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Tracking Bennu

April 12, 2019

OSIRIS-Rex is starting a new job about now. The spacecraft has spent the last couple of months mapping the surface of a small asteroid known as Bennu. Now, it’s beginning to zero in on the equator. It’ll identify as many as a dozen areas that might be good spots for picking up a couple of ounces of dirt. It’ll swoop down on the winning location next summer, then bring its bounty to Earth in 2023.

OSIRIS-Rex entered orbit around Bennu in December. The asteroid is only about a third of a mile in diameter, and it looks a bit like a spinning top. The craft’s instruments have found minerals on Bennu that formed in the presence of water.

Bennu’s orbit periodically brings it close to Earth. It’ll come within half a million miles in 2060, for example. Predicting its orbit beyond that is tricky. But astronomers give Bennu about a one-in-2700 chance of hitting Earth in the late 22nd century.

OSIRIS-Rex will help narrow down those odds. Precise tracking of the spacecraft will reveal how Bennu’s orbit changes as a result of the Yarkovsky effect. It produces a slight “thrust” as areas warmed by sunlight cool off on the asteroid’s nightside.

Ground-based tracking has shown a “push” of about a hundred miles in 12 years — enough to alter calculations of Bennu’s position. The time OSIRIS-Rex spends with Bennu will refine estimates of the Yarkovsky effect, helping astronomers predict the motions of this possible threat to Earth.

Script by Damond Benningfield

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