NASA

Many millions of miles from Earth, an asteroid and a spacecraft are traveling together. The asteroid, as wide as a skyscraper is tall, is ancient, almost as old as the solar system itself. The spacecraft, dispatched more recently, circles the asteroid like a tiny mechanical moon. Tonight, if everything goes as planned, the spacecraft will swoop toward the asteroid, touch its surface, and snatch some rocks before backing away again.

As NASA counts down the hours to this maneuver, a simple question comes to mind: Why? Asteroids have historically not been kind to Earth, as the truncated story of the dinosaurs can attest. No Hollywood scenario involving asteroids bodes well for us, either. This particular asteroid, known as Bennu, has a 1-in-2,700 chance of striking Earth in the late 22nd century, and would probably cause catastrophic damage across the planet.

For most of human history, the only way for scientists to get their hands on an asteroid was to wait for small chunks of one to fall through Earth’s atmosphere and smash into the ground. Incoming rocks can break apart and even vaporize during their fiery descent, so the world’s inventory of meteorites—the names given to asteroids once they’ve made it through the atmosphere—consists of only the hardiest samples. On Earth, too, meteorites are exposed to the same environmental conditions that smooth down terrestrial rocks over time, and the details of their cosmic origins erode.

The spacecraft circling Bennu is designed to bring asteroid samples home in pristine condition, without the drama and damage, so that scientists can explore the mystery of how we got here in the first place. Right now, Bennu isn’t a threat to our existence, but rather a thrilling target for discovery: The asteroid samples could contain hints about the early forces that shaped Earth, flooded its surface with oceans, and enriched the water with the molecules that helped life emerge.

“We’re probably missing a ton of information about these asteroids just because we have this awesome atmosphere that’s protecting us from these things,” says Hannah Kaplan, a planetary scientist at NASA’s Goddard Space Flight Center and a member of the Bennu mission.

A trip to an asteroid and back is the only way to bring home these cosmic souvenirs intact. Like astronauts, samples return to Earth tucked safely inside a heat-shielded capsule. Japan’s space agency, JAXA, has deployed such missions to two asteroids; the first mission returned with samples in 2010, and the second is currently on its way back to Earth. NASA’s mission to Bennu, known as OSIRIS-REx, which stands for—take a deep breath here—Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer—is expected to bring home the biggest haul from an asteroid yet, the largest sample return since the Apollo astronauts came home with pieces of the moon.

OSIRIS-REx has spent nearly two years in orbit around Bennu, accumulating a trove of data about its surface. Observations suggest that the landscape is covered in organic molecules, and the terrain bears the markings of a watery past. Kaplan and her colleagues have spotted bright streaks across some of Bennu’s boulders that might be made of the mineral carbonate, left behind by flowing water. Several billion years ago, as the solar system was swirling into shape, Bennu was part of a much bigger asteroid, with ice flecked throughout the rock. In the heat of those early years, some of the asteroid’s ice melted and flowed through its interior, leaving behind the hollowed tracks that scientists can see in boulders today.

Scientists believe that the solar system’s more ancient asteroids might have been responsible for delivering water to early Earth. Remarkably, the origin of our oceans, unmatched in the solar system, remains a mystery, one that bits of Bennu could help solve. “We are really curious to see if the water that is bound up in Bennu’s hydrated minerals has signatures that are similar to water on Earth,” Daniella DellaGiustina, an OSIRIS-REx scientist who works at the University of Arizona’s Lunar and Planetary Laboratory, told me. Researchers are also keen to see whether the organic materials they snag from Bennu resemble the ancient precursors that led to life on Earth.

Asteroids likely shaped the trajectories of other worlds in the solar system and beyond. “If they really did bring water and organic materials to Earth, then presumably they would have brought them to Mars. They would have brought them to Venus,” Andy Rivkin, a planetary astronomer at the Johns Hopkins Applied Physics Laboratory who studies asteroids, told me. “And those sorts of processes would presumably be going on in other solar systems."

Sampling an asteroid is a dangerous task, and, in some sense, the OSIRIS-REx isn’t equipped for the particular challenges of Bennu. Telescope observations from afar had suggested that Bennu’s surface would resemble a sandy shore, and engineers designed the mission with that image in mind. The spacecraft instead revealed a rugged, boulder-filled landscape that occasionally ejects coin-size particles into space. “That was a scary moment,” Kaplan said of the team’s reality check. “How are we going to get a sample back from this thing?”

The team eventually selected a sampling site that appeals to not only the scientists on the team, who want to collect the most intriguing samples, but also the engineers, who would like to avoid destroying the spacecraft. Propelled by its thrusters, OSIRIS-REx will leave its cozy orbit around Bennu and navigate to a small clearing about the size of a few parking spots, surrounded by boulders the size of buildings. In a matter of seconds, a robotic arm will stir up the regolith with nitrogen gas and then sweep the floating detritus into its grasp, before the spacecraft returns to its orbit.

NASA won’t know how much material the spacecraft will have scooped up right away. Later this week, engineers will command OSIRIS-REx to spin itself around, a clever move to calculate how much new mass the probe has acquired. If mission managers feel they have enough, the samples will be stowed away until the spacecraft’s return to Earth in late 2023. If not, they will need to decide whether to attempt a second—or even a third—touchdown. Olivia Billett, a systems engineer and the OSIRIS-REx science lead from Lockheed Martin, which built the spacecraft, considers this the worst-case scenario, since the descent puts the spacecraft at risk. “That’s a decision that I really hope we don’t have to make,” she told me. The mission is designed to fetch just more than two ounces (about 60 grams) of material. In astronomy, scientists are used to working with tiny samples, extracting cosmic insights from even the smallest grains. But when you’ve traveled millions of miles, you want to bring home as many souvenirs as possible.

The journey home will be more familiar, but still risky. Keiko Nakamura-Messenger, a NASA scientist who will curate the Bennu samples, remembers the pain of one NASA mission in the early 2000s, the agency’s first sample-return attempt since the Apollo era. A spacecraft had successfully sampled solar wind, the sun’s constant stream of high-energy particles, but the return capsule crash-landed after its parachute failed to deploy. The team spent days scouring a Utah desert for remnants of the capsule and washing away contaminants. They recovered enough material, even after this minor disaster, both to reveal new information about the sun and the solar system, and to store some particles for future scientists to study. In comparison, the haul from Bennu could be a treasure trove.

“No matter what, it’s going to be really, really precious,” Nakamura-Messenger told me. “We’re not going to waste a single grain.”

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