Scientists are wondering why asteroid Bennu is ejecting rock particles off to space. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, & Security-Regolith Explorer) NASA spacecraft has been to Bennu, an asteroid with a rough diamond shape and a surface with much activity.

Scientists found that it is shedding small rock pieces off to space, and after studying the phenomenon for over one and a half years, they have begun to understand these events.

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Various Studies on Asteroids

The Journal of Geophysical Research has recently published a unique collection of Bennu studies edition. This is a compiled volume of studies that showed a detailed profile on how the particles move in space; the ways on how they could be ejected; and the trajectories they take, which can estimate the weak gravitational field of Bennu.

Asteroids are mainly composed of dust and rock, with ice in possibly smaller quantities. Bennu's equatorial width measures approximately 565 meters.

According to the University of Arizona professor and principal investigator of OSIRIS-REx Dante Lauretta, they thought that the boulders covering Bennu's surface were already a surprising discovery before discovering the particle ejection events.

He says Bennu allowed them to expand their knowledge of the behavior of active asteroids.

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Ejection of Rock Particles

The cameras carried by OSIRIS-REx recorded repeatedly ejected rock particles when it surveyed Bennu in January 2019.

A study led by Steve Chesley, Jet Propulsion Laboratory, senior research scientist, found that most of the pebble-sized rocks measured roughly seven millimeters, have been pulled back by Bennu's weak gravity field after their short stint into space. They sometimes even ricochet back to space again after they hit the asteroid's surface.

For other rocks, their return to the mother asteroid's surface took a long time. They sometimes stay in orbit on Bennu for several days at a maximum of 16 complete revolutions. And finally, some rocks are ejected forcefully enough to break free from Bennu's influence and pull.


Tracking the Ejected Rocks

Chesley's research team found the possible reason for the particles' launch from Bennu. Their small size corresponds to the expected size produced by thermal fracturing, which is caused the asteroid's repeated heating and cooling in its rotation.

The ejection location also matches model impact locations from meteoroids that hit Bennu's surface during its orbit. Chelsey says that these phenomena may all contribute to the ejection, but they need more observations for a conclusive answer.


Measuring Bennu's Gravitational Field

The particles are also useful in serving as high-fidelity probes that help scientists measure Bennu's gravity field. Many of the particles orbiting the asteroid have trajectories that are highly sensitive to Bennu's irregular gravity, allowing researchers to produce a more precise estimate of the asteroid's gravity than the estimate that the instruments of OSIRIS-REx can produce.

Chelsey says the particles helped them see the tiny variations in the gravity field of Bennu that they could not have otherwise known.

OSIRIS-Rex is scheduled to land on Bennu briefly on October 20 to obtain sample material from the surface. This could include previously ejected particles that returned to Bennu's surface. It is scheduled to go back home in September of 2023, bringing some of the rock particles ejected from asteroid Bennu.

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