For decades, scientists have dreamed of getting Martian rocks into Earth’s laboratories — a Herculean feat that would revolutionize our knowledge of the Red Planet. An audacious mission from the Japanese space agency might offer the next best thing: rocks from Mars’ larger moon, Phobos.

That’s the task confronting the Martian Moons eXploration (MMX) spacecraft. The mission, which launched October 19, aims to crack the long-standing mystery of how Mars got its two odd moons by delivering a sample of Phobos to Earth in 2031. These rocks are no substitute for true Martian material — but given that NASA’s plans to fetch the Perseverance rover’s prized samples are in limbo , scientists are eager for even an indirect glimpse of Mars.

“It’s a chance to get a totally different sample and view of Martian materials,” says planetary scientist Briony Horgan of Purdue University in West Lafayette, Ind., who is not involved in the MMX mission.…