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This asteroid is ejecting particles into space. A spacecraft could tell us why

Bennu is a near-Earth asteroid that is currently in orbit around NASA’s OSIRIS-REx mission. And in October, the spacecraft will play “tag” the asteroid, collect samples from the surface and return them to Earth by 2023.

While many asteroids seem dull and lifeless, this one is surprisingly active. And he may not be alone.

The first detailed images of the spacecraft revealed that Bennu is an asteroid filled with rubble in the shape of a spinning top and covered with boulders, not at all the smooth asteroid they expected. Rubble heaps asteroids are literally piles of rocks held together by gravity.

Shortly after OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security ̵

1; Regolith Explorer) arrived in Bennu, the spacecraft’s cameras caught something else unexpected.

The asteroid was releasing small plumes of material into space, mostly small pebbles, and on a regular basis.

“We thought the Bennu boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us,” said Dante Lauretta, principal investigator of OSIRIS-REx and a professor at the University of Arizona. in a statement.

This image shows the asteroid Bennu ejecting rock particles from its surface on January 19, 2019.

“We have spent the last year investigating Bennu’s active surface, and it has provided us with a tremendous opportunity to expand our knowledge of how active asteroids behave.”

This week, a series of studies detailing Bennu particle ejections were published in the Journal of Geophysical Research: Planets.

At first, the researchers didn’t realize that Bennu was throwing small pebbles into space.

NASA's mission captures a nearby asteroid that ejects material into space

But Carl Hergenrother, the mission chief astronomer and scientist at the University of Arizona’s Lunar and Planetary Laboratory, took a closer look at the images captured by OSIRIS-REx. They were taken a few days after the spacecraft arrived for its first close encounter with the asteroid.

OSIRIS-REx used the stars to help it reach the asteroid after launch in 2016. The images from its navigation camera are stars in the background, which are compared to star maps to keep it on track.

Hergenrother was analyzing the images sent by the spacecraft when he realized that the asteroid was surrounded by too many stars.

“I was looking at the star patterns in these images and thought, ‘huh, I don’t remember that star cluster,'” Hergenrother, also an author of the study collection, said in a statement.

“I only noticed this because there were 200 points of light where there should be about 10 stars. Other than that, it seemed to be just a dense part of the sky.”

It turned out they weren’t stars at all. Those points of light represented clouds of particles ejected from the surface of the asteroid.

The Oddities of Bennu

Since OSIRIS-REx began observing Bennu’s unusual behavior, the probe has witnessed more than 300 of these particle ejection events. But they are not all the same.

Sometimes, particles are ejected with enough force to escape into space. Others briefly orbit the asteroid. And many of the particles fall back onto the asteroid.

Much of this activity occurs over a two-hour afternoon to evening period on the asteroid.

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These events have been witnessed by the spacecraft’s “eyes”, or by the suite of navigation cameras that keep an eye on the fleeing particles around Bennu. The observations helped scientists realize that Bennu regularly loses material.

But these particles, the largest of which are about 2 inches in diameter, pose no threat to the spacecraft. Bennu’s weak gravity means that the particles move in slow motion relative to the spacecraft.

“Space is so empty that even when the asteroid is launching hundreds of particles, as we have seen in some events, the chance of one of them hitting the spacecraft is extremely small,” Hergenrother said. “And even if that does happen, the vast majority of them aren’t fast enough or big enough to cause damage.”

NASA's OSIRIS-REx mission prepares for landing on an asteroid

Between January and September 2019, scientists studied about 668 particles that emerged from Bennu. They measured between 0.2 and 0.4 inches and only moved about 8 inches per second, or the equivalent of a cockroach moving across the ground. The fastest reached 9.8 feet per second, but there was only one case of this.

On average, one or two particles are ejected from the asteroid each day, with most of them returning to the asteroid. This means that Bennu isn’t losing a lot of mass.

“To give you an idea, all of those 200 particles that we observed during the first event after arrival would fit on a 4 x 4 inch tile,” he said. “The fact that we can even see them is a testament to the capabilities of our cameras.”

What causes it?

To understand the cause behind these particle powders, scientists studied the possible release of water vapor on the asteroid, the impacts of tiny meteoroids or rocks breaking up on the surface.

Bennu completes one rotation every 4.3 hours, which constantly exposes the boulders on the asteroid to daytime heating and nighttime cooling. This would cause rocks to crack and crack over time, launching their particles into space.

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It is also possible that small rocks like meteoroids hit the asteroid in the afternoon, which would push the material on the asteroid into space.

Given that most of the activity has been observed during the afternoon, when the rocks are warming, it suggests that one or both of these are strong possibilities. Based on the gravity of the asteroid, it doesn’t take long to disturb its surface.

However, further observations are needed to know for sure. But the particles revealed information about the asteroid’s gravity that the spacecraft would only know if it got too close – dangerously close – to the asteroid.

“The particles were an unexpected gift for gravity science to Bennu as they allowed us to see tiny variations in the gravitational field of the asteroid that we wouldn’t have known otherwise,” said Steve Chesley, lead author of one of the studies and senior researcher. at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement.

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By tracking the trajectories of the particles, the scientists noticed that some hovered around the asteroid for a few hours before landing again, while others are orbiting the sun on their own.

The researchers also witnessed interesting events such as a particle that orbited the asteroid for a week and others that bounced into space after hitting the surface again.

“A particle fell, hit a boulder and went back into orbit,” Hergenrother said. “If Bennu has this kind of business, then there’s a good chance all asteroids do, and that’s really exciting.”

These findings have suggested why it is so important to send a spacecraft to study an asteroid, which is revealing that these celestial bodies can be dynamic, active, and completely unexpected.

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