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OSIRIS-REx Target Bennu Contains Bits Of Previously Visited Asteroid

By Nicholas Gerbis, Mark Brodie
Published: Tuesday, October 20, 2020 - 4:34pm
Updated: Wednesday, October 21, 2020 - 12:16pm

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NASA’s OSIRIS-REx mission readies itself to touch the surface of asteroid Bennu
NASA/Goddard/University of Arizona
NASA’s OSIRIS-REx mission readies itself to touch the surface of asteroid Bennu.

The OSIRIS-REx mission is designed to retrieve a sample from the asteroid Bennu. But Bennu itself holds samples from another asteroid NASA visited not long ago.

In September, University of Arizona scientists reported bright rocks on Bennu containing pyroxene. There was just one problem: the mineral forms under conditions incompatible with Bennu's makeup.

If the asteroid had ever been exposed to the kind of heat and pressure it takes to make pyroxene, Bennu would no longer have the water-bearing minerals that make it such an appealing target for exploration. 

A more likely explanation is that pieces of another rocky body were scattered on Bennu's surface. 

In fact, the boulders look like they belonged to the large asteroid Vesta, which NASA's Dawn mission visited in 2011.

"We've not visited that many asteroids with spacecraft. So for material from one asteroid we visited with a spacecraft to be on another was a bit of a puzzle," said UA's Dani DellaGiustina, Image Processing Lead Scientist on OSIRIS-REx.

Bennu
NASA/Goddard/University of Arizona
The asteroid Bennu.

Scientists believe material from broken-off bits of Vesta collided with Bennu's larger "parent asteroid," a testament to the complexity of gravity — and of asteroid family trees.

The surface of Bennu is extremely rocky and unstable — Heather Enos, deputy principal investigator on the OSIRIS-REx Mission at the University of Arizona, spoke of the challenges of landing the craft on an asteroid 200 million miles from Earth.

“We thought we would need to go somewhere that was 25 meters in size, but we are navigating more to like a 5-meter space.”

The OSIRIS-REx craft touched down on that parking-space sized spot on Bennu’s surface after a four-and-a-half hour approach.

The lander spent about ten seconds on the surface of the asteroid, collecting surface samples of rock with an 11-foot probe, before taking off again to return the space-booty to Earth.

When asked to describe the touch-and-go landing, UA principal mission investigator Dante Lauretta said “exactly perfect.” 

The samples will be delivered to Earth in 2023; researchers hope to use them to learn more about the origins of our solar system.

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