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Mars’s twin moons may soon get a visitor. We’ve never landed anything on Phobos and Deimos, but we have taken pictures of their surfaces from orbiters around Mars. Now, a mission headed by JAXA, the Japanese space agency, is set to launch a rover to one of these small moons in 2024 – the final destination is yet to be decided.

The Martian Moons eXploration (MMX) mission is a spacecraft intended to orbit both moons. The plan is to enter Mars orbit in 2025 and return samples of the moons to Earth in 2029. JAXA is also partnering with the German and French space agencies to building a rover to explore one of them. It will be the first rover to ever land on a minor body in the solar system.

“My guess is that they would go to Phobos unless there was some kind of spacecraft engineering reason not to, because it’s a bigger target and has more gravity,” says Tim Glotch at Stony Brook University in New York. Landing there could help us solve the mystery of where these moons come from.

There are two leading theories: either Phobos was created when an impactor hit Mars, or it could be a captured asteroid. “If you do spectroscopy on it, it’s similar to an asteroid’s carbonaceous chondrite material,” says Glotch. It also has a weird potato-like shape that is reminiscent of some asteroids we’ve seen.

But the details of Phobos’s orbit are such that it would be almost impossible for it to have been captured if it was coming in from the asteroid belt, he says. Some data suggests the moon has a similar make-up to Mars, but it’s not conclusive.

“A rover could sample the rocks and tell us what the surface is made out of. If it has minerals that are similar to the Martian crust, that could support that idea, or if it has minerals closer to carbonaceous chondrites, it could be more like a captured asteroid,” says Glotch.

Unlike the asteroids Bennu and Ryugu, the two most recent space rocks Japan has sent spacecraft to explore, Phobos isn’t particularly rocky from what we can tell. “It’s mostly covered by a fine-grain dusty material which makes landing a bit easier. It should be relatively easy to traverse, and there’s plenty of safe places to land on it,” says Glotch.

Knowing how the make-up of these moons will tell us more about how our solar system formed, but could also come in handy for future crewed missions to Mars.

“In some of the potential plans to eventually send humans to Mars, Phobos is a waystation. If it ends up being volatile-rich, meaning we could extract water for fuel, that could potentially support human expeditions to Mars,” says Glotch.