The asteroid, which is being studied by NASA‘S OSIRIS-REx, shows surprising activity on its surface, and scientists are beginning to understand what could be causing it.
When NASA’s OSIRIS-REx spacecraft landed on asteroid (101955) Bennu, mission scientists knew their probe was orbiting something special. Not only was the boulder-strewn asteroid shaped like a rough diamond, but its surface crackled with activity, throwing small pieces of rock into space. Now, after more than a year and a half in close contact with Bennu, they are beginning to better understand these dynamic particle ejection events.
A collection of studies in a special edition of the Journal of Geophysical Research: Planets deals with the asteroid and these enigmatic particles. The studies provide a detailed look at how these particles act in space, possible clues as to how they are ejected, and even how their trajectories can be used to approximate Bennu’s weak gravitational field.
Generally, we consider comets, not asteroids, to be active ones. Comets are made up of ice, rock and dust. When those ices are heated by the Sun, vapor sizzles from the surface, the dust and pieces of the comet’s core are lost in space and a long dusty tail forms. Asteroids, on the other hand, are made up mostly of rock and dust (and possibly a smaller amount of ice), but it turns out that some of these space rocks can be surprisingly vibrant too.
“We thought Bennu’s boulder-covered surface was the wild card discovery of 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. “We have spent the last year investigating Bennu’s active surface and it has provided us with an extraordinary opportunity to expand our knowledge of how active asteroids behave.”
Cameras on OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer) spotted rock particles repeatedly thrown into space during a January 2019 investigation of the asteroid, which is about a third of a mile away. (565 meters) wide at its equator.
One of the studies, led by senior researcher Steve Chesley of NASA’s Jet Propulsion Laboratory in Southern California, found that most of these pebble-sized pieces of rock, typically measuring about a quarter of an inch (7 millimeters), are were withdrawn to Bennu under the asteroid’s weak gravity after a short jump, sometimes even bouncing into space after colliding with the surface. Others took longer to return to the surface, remaining in orbit for a few days and up to 16 revolutions. And some were expelled with enough grit to escape Bennu’s environs entirely.
By tracing the journeys of hundreds of ejected particles, Chesley and his collaborators were also able to better understand what could cause the particles to be launched from Bennu’s surface. The particle sizes match what is expected for thermal fracture (as the surface of the asteroid is repeatedly heated and cooled as it rotates), but the locations of the ejection events also match the modeled impact locations of the meteoroids (small rocks hitting the surface of Bennu as it orbits the Sun). It could also be a combination of these phenomena, Chesley added. But to arrive at a definitive answer, further observations are needed.
While their very existence poses numerous scientific questions, the particles also served as high-fidelity probes of Bennu’s gravitational field. Many particles orbited Bennu much closer than would have been safe for the OSIRIS-REx spacecraft, and thus their trajectories were highly sensitive to Bennu’s erratic gravity. This allowed the researchers to estimate the gravity of the Bennu even more accurately than was possible with the tools of OSIRIS-REx.
“The particles were an unexpected gift for gravity science to Bennu as they allowed us to see tiny variations in the asteroid’s gravitational field that we wouldn’t have known otherwise,” Chesley said.
On average, only one or two particles are ejected per day, and because they are in a very low gravity environment, most move slowly. As such, they pose a small threat to OSIRIS-REx, which will attempt to briefly land on the asteroid on October 20 to collect surface material, which may even include particles that were ejected before falling back to the surface.
If all goes as planned, the spacecraft will return to Earth in September 2023 with a cache of Bennu’s material that scientists can study further.
NASA’s Goddard Space Flight Center in Greenbelt, Maryland provides overall mission management, systems engineering, and mission security and assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona at Tucson is the principal investigator, and the University of Arizona also leads the science team and the scientific observation planning and data processing of the mission. Lockheed Martin Space in Denver built the spacecraft and provides flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission of NASA’s New Frontiers Program, which is managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington.