OSIRIS-REx is ready to get the goods. On 20 October, after several years of patient study of its enigmatic target, NASA’s $800 million spacecraft will finally stretch out its robotic arm, swoop to the surface of the near-Earth asteroid Bennu, and sweep up some dust and pebbles. The encounter, 334 million kilometers from Earth, will last about 10 seconds. If it is successful, OSIRIS-REx could steal away with up to 1 kilogram of carbon-rich material from the dawn of the Solar System for return to Earth in 2023.
Since OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) arrived in 2018, Bennu has yielded surprises, not all of them welcome. The 500-meter-wide asteroid was not smooth, as expected, but studded with more than 200 large boulders that could upset the sampling maneuver. And every so often, the asteroid ejected coin-size pebbles, probably propelled by meteoroid impacts or solar heating. The boulder hazard, in particular, forced the team to target an area just 16 meters across for sampling, 10 times smaller than planned. “Bennu has not made things easy for us,” says Mike Moreau, the mission’s deputy project manager at NASA’s Goddard Space Flight Center.
Despite the logistical challenge, the boulders contain a prize: veins of carbonate minerals thicker than your hands, the team reports in one of six studies published today in Science and Science Advances. The minerals, which precipitate out of hot water, popped out of data gathered during a close flyby of light-colored boulders near the target site, called Nightingale. Researchers believe the veins grew in channels of fluid circulating within Bennu’s parent body, a larger planetesimal thought to have formed beyond Jupiter’s orbit soon after the dawn of the Solar System 4.56 billion years ago, before being smashed apart in the asteroid belt within the last billion years. Heat from the decay of radioactive elements in its interior presumably drove the churning, and the presence of so much carbonate “suggests large-scale fluid flow, possibly over the entire parent body,” says Hannah Kaplan, a planetary scientist at Goddard who led the work.
This ancient water world is consistent with the idea that objects like Bennu delivered much of Earth’s water when they struck the planet billions of years ago, says Dante Lauretta, the mission’s principal investigator and a planetary scientist at the University of Arizona. The veins also suggest watery bodies like Bennu were a cauldron for the organic chemistry that generated the amino acids and other unusual prebiotic compounds found in carbon-rich meteorites.
OSIRIS-REx won’t be sampling the carbonate veins directly: The chamber at the end of its robotic arm is designed to suck up grit smaller than a penny. That’s all right, however, because the small pebbles strewn across Nightingale also contain signs of carbonates and other organic molecules, the team reports today. “This gives me a hint that my dream is going to come true,” Lauretta says. “I want to bring back something we’ve never seen before.”
The team picked Nightingale for its abundant pebbles and because the site appears young, probably because an impact exposed it in recent geological time, leaving it largely unaltered by bombarding cosmic rays. But navigating the van-size spacecraft to a safe touchdown still won’t be easy; the site is ringed with building-size rocks, including one nicknamed Mount Doom, along with smaller boulders throughout. Observations suggest many of these boulders are porous, almost fluffy, and would crumble if touched. But the team doesn’t want to take that chance: Using its cameras to navigate, the spacecraft will automatically abort its approach at an altitude of 5 meters if the site appears hazardous.
The entire sampling attempt, lasting 4.5 hours, needs to be autonomous; Bennu is currently five times farther from Earth than Mars, and radio signals take 18 minutes to reach it. After thruster maneuvers bring it to the touch point with Bennu, blasts of nitrogen should push dust and pebbles into the doughnut-shaped collector at the end of the robotic arm. It will be several days before NASA can judge how much was gathered, based on images of the target site and sampling head, and changes in how the spacecraft spins. By the end of the month, managers will decide whether to make a second attempt at a backup site in January 2021. Either way, the spacecraft will leave Bennu next year and head back to Earth. It will arrive in September 2023 and eject the sample capsule, which will parachute to a landing in the Utah desert.
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