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OSIRIS-REx orbiting Bennu
An artist’s conception shows the OSIRIS-REx probe circling in to enter a close-in orbit around asteroid Bennu. (Univ. of Arizona / NASA Graphic)

NASA’s OSIRIS-REx spacecraft today maneuvered into an orbit that takes it within 4,000 feet of the surface of Bennu, a diamond-shaped asteroid that’s 70 million miles from Earth.

The orbit sets a record for interplanetary travel. The quarter-mile-wide asteroid is now the smallest body ever orbited by a spacecraft, and the spacecraft is tracing the closest sustained orbit around a celestial body.

Bennu beat out Comet 67P/Churyumov-Gerasimenko, the 2.5-mile-wide comet that the European Space Agency’s Rosetta probe circled from 2014 to 2016. OSIRIS-REx orbits about a mile from Bennu’s center, while Rosetta’s orbit was 4 miles out from the comet’s center.

Today’s crucial eight-second burn of OSIRIS-REx’s thrusters, built by Aerojet Rocketdyne in Redmond, Wash., was executed perfectly, said University of Arizona planetary scientist Dante Lauretta, who serves as the mission’s principal investigator.

“Entering orbit around Bennu is an amazing accomplishment that our team has been planning for years,” Lauretta said in a news release.

OSIRIS-REx made its rendezvous with Bennu on Dec. 3 after a two-year cruise. Since the spacecraft’s arrival, the mission team has been carefully gauging the near-Earth asteroid’s mass distribution and lowering its orbit, step by step.

The spacecraft has to follow such a close orbit in part because Bennu’s gravitational pull is so weak. Earth’s pull is about 200,000 times stronger, the mission team says. The closeness also facilitates the task that lies ahead for OSIRIS-REx: mapping the asteroid’s surface in detail during a series of leisurely 62-hour orbits.

The orbit will have to be adjusted periodically for stability’s sake, said Dan Wibben, OSIRIS-REx maneuver and trajectory design lead at KinetX Aerospace in Simi Valley, Calif.

“The gravity of Bennu is so small, forces like solar radiation and thermal pressure from Bennu’s surface become much more relevant and can push the spacecraft around in its orbit much more than if it were orbiting around Earth or Mars, where gravity is by far the most dominant force,” he explained.

In February, the spacecraft will take on a series of even closer flybys to search for the best site to sample during a series of touch-and-go maneuvers scheduled for 2020.

OSIRIS-REx’s name is an acronym that stands for “Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer.” The mission’s objectives include characterizing the composition of the asteroid to shed more light on the makeup of the solar system’s primordial material; identifying resources that could be useful to future space travelers; and helping scientists figure out how to divert a potentially threatening asteroid if necessary.

The samples gathered by OSIRIS-REx are due to be delivered back to Earth for laboratory study in 2023.

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