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Asteroid-bound probe to slingshot past Earth Friday

One year after launch, NASA's OSIRIS-REx spacecraft is racing back toward Earth for a critical flyby Friday, using the planet's gravity to change course and set up a rendezvous with the asteroid Bennu late next year in a billion-dollar mission to grab a sample from the surface and return it to waiting scientists.

Streaking through space at some 19,000 miles per hour, OSIRIS-REx will make its closest approach to Earth at 12:52 p.m. ET Friday, passing some 11,000 miles above Antarctica just south of Cape Horn, Chile, in a long-planned gravity-assist flyby that will boost the probe's velocity and, more important, put it in the same orbital plane as Bennu.

"It's mostly a change in direction," Mike Moreau, the flight dynamics lead at NASA's Goddard Space Flight Center, said in an interview. "Flying under the pole kind of tips us up and out of the Earth [orbital] plane and gets us aligned to Bennu's plane. It's about a six-degree difference."

The OSIRIS-REx spacecraft will use a sample collector on the end of an extended robot arm to capture soil and rocks from the surface of asteroid Bennu so they can be returned to Earth for analysis. NASA

The Earth flyby also will give OSIRIS-REx a needed velocity boost. The total change in velocity after the flyby is about 8,400 miles per hour. To put that in perspective, the most the spacecraft could do on its own, using its entire load of propellant, would be about 4,250 mph.

"So the size of this maneuver, if you will, is much larger than the total propulsive capability of the vehicle," Moreau said. "That's why we use the Earth flyby. It opens up trajectory options that would not be available just with the fuel you carry on board."

Four hours after close approach, OSIRIS-REx will begin a series of observations of the Earth and moon to help engineers calibrate its instruments as it begins the final leg of its two-year voyage to Bennu, arriving in October 2018 or shortly thereafter.

The spacecraft will spend two years mapping the small asteroid in extraordinary detail, using a sophisticated camera, two mineral sniffing spectrometers, a laser altimeter and a student-managed X-ray imaging system before unlimbering a robot arm in July 2020 to collect a soil sample from the surface.

The goal is to capture at least 2.1 ounces of material and, depending on the nature of the surface, up to 4.4 pounds.

With the collected material safely stowed in a re-entry capsule, OSIRIS-REx will bid Bennu farewell in March 2021, setting off on a two-year voyage back to Earth. The 100-pound sample capsule will be released in September 2023, hitting Earth's atmosphere at a blistering 27,000 mph before a gentle parachute-assisted touchdown in Utah.

Appropriately enough, the spacecraft's name is a convoluted acronym standing for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer.

Moreau credited a team of engineers at KinetX Aerospace for the precise navigation that is guiding OSIRIS-REx to its target.

"The KinetX guys have this down, the deep space navigation part of it," he said. "Right now, just in the last few days as we've been approaching [Earth], our uncertainty in the position is less than a kilometer and a few tenths of a second at the time of closest approach."

He said the flyby "is cool, but many people don't realize OSIRIS-REx is an extremely challenging mission from a navigation perspective."

"When we arrive at Bennu, it's one of the smallest objects that's ever been visited, and we're going into these really tight, small orbits around it where we'll be as close as 700 meters from the surface," he said. "The challenges of doing that are really extreme, you have to model all of these really small forces on the spacecraft extremely precisely."

The pressure of sunlight on the spacecraft, for example, can cause very slight changes that have to be taken into account, along with subtle thermal effects.

"When the spacecraft slews and there's a hot surface that was exposed to the sun and now it's in shadow, that radiates heat, and that makes a small acceleration," Moreau said. "We have models for all that stuff. I think you could probably say we've characterized OSIRIS-REx at this point better than almost any planetary mission."

As for the Earth flyby Friday, the navigation team will monitor the spacecraft's trajectory "just to make sure everything is on track," Moreau said. At the same time, spacecraft engineers will monitor telemetry to make sure the science instruments are ready for the planned Earth-moon calibration observations.

And while OSIRIS-REx will fly through a region of near-Earth space during the flyby that is populated by a variety of satellites, the odds of a collision are extremely low. Moreau said a detailed analysis showed a half-dozen satellites could be within 600 miles of so of the spacecraft at one point or another, "but there was really never a significant chance of having a conjunction with anything."

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