Asteroid Bennu turns out to be cooler than we have thought. NASA’s OSIRIS-REx spacecraft observes particle plumes erupting from the asteroid’s surface.

OSIRIS-REx spacecraft has been observing the near-Earth asteroid Bennu since December 31st, 2018. The space rock is slightly wider than the Empire State Building.

Observations from the spacecraft reveal the 1,650-foot-wide (500 meters) space rock ejected particles of dust and gravel into space multiple times over the past few months. So, Bennu is an active asteroid, of which scientists have found only a dozen among the nearly 800,000 others observed. It is also the only asteroid humans have observed up close.

“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”

OSIRIS-REx scientists discovered the first particle explosion on Bennu’s surface on January 6th. Nearly a dozen more would come in the next couple of months. After deciding that the particles wouldn’t pose a risk to the spacecraft, the mission team increased the frequency of observations in order to analyze their potential causes. The probe subsequently detected additional particle plumes during the following two months.

“The first three months of OSIRIS-REx’s up-close investigation of Bennu have reminded us what discovery is all about—surprises, quick thinking, and flexibility,” said Lori Glaze, acting director of the Planetary Science Division at NASA Headquarters in Washington. “We study asteroids like Bennu to learn about the origin of the solar system. OSIRIS-REx’s sample will help us answer some of the biggest questions about where we come from.”

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OSIRIS-REx’s observations will additionally shed light on how potentially dangerous asteroids move through space and which of the space rocks miners may want to target down the road, among other things.

Photos from the mission also show Bennu is more rugged than expected. Therefore, challenging the mission team to alter its flight and sample collection plans, due to the rough terrain.

The OSIRIS-REx team didn’t anticipate the number and size of boulders on Bennu’s surface. Based on observations from Earth, scientists expected Bennu to have a smooth surface. Instead, a closer look revealed that Bennu is rough and rugged. So, this will make sample collection more difficult. But NASA is currently working on an alternative approach.

The mission team is developing an updated approach to accurately target smaller sample sites. NASA calls Bullseye TAG.

“Throughout OSIRIS-REx’s operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements,” said Rich Burns, the project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Bennu has issued us a challenge to deal with its , and we are confident that OSIRIS-REx is up to the task.”

The spacecraft also detected magnetite on Bennu’s surface. Thus, supporting an earlier finding that rock interacted with liquid water on Bennu’s parent body.

However, if all goes according to plan, in mid-2020, the probe will dip down and grab a sample of Bennu material, which will come down to Earth in a return capsule in September 2023.

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Thumbnail image: Asteroid Bennu ejecting particles from its surface on January 19. Credit: NASA/Goddard/University of Arizona/Lockheed Martin