NASA’s OSIRIS-REx observes Particles as Big as Softballs ejected from Asteroid “Bennu”

Researchers in charge of NASA‘s OSIRIS-REx mission has regularly recorded detailed observations of shedding material from asteroid Bennu. The spacecraft has, for the first time, given scientists the ability to observe the behavior of the asteroid at close range. The asteroid surface is extremely active and creates a picture of asteroids as being very dynamic.

For the first time, the spacecraft has provided an in-depth look at the nature of particle ejection events on Bennu’s surface, providing researchers with an opportunity to explore the possible mechanisms that cause the asteroid to eject particles into space. The first observations of particles ejected into space from the asteroid surface were made in January 2019, just days after OSIRIS-REx arrived at the asteroid.

OSIRIS-REx uses star location to navigate in space, making sure that it’s on the right path. One researcher looking at photos the spacecraft beamed back to Earth and found something unusual. Photographs of the asteroid silhouetted against the black sky with stars behind it seemed to reveal so many stars. The researcher says he looked at the patterns and pictures of the stars and realized he didn’t know “that star cluster.”

What OSIRIS-REx has observed?

He says he just noticed that there were 200 light dots where there should’ve been about ten stars. But other than too many stars, it looked like a dense part of the sky. He discovered, through closer observation, what he thought was a star cluster was a cloud of tiny particles ejected from the asteroid surface. Researchers have used software algorithms designed to detect and track near-earth asteroids by detecting their motion against background stars on the images. The algorithm found that the largest particles to be ejected from the surface of the asteroid were around two inches in diameter.

The small size and low velocity of the particles appear like a shower of tiny pebbles in super slow motion. According to the researchers on the mission, none of the particles poses a threat to the spacecraft. A team dedicated to monitoring objects ejected from the asteroid counted 668 particles between January and September 2019, with the majority measuring between 0.5 and one centimetres travelling at around eight-inch per second. On average, 1 to 2 particles are kicked up per day, with most of the material falling down on the asteroid. Many-particle ejections occur late in the afternoon while the rock is heating up, suggesting thermal cracking is a driver.