Shortly after the NASA OSIRIS-REx spacecraft arrived at the asteroid known as Bennu, it discovered something mysterious. The asteroid is active and is continuously discharging particles into space. NASA wants to gather samples from the asteroid, but it also wants to figure out why Bennu is continuously discharging particles into space.
The OSIRIS-REx team first observed the particle-ejection event in images that were captured by the spacecraft navigation cameras on January 6, only a week after it enters orbit. The team thought at first that the particles were stars behind the asteroid. Closer inspection showed that the asteroid was ejecting material from its surface.
The team spent time to ensure that the ejection didn’t pose a threat to the spacecraft and then began dedicated observations to document the activity fully. The team behind the research says that it has spent the last several months investigating the mystery and notes that it is a good opportunity to expand the knowledge of how asteroids behave.
The three largest particle emission events that the team observed were January 6, January 18, and February 11. The events originated from different locations on the surface of the asteroid, and all three events took place in the late afternoon on the surface of the asteroid. The team also found that after ejection from the surface, the particles either briefly orbited Bennu and fell back to its surface or escaped from Bennu into space.
The particles traveled at 10 feet per second and were smaller than an inch up to four inches in size. About 200 of them were ejected during the largest event on January 6. As for causes, there are three possibilities, including meteoroid impacts, thermal stress fracturing, and released water vapor. The team says it could be more than one mechanism at play. Study will continue, and sample collection is set for the summer of 2020 with samples to be delivered to Earth by September 2023.