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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.
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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.”