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.


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…

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