It’s difficult to imagine a more daunting journey than one to an alien moon named after the Greek god of fear—especially when three other attempts to make that journey have failed.
And yet, as Japan’s space agency, JAXA, and its collaborators gear up to send a spacecraft to Mars’ formidably named moon, “all the members are filled with a quiet sense of excitement,” says Kiyoshi Kuramoto, who leads the mission’s slew of international scientists.
This mission, called the Martian Moons eXploration (MMX), is set to launch from Japan’s Tanegashima Space Center in the wee hours of Tuesday in Japan (Monday afternoon in the US). After almost a year of travel, the spacecraft will reach Mars and its two moons: Phobos, meaning fear in Greek, and Deimos, meaning dread. MMX will observe the pair from afar and collect samples from Phobos in a bid to answer a looming scientific question: How, exactly, did the moons come to be?
Researchers have been attempting similar gambits for decades. Since the 1980s, Russia has sent three probes to Phobos, all of which failed. Still, researchers are anxious to achieve this mission impossible because the origin of Phobos and its sibling remains a mystery. Some of their attributes, such as the pattern of visible light they reflect, suggest they are asteroids captured into Mars’ orbit by its gravity. But the moons’ circular orbits in line with the Martian equator indicate that they could have formed from debris left over from a collision between a nascent Mars and a gigantic asteroid, Tom Statler, MMX program executive at NASA, said at a press briefing.
The new mission is designed to help settle the debate by surveying Mars and the moons in detail using a suite of scientific instruments. With MMX’s MEGANE instrument, for instance, researchers will determine the elemental makeup of Phobos by measuring the neutrino particles and gamma rays it emits. If Phobos ends up having more volatile elements such as hydrogen or potassium, it could lean closer to the captured-asteroid hypothesis, while fewer volatiles could support the impact idea.
“We know nothing about Phobos' elemental composition,” says David Lawrence, who leads the MEGANE team from Johns Hopkins University. “For me, that’s what’s exciting—making a measurement nobody’s done before.”
These observations are “going to give us a hint at some of the answers to the really big questions we have about Phobos,” says Abigail Fraeman, an MMX team member at NASA’s Jet Propulsion Laboratory. “But really, the return samples are going to nail those questions.” After scoping out Phobos with its onboard instruments and a rover—which will land ahead of the spacecraft to explore the new terrain—the spacecraft will land on the moon to collect at least 10 grams of material using two different sampling methods. The goal is to bring them to Earth by 2031, which would make them the first ever samples returned from a moon other than our own.
Researchers suspect Phobos’ surface soil contains Martian material, and analyzing it could paint a better picture of how the planet evolved over billions of years. But MMX will also provide insight into our solar system at large. Laboratory analysis of the samples will help determine the origin of leftover material from the early solar system and how it was transported, Statler said during the briefing.
Undertaking this research will require overcoming daunting obstacles. Because scientists currently know little of Phobos’ surface, they can’t pick a landing spot until taking up-close observations of the moon, Kuramoto says. The mission also uses two sampling methods as a safeguard in case one fails, he adds. “Returning samples from a Martian moon is a world-first challenge that is not easy.”
Still, the number of researchers involved and their depth of expertise give the team confidence that it can succeed where others have failed. MMX is a feat of international collaboration with scientists from NASA, the European Space Agency, and Germany’s and France’s space agencies. “That kind of compounded set of expertise and contributions really helps make the mission objectives very achievable,” says Elliot Sefton-Nash, MMX project scientist at ESA.
JAXA also has a proven track record of successful sample-return missions. Despite some setbacks, its Hayabusa mission became the first mission to return a sample from the surface of a near-Earth object when it brought back material from asteroid Itokawa in 2010. Its successor, Hayabusa2, delivered samples from the primitive asteroid Ryugu in 2020. Many Hayabusa2 members have helped design the MMX mission, but sampling Phobos will be different—its orbit and gravity are different from Ryugu’s, for instance, Kuramoto says.
Still, “JAXA's learned and achieved so much with their sample-return missions that they are really well positioned to be able to do this,” Fraeman says.