Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study. On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. Since OSIRIS-REx has been monitoring Bennu, the spacecraft has witnessed plumes of dust shooting out of the asteroid at least 11 times. This uncharacteristic asteroid behavior has led scientists to believe that all prior knowledge of asteroids could be false or more likely, lacking information.
"The discovery of plumes is one of the biggest surprises of my scientific career," said principal investigator Dante Lauretta of the University of Arizona.
OSIRIS-Rex positioned itself in orbit near Bennu and has since been observing the asteroid. Scientists believe Bennu holds evidence of a much younger Solar System, as it is thought to have been formed at that time.
Scientists witnessed the initial plume on 6 January, and the team then watched it very closely for any similar behavior. Over the following two months, they observed the plumes a few more times, and kept track of the particles of dust therein. Some of the dust got blown out to space, while other particles were recaptured in Bennu's orbit, most of them falling back down to the asteroid's surface. At least four chunks, however, stayed in Bennu orbit and are possibly on their way to becoming miniature asteroid moons.
The plumes don't pose a risk to OSIRIS-REx - but that's about all we know about them other than the fact that maybe asteroids are much more active than previously considered. The OSIRIS-REx team is trying to find out more, such as where they come from, and what triggers them.
"We have had spacecraft around other asteroids, and nothing like this was ever reported,", stated planetary astronomer Andrew Rivkin of Johns Hopkins University in Maryland. "The question is, why is this asteroid different?"
Although dust plumes are normally associated with comets, due to ice pockets collapsing under the surface, asteroids do not contain ice, or so we thought. It's not impossible that Bennu contains ice under its surface, although its orbital position is too warm for it to have formed there. Unless, of course, Bennu was formed much farther out in the Solar System and somehow made its way closer with the subterranean ice particles still somewhat intact.
With scientists and NASA eagerly monitoring Bennu, we here on Earth can expect many more exciting discoveries in regards to asteroids' behavior.
As planetary scientist Lori Glaze of NASA expressed her excitement in a recent statement. "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..."
