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On the Asteroid Bennu, Surprising Activity and Rugged Terrain

OSIRIS-REx’s mission to the asteroid has come across some surprising findings that might affect its ability to collect samples to bring back to Earth.

ByKatherine J. WuNOVA NextNOVA Next

The asteroid Bennu, ejecting particles from its surface on January 19. Image Credit: NASA/Goddard/University of Arizona/Lockheed Martin

The asteroid Bennu is proving tough to tame.

According to a collection of studies published yesterday in the journal Nature, the 1700-foot-wide rocky body, which orbits the Sun in a rough loop that’s often close to Earth, has an unexpectedly rubbly and active surface that appears to be spewing gravel-sized particles into space. These geologic surprises could throw a serious wrench into NASA’s mission to scoop up samples of the asteroid to ferry back to Earth for further analysis, via the OSIRIS-REx spacecraft.

“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,” Lori Glaze, acting director of NASA’s Planetary Science Division, said in a statement.

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OSIRIS-REx has been lurking in Bennu’s neighborhood since December 2018. Its ultimate objective is to collect dust from the asteroid in 2020 and shepherd it back to Earth by 2023, allowing researchers to analyze the material for clues about the origins of life in our Solar System. Asteroids like Bennu are thought to be million- to billion-year-old relics preserved by the vacuum of space. With their hydrated minerals, Bennu or similar objects might have even gifted our planet with some of its water in the distant past.

But the asteroid’s rugged, dynamic physique might make for an obstacle course that OSIRIS-REx wasn’t designed to handle. When the spacecraft was originally dispatched, researchers had expected Bennu to be cloaked in dust that OSIRIS-REx could vacuum up after touching down on its face.

Now it’s clear that what actually speckles Bennu’s surface is a shocking series of hefty boulders, some of which could be as large as 150 feet across. Without the equipment to pulverize these hunks of rock, OSIRIS-REx might be ill-equipped to navigate the terrain for touchdown. But not all hope is lost—and the NASA team has already proposed some workarounds. One option might be for the spacecraft to instead shoot a puff of gas at the asteroid and snatch up any dust particles that are jostled loose, reports Sarah Kaplan at The Washington Post.

These contingency plans come on the heels of another dusty discovery: It turns out Bennu is no stranger to spraying its surroundings with dust. Since its first encounter with the asteroid, OSIRIS-REx has taken note of 11 instances in which Bennu has belched out plumes of particles and pebbles ranging from inches to perhaps feet in diameter.

Bennu isn’t the only asteroid that’s been caught blasting material out into space, but it joins a very exclusive club. Of the 8,000 asteroids discovered to date, only 12 have been classified as “active.” And of these dozen, Bennu is still a bit of an oddball: Some of its debris appears to have been locked into strange, cloudy orbits around the asteroid like tiny, transient moons.

“That has never been seen before in any solar-system object,” Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson told Marina Koren at The Atlantic.


Bennu’s surface is covered in a surprising number of large boulders. The large, light-colored boulder just below the center of the image is about 24 feet wide, which is roughly half the width of a basketball court.

Image Credit: NASA/Goddard/University of Arizona

It’s still unclear what’s causing the ejections, but astronomers already have theories. The plumes might be byproducts of Bennu’s aggressive spinning, or minor collisions between the asteroid and wayward debris. Another possibility is that water ice beneath its surface (which itself isn’t a guarantee) sublimates into vapor when the asteroid approaches the Sun, jettisoning particles in response to the heat.

Whatever the source of these shake-ups, this space rock’s story is only just beginning to unfold. “Bennu is already surprising us, and our exciting journey there is just getting started,” Lauretta said in a statement.

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