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article imageSurprise! — Asteroid Bennu caught spewing particles into space

By Karen Graham     Mar 20, 2019 in Science
NASA’s OSIRIS-REx spacecraft has spotted something that hasn’t been seen up close on an asteroid before: plumes of particles erupting into space.
We would need to go back in time billions of years — to a time when a cloud of cosmic gases and dust turned into a spinning disc, eventually heating up and igniting, forming our sun — to see something similar The particles and pieces that remained ended up forming planets and moons, while the very smallest particles became comets and asteroids.
And as the Atlantic notes, these comets, and asteroids were the crumbs left over from a magnificent feast. Now, one of those crumbs is exploding: the near-Earth asteroid Bennu.
The 1,650-foot-wide (500 meters) asteroid has been caught in the act of spewing particles into space, not once, but multiple times by NASA's OSIRIS-REx spacecraft.
The $800 million OSIRIS-REx mission (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) began with the launch of the spacecraft on September 8, 2016, from Cape Canaveral Air Force Station in Florida. The spacecraft arrived in orbit around Bennu on December 31, 2018.
As OSIRIS-REx surveys asteroid Bennu in detail  the mission team is beginning to identify potential ...
As OSIRIS-REx surveys asteroid Bennu in detail, the mission team is beginning to identify potential locations for the spacecraft to collect a sample.
NASA/Goddard/University of Arizona
If everything goes according to plan, the probe will spend the next two years studying Bennu, then secure a sample from the surface before heading back to Earth in March 2021.
The OSIRIS-REx team initially spotted the particle plumes in images taken on January 6, while the spacecraft was orbiting Bennu at a distance of about one mile (1.61 kilometers).
The discovery "is probably the biggest surprise of the early stages of the OSIRIS-REx mission and, I would say, one of the biggest surprises of my scientific career," OSIRIS-REx principal investigator Dante Lauretta, of the University of Arizona, said during a news conference March 19, reports Geek Wire.
This view of the asteroid Bennu ejecting particles from its surface on Jan. 19  2019  was created by...
This view of the asteroid Bennu ejecting particles from its surface on Jan. 19, 2019, was created by combining two images taken by NASA's OSIRIS-REx spacecraft: a short-exposure photo, which shows the asteroid clearly, and a long-exposure version, which shows the particles clearly.
NASA/Goddard/University of Arizona/Lockheed Martin
It was determined that the particles ejecting off the asteroid didn't pose any danger to the spacecraft, so the team continued to observe the plumes and their possible causes. “We don’t know the mechanism that is causing this right now,” Lauretta said.
Lauretta says the team has observed 11 ejection events, three of them "substantial," involving dozens to more than 100 particles. The particles vary in size from a few centimeters to up to tens of centimeters in diameter, Lauretta said. And the particles' velocities vary widely, too. Some move at up to 7 mph (11 kilometers per hour), fast enough to escape Bennu's weak gravity and cruise into interplanetary space.
"Basically, it looks like Bennu has a continuous population of particles raining down on it from discrete ejection events across its surface," Lauretta said. "This is incredibly exciting."
Interestingly, while it is not known for sure what is causing the ejection events, NASA has noticed they have occurred around the time of Bennu's closest passage to the sun during the asteroid's elliptical orbit. That occurred on January 10 this year. This has led to speculation that solar heating of the surface of the asteroid may be a driver.
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