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

This asteroid has been full of surprises since a NASA spacecraft arrived to orbit it in December 2018.

Bennu is a near-Earth asteroid that is currently being orbited by 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 it may not be alone.

The spacecraft’s first detailed images revealed that Bennu is a rubble-pile asteroid shaped like a spinning top and covered with boulders, not at all the smooth asteroid they expected. Rubble-pile asteroids are literally piles of rocks held together by gravity.

Shortly after the arrival of OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security — Regolith Explorer) at Bennu, the spacecraft’s cameras captured something else unexpected.

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

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

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

This week, a suite of studies detailing Bennu’s particle ejections published in the Journal of Geophysical Research: Planets.

At first, researchers didn’t realize that Bennu was launching little pebbles into space.

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

OSIRIS-REx used stars to help it reach the asteroid after launching in 2016. Its navigation camera images stars in the background, which are compared with star charts to keep it on track.

Hergenrother was analyzing images sent back by the spacecraft when he realized 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 on the study collection, said in a statement.

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

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

Bennu’s oddities

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

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

Much of this activity happens during a two-hour afternoon to evening time period on the asteroid.

These events have been witnessed by the spacecraft’s “eyes,” or the navigation camera suite that keeps a lookout for runaway particles around Bennu. The observations helped scientists realize that Bennu sheds material regularly.

But these particles, the largest of which are about 2 inches in diameter, don’t pose a threat to the spacecraft. Bennu’s weak gravity means that the particles are moving in slow motion compared to the spacecraft.

“Space is so empty that even when the asteroid is throwing off hundreds of particles, as we have seen in some events, the chances of one of those hitting the spacecraft is extremely small,” Hergenrother said. “And even if that were to happen, the vast majority of them are not fast or large enough to cause damage.”

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

This averaged out to one or two particles being ejected from the asteroid each day, with the majority of them returning to the asteroid. This means Bennu isn’t losing much mass.

“To give you an idea, all of those 200 particles we observed during the first event after arrival would fit on a 4-inch 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 dust-ups, scientists investigated the possible release of water vapor on the asteroid, impacts by tiny meteoroids or rocks cracking on the surface.

Bennu completes a rotation every 4.3 hours, which constantly exposes boulders on the asteroid to day heating and night cooling. This would cause the rocks to crack and break down over time, flinging their particles out into space.

It’s also possible that small rocks like meteoroids are striking the asteroid in the afternoon, which would kick material on the asteroid out into space.

Given the fact that the most activity was seen during the afternoon when the rocks are experiencing heating suggests either or both of these are strong possibilities. Based on the gravity of the asteroid, it doesn’t take much to disturb its surface.

However, more 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 at Bennu since they allowed us to see tiny variations in the asteroid’s gravity field that we would not have known about otherwise,” said Steve Chesley, lead author of one of the studies and senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement.

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

The researchers also witnessed interesting events like one particle that orbited the asteroid for as long as a week, and others that ricocheted back into space after they hit the surface again.

“One particle came down, hit a boulder and went back into orbit,” Hergenrother said. “If Bennu has this kind of activity, then there is a good chance all asteroids do, and that is really exciting.”

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