The asteroid that is being studied by NASA‘s OSIRIS-REx, shows some surprising activity on its surface and scientists are beginning to understand what could be causing it.
When NASA’s OSIRIS-REx spacecraft arrived on asteroid (101955) Bennu, the scientists on the mission knew that their spacecraft was orbiting something special. The rock-strewn asteroid not only had the shape of a rough diamond, but its surface burst with activity, throwing small pieces of rock into space. Now, after more than a year and a half with Bennu, they are beginning to better understand these dynamic particle ejection events.
A collection of studies in a special issue of the Journal of Geophysical Research: The planets settle on the asteroid and these mysterious particles. The studies provide a detailed look at how these particles act when in space, possible clues as to how they were ejected, and even how their trajectories can be used to approximate Bennu’s weak gravitational field.
We usually consider comets, not asteroids, to be active. Comets are made up of ice, rocks and dust. As these ice is heated by the sun, the vapor rises from the surface, dust and pieces of the comet’s nucleus are lost in space and a long dusty tail forms. Asteroids, on the other hand, are made up mostly of rocks and dust (and perhaps less ice), but it turns out that some of these cosmic rocks can also be surprisingly lively.
“We thought Bennu’s rocky surface was the discovery of a wild card in the asteroid, but these particle events definitely surprised us,” said Dante Laureta, chief researcher at OSIRIS-REx and a professor at the University of Arizona. “We spent the last year investigating Bennu’s active surface, and it provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”
OSIRIS-REx cameras (short for Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer) note that rock particles were repeatedly launched into space during an asteroid exploration in January 2019, which is about a third of a mile (565 meters)) wide at its equator.
One study, led by senior scientist Steve Chesley of NASA’s Southern California Jet Propulsion Laboratory, found that most of these pebbles the size of pebbles, usually about a quarter of an inch (7 millimeters) in size, were pulled back to Bennu under the asteroid’s weak gravity after a short jump, sometimes even ricocheting back into space after colliding with the surface. Others took longer to return to the surface, remaining in orbit for several days and up to 16 revolutions. And some were thrown out with enough noise to escape completely from the surroundings of Bennu.
By tracking the travels of hundreds of ejected particles, Chesley and his associates were also able to better understand what could cause the particles to launch from Bennu’s surface. The size of the particles coincides with what is expected for thermal fracturing (because the asteroid’s surface is repeatedly heated and cooled as it rotates), but the location of the ejection events also corresponds to the modeled impact points of the meteoroids (small rocks hitting the surface of Bennu as it orbits the Sun). It could even be a combination of these phenomena, Chesley added. But more observations are needed to reach a definitive answer.
Although their very existence raises many scientific questions, the particles have also served as a high-precision probe in Bennu’s gravitational field. Many particles orbited Benu much closer than they would have been safe for the OSIRIS-REx spacecraft, so their trajectories were highly sensitive to Bennu’s irregular gravity. This allowed the researchers to estimate Bennu’s gravity even more accurately than was possible with the OSIRIS-REx instruments.
“Particles were an unexpected gift to the science of gravity at Bennu, as they allowed us to see small variations in the asteroid’s gravitational field that we wouldn’t otherwise know about,” Chesley said.
On average, only one or two particles are ejected per day, and because they are in a medium with very low gravitational motion, most move slowly. As such, they pose a small threat to OSIRIS-REx, which will attempt to briefly touch the asteroid on October 20 to scoop up surface material, which may even include particles that have been ejected before falling back to the surface.
If all goes according to plan, the spacecraft will return to Earth in September 2023 with a cache of Bennu materials so that scientists can study further.
NASA’s Goddard Space Flight Center in Greenbelt, Maryland, provides overall mission management, systems engineering, and safety and security for OSIRIS-REx. Dante Laureta of the University of Arizona at Tucson is the lead researcher, and the University of Arizona also leads the research team and the planning and processing of mission data. Lockheed Martin Space in Denver built the spacecraft and provided flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA’s New Borders Program, which is operated by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s scientific mission directorate in Washington.