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NASA Osiris-rex reveals secrets of ‘rubble pile’ asteroid Bennu ahead of daring heist

The past 4.5 billion years have been an incredibly lonely period for the asteroid 101955 Bennu. A gigantic impact in the early days of the solar system smashed an ancient cosmic rock to pieces, ejecting dust and debris into the void. Gravity forced the rubble pile to clot together and, ever since, it’s been wandering alone as Bennu, the space rock shaped like a spinning top. For billions of years, it’s drifted around the sun between Earth and Mars, untouched and unaccompanied.

Until NASA’s Osiris-rex spacecraft greeted it in orbit on Dec. 3, 2018. 

After a 27-month journey from Earth, NASA’s asteroid-chasing spacecraft sidled up to Bennu for a closer look. Bennu finally had company. The spacecraft is part of an ambitious plan to return pieces of Bennu to Earth, the first time a NASA mission has attempted such a feat. 

Since arriving at the asteroid, Osiris-rex has been busy taking measurements and sizing Bennu up. It performed close flybys to get a high-resolution look at the surface and caught the asteroid unexpectedly spewing debris into space in late 2019. Its five instruments have been gathering data, mapping Bennu’s surface and slowly piecing together the asteroid’s story. Where did it come from? What is it made of? Will it collide with the Earth? (That last one isn’t likely, but Bennu is expected to pass close-by next century.) 

On Thursday, a suite of new studies, published in the journals Science and Science Advances, shed light on these questions, revealing more about Bennu’s boulder-riddled surface. In addition, Osiris-rex has allowed for a detailed examination of “Nightingale” crater, the target of Osiris-rex’s daring heist set for Oct. 20.

This artist’s impression shows Osiris-Rex reaching for Bennu’s surface.

NASA/Goddard/University of Arizona

“As a set, these papers help us to fill in more about Bennu’s history and allow us to anticipate what will be returned in the sample,” says Hannah Kaplan, a space scientist at NASA’s Goddard Space Flight Center.

And the collection of studies helps answer even bigger questions about the early solar system. Bennu may appear boring, a dull gray space rock spinning through infinity. But it’s actually a message in a bottle. Adrift on the cosmic seas for eons, it contains secrets and clues about the solar system’s formation and evolution locked within its rocky exterior. 

A castaway

Bennu is, unflatteringly, described as a “rubble pile.” It’s about as wide as the Empire State Building is tall. From a distance, it looked smooth — but as Osiris-rex approached, the truth became clearer. “When we got there, we found the surface was covered in boulders,” explains Kaplan.

Officially named “Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer,” Osiris-rex has been circling Bennu, using the asteroid’s feeble gravity to pass around it, for almost two years. In that time, it has pointed an array of instruments at its surface that can see in visible light, infrared and X-rays. In totality, they allow scientists to get a clear visual of the asteroid and determine the