University of Arizona-led OSIRIS-REx mission sets eyes on asteroid Bennu, begins approach phase

Nearly two years after launch, the University of Arizona-led OSIRIS-REx mission caught its first glimpse of the asteroid Bennu on Aug. 17 from 1.4 million miles away.

The asteroid appeared as a speck of light to the spacecraft’s UA-built PolyCam camera, but Bennu will seem to grow now that OSIRIS-REx has begun the approach phase of the mission.

“Now that OSIRIS-REx is close enough to observe Bennu, the mission team will spend the next few months learning as much as possible about Bennu’s size, shape, surface features and surrounding before the spacecraft arrives at the asteroid,” said Dante Lauretta, principal investigator for the mission and UA professor of planetary science.

OSIRIS-REx is short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer. It launched in September 2016 for the seven-year mission to Bennu and back. The UA leads the science team and the mission’s science observation planning and data processing.

Beginning Oct. 1, the spacecraft will put on the brakes, matching Bennu’s speed and trajectory.

In mid-October, the sampling arm — responsible for collecting a piece of the asteroid — will be extended and photographed for the first time in flight.

By late October, on-board cameras will reveal the asteroid’s shape. In November, it’s expected Bennu will appear large enough for its craters and boulders to be visible to onboard cameras.

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“You could say that’s when our asteroid will transition from being an astronomical object to an actual geological object,” said Carl Hergenrother, a staff scientist at the UA’s Lunar and Planetary Laboratory, in a news release. He was one of the scientists who proposed Bennu as a mission target.

The spacecraft has traveled more than a billion miles since leaving Earth. It is expected to arrive at Bennu on Dec. 3.

Last September, OSIRIS-REx used Earth’s gravity to slingshot itself into alignment with Bennu’s orbit. The spacecraft is now traveling 32,000 mph relative to Earth as it approaches its target.

Upon arrival, it will spend the first month collecting data on the mass and surface features as it skirts the asteroid at distances ranging from about 12 to 4½ miles from the surface.

Because Bennu is less than a half-mile wide, its gravitational tug is very weak. It is the smallest object any spacecraft has ever orbited, making navigation a challenge.

For every planetary body that has been orbited by spacecraft until now, gravity has been the main force on the spacecraft, making navigation relatively straightforward. But Bennu is small, so the main force operating against the spacecraft is not gravity, but heat from the sun that is absorbed by the asteroid’s surface during the day and dissipated at night. That heat pushes the craft away, so the OSIRIS-REx team is closely studying the asteroid to learn to navigate in these alien conditions.

OSIRIS-REx will scoop a sample of the asteroid in July 2020. It will return to Earth and eject the Sample Return Capsule for landing in the Utah desert in September 2023.