OSIRIS-REx Observes Asteroid Bennu Particle Ejection Events Up Close

Researchers in charge of the OSIRIS-REx mission have reported detailed observations of asteroid Bennu shedding material regularly. The spacecraft has provided scientists with the ability to observe the asteroid's activity at close range for the first time. The surface of the asteroid is highly active and builds a picture of asteroids as being very dynamic.The spacecraft has provided an in-depth look for the first time at the nature of particle ejection events on the surface of Bennu, providing researchers an opportunity to talk about the likely mechanisms causing the asteroid to eject particles into space. The first observations of particles from the asteroid's surface being ejected into space were made in January 2019, only days after OSIRIS-REx arrived at the asteroid.

OSIRIS-REx uses the position of stars to navigate in space, ensuring it's on the correct path. One researcher looking at images the spacecraft beamed back to Earth and noticed something strange. Photos of the asteroid silhouetted against the black sky with stars behind it appeared to show too many stars. The researcher says that he was looking at the star patterns and images and thought that he didn't remember "that star cluster."

He says he only noticed because there were 200 dots of light where there should've been about ten stars. But other than too many stars, it looked like a dense part of the sky. With closer inspection, he determined what he thought was a star cluster was a cloud of tiny particles ejected from the asteroid surface. Researchers used software algorithms designed to discover and track near-Earth asteroids by detecting that motion against background stars on the images. The algorithm determined that the largest particles being ejected from the asteroid's surface were about two inches in diameter.

The small size and low velocity of the particles look like a shower of tiny pebbles in super slow motion. None of the particles are a threat to the spacecraft, according to the researchers on the project. Between January and September 2019, a team dedicated to tracking objects ejected from the asteroid counted 668 particles, with the majority measuring between 0.5 and one centimeter moving at about eight-inches per second. On average, 1 to 2 particles are kicked up per day, with most of the material falling back on the asteroid. Most particle ejections happen in the late afternoon when the rock heats up, suggesting that thermal cracking is a driver.