Asteroid Bennu: Mystery behind the ejection of particles

The asteroid Bennu, which was discovered in 1999 during a Near-Earth asteroid survey by Lincoln Near-Earth Asteroid Research(LINEAR) has been a subject of curiosity to many scientists and are now beginning to understand the mystery behind the surprising activities on the surface of the asteroid.

The asteroid was studied by NASA’s OSIRIS-REx spacecraft which arrived at Bennu in 2018 after a two-year journey. Not only was the boulder-strewn asteroid shaped like a rough diamond, but also, its surface was frizzling with activity, casting small pieces of rocks into space. Now, more than one year of studies has made scientists understand that these are dynamic particle-ejection events.

In 2019, the OSIRIS-REx spacecraft first observed episodes of particle ejection from 101955 Bennu shortly after entering the orbit of the asteroid, thus making it the first time that an asteroid activity had been observed up close by a spacecraft. The cameras on-board spotted rock particles being ejected into space which was about a third of a mile (565 meters) wide at its equator. The scale of these mass loss events is much smaller than those previously observed at other active asteroids. Generally, most of the asteroids are not active but Bennu is an active asteroid. Active asteroids have asteroid-like orbit but show comet-like visual characteristics. Unlike comets, which are composed of ice, rock, and dust, asteroids are mainly of rock and dust. But some of these rocks can be very lively too.

According to Dante Lauretta, the OSIRIS-REx principal investigator and a professor at the University of Arizona, “We thought that Bennu’s boulder-covered surface was the wild card discovery at the asteroid, but these particle events surprised us”.

A collection of studies in a special edition of the Journal of Geophysical Research shows how these particles act when in space, how they are ejected, and how their trajectories can be used to approximate Bennu’s weak gravitational field. One of the studies led by senior research scientist Steve Chesley in NASA’s Jet Propulsion Laboratory found that most of the pebble-sized rocks from the asteroid were pulled back to Bennu under its gravity or sometimes it bounces back into space after colliding with the surface. Some remain in the orbit for a few days and some completely escape from Bennu’s vicinity.

After tracking hundreds of ejected particles, the team was able to better understand What might be causing the particles to eject from the surface of Bennu and they listed 2 main reasons: thermal stress fracturing and meteoroid impacts. Bennu’s surface temperatures vary over a 4.3 hour rotation period. Although it is extremely cold during the night hours, the asteroid surface warms in the mid-afternoon. As a result of this temperature change, rocks may begin to crack and break down. Eventually, particles could be ejected from the surface and this cycle is called thermal stress fracturing.

The team also determined that meteoroid impacts are another reasonable explanation. It is possible that these small fragments of space rocks could be hitting Bennu where OSIRIS-REx is not observing and shaking those particles with the momentum of their impact. It may even be a combination of these two events, added Chesley. But for more accurate results further observations are needed.
These ejected particles also serve as high-fidelity probes of Bennu’s gravity field.

Many particles orbiting the Bennu are far closer than OSIRIS-REx spacecraft and their trajectories were highly sensitive to the irregular gravity of Bennu. This allowed the team to estimate Bennu’s gravity more precisely than the instruments in OSIRIS-REx.Bennu’s activity can be studied further once the sample is collected and returned to Earth. The spacecraft with the sample is scheduled to return in September 2023.

References, Images and Videos: NASA

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