For the Second Time Ever, an Asteroid Sample Returns to Earth

Early on Sunday morning, the skies above a secluded military complex in central Australia will be brightened by a fireball plummeting to Earth. It will be a flamboyant homecoming for the sample return capsule from Hayabusa2, a Japanese spacecraft launched almost exactly six years ago on a mission to shoot an ancient asteroid and steal some of its dirt. If the capsule survives its fiery descent, its payload of pristine space rock will help scientists understand the earliest days of our solar system, shed light on the mysterious origins of meteorites, and may even provide clues about the emergence of life on Earth.

By the time it lands under parachute in the Australian outback, the sample will have traveled more than 180 million miles from Ryugu, a diamond-shaped asteroid orbiting the sun between Earth and Mars. Scientists believe that Ryugu broke off from a larger parent body only a few million years ago, but the rocks that compose it are closer to 4 billion years old. Hayabusa2 camped out around Ryugu for more than a year and a half, studying the asteroid from a distance and sending robotic scouts to its surface to prepare for a sample collection. It’s main mission was to collect just a few grams of dust and pebbles from this cosmic time capsule that has been preserved for eons in the frigid vacuum of space.

“We’re hoping to learn a lot about how a giant cloud of gas and dust turned into planets 4.5 billion years ago in our solar system,” says Larry Nittler, a cosmochemist at the Carnegie Institution for Science and one of nine American scientists selected by NASA to participate in the Japanese mission. “Ryugu and other asteroids like it are basically the leftover building blocks that didn’t grow into planets and have been floating around ever since.”

Ryugu looks like a piece of charcoal the size of several city blocks, and it spins like a top once every eight hours. It is one of the darkest asteroids ever discovered, its inky complexion a result of all the carbon trapped in organic compounds smeared across its surface. Some of these prebiotic compounds, such as amino acids, are the building blocks of life, and it may very well have been asteroids like Ryugu that seeded Earth with the molecular grist that kicked evolution into gear.

Carbonaceous asteroids like Ryugu are abundant in our solar system, but they mostly hang out around the outer planets. Every now and then, they bump into each other, break apart, and the pieces are sent on a trajectory toward the sun’s inner sanctum. If those pieces happen to collide with Earth, we call them meteorites. Almost everything we know about them is from the bits and pieces that make it to the surface. But by the time these stones have crash-landed on Earth, they have been cooked to a crisp and have been corrupted by terrestrial chemistry. Sending a probe to a still-orbiting asteroid is the best way to collect a clean sample. As the first spacecraft to visit a carbonaceous asteroid, Hayabusa2 can help determine the provenance of meteorites discovered on Earth and shed some light on the processes that formed the organic compounds in the early solar system.

“Are there samples of the organics that we don’t have in our collection because they didn’t survive going through the atmosphere? We don’t know,” says Harold Connolly, a geologist at Rowan University and a member of the sample analysis team for Hayabusa2 and NASA’s own asteroid sample return mission, OSIRIS-REx. But he hopes Hayabusa2 can help solve the mystery.

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