that flings rock into orbit is a mechanism also invoked for Earth's Moon. The Martian version has its own wrinkle, since the moons born from the debris evolved, and only two of them are left to study ^[4].

The moons' looks favor the first story. Their irregular shapes and low density fit porous, rubble-like bodies, the sort of thing an asteroid might be ^[4].

Their orbits favor the second. Both moons travel on nearly circular paths, while captured asteroids would typically be expected to end up on eccentric, stretched-out ones ^[4]. Capture would also require a much denser early Martian atmosphere to slow an incoming body enough to hold it, according to the account by researcher Lana Williams published by The Conversation ^[4].

So each theory has a fact it struggles to explain. That is why a sample matters more than another round of photographs.

What the spacecraft will do

MMX is expected to reach Mars in August 2027 and survey both moons ^[1]. Collection of Phobos material comes no earlier than December 2028, and the sample is due back on Earth in 2031 ^[1].

That leaves at least 16 months between arrival and the earliest sampling date. The spacecraft will spend that time looking at the moons up close before it commits to a spot.

It carries two ways of getting material. One uses high-pressure nitrogen to gather surface material. The other takes a core from a few inches below the surface ^[1]. The second matters because a surface bathed in sunlight and radiation for billions of years may not look like what lies just under it.

NASA is contributing an instrument called MEGANE, which measures neutrons and gamma rays coming off Phobos to work out which elements it contains ^[1]. It is also providing a sampling technology demonstration called P-Sampler, communications and navigation support, and funding for US scientists on the mission team ^[1].

The mission also plans to put a small rover on Phobos. The rover, IDEFIX, was developed by the French and German space agencies ^[3].

Why a sample settles what a telescope cannot

The logic of the experiment is simple. If the returned material contains a substantial share of Mars itself, that would favor the impact explanation. If it looks like known asteroid meteorites, that would favor capture ^[4]. These are clues scientists hope to read, not results anyone has.

They also may not be clean. Nobody yet knows how much material the spacecraft will collect, or whether the answer will point in one direction.

The case for bringing rock home rests on past missions. Laboratory work on returned asteroid samples has revealed details that remote observation cannot reach, including delicate mineral features ^[2][5]. A spacecraft can measure a surface from orbit. A lab on Earth can take it apart grain by grain.

NASA's Tom Statler, a program executive and scientist, said the moons may be remnants of a massive impact or captured asteroids, and that an answer could shed light on the early era when the inner planets may have acquired water and organic compounds ^[1]. A small, dark moon is being asked to speak for the early history of the planets closest to the Sun.

A shared haul

JAXA leads the mission, with NASA and the French and German agencies as partners ^[1][3]. The sharing extends to the sample. NASA says it will receive and curate about 15% of the returned material at the Astromaterials Lab at its Johnson Space Center, for future research ^[1].

That arrangement says something about how the work will unfold. The first analyses will not come from a single team and will not come soon. Even a perfect launch on October 20 starts a clock that runs to 2031.

The next fixed point after liftoff is arrival at Mars in August 2027. Only then will the spacecraft begin the close survey of Phobos and Deimos that will tell the team where to collect ^[1].