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Study: Asteroid Wears Boulder ‘Body Armor’ For Protection

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Bennu asteroid. (Credit: NASA)

A University of Hawaiʻi at Mānoa scientist was part of a study that discovered that the boulder-covered surface of an asteroid within Earth space that came from the asteroid belt in our solar system protects it from small meteoroid impacts.

Researchers, including study co-author UH-Mānoa planetary geologist David Trang, made the discovery about asteroid Bennu’s use of “body armor” through observations by NASA’s OSIRIS-REx spacecraft. Their findings were reported in the study recently published in Nature Geoscience.

Surface of Bennu. (Credit: NASA/Goddard/University of Arizona)

“Measuring craters and their population on Bennu was exceptionally exciting,” Trang, an assistant researcher at the Hawaiʻi Institute of Geophysics and Planetology at the UH-Mānoa School of Ocean and Earth Science and Technology, said in a press release. “At Bennu, we discovered something unique to small and rocky bodies, which expanded our knowledge of impacts.”


The research team used unprecedented, high-resolution global data sets to examine craters on Bennu: images from the OSIRIS-REx Camera Suite and surface-height data derived from the OSIRIS-REx Laser Altimeter, a laser-ranging instrument on the spacecraft.

“These observations give new insight into how asteroids like Bennu respond to energetic impacts,” Edward “Beau” Bierhaus of Lockheed Martin Space in Littleton, Colo., and lead author of the study, said in the press release.

Scientists estimate the age of planetary surfaces by measuring the abundance and sizes of craters, if the craters are not being erased by erosion from wind and water or buried by lava flows or other processes. Impact craters accumulate with time, so a surface with many craters is older than a surface with few craters.


The size of a crater depends on the size of the impactor, with larger impactors generally making larger craters. Since small meteoroids are far more abundant than large meteoroids, celestial objects such as asteroids usually have many more small craters than large ones.

Bennu’s large craters follow this pattern, with the numbers of craters decreasing as their size increases — but only to a point. However, for craters smaller than about 6.6 feet, the trend is backward, with the number of craters decreasing as their size decreases.

This indicates something unusual is happening on Bennu’s surface.


The researchers think Bennu’s profusion of boulders acts as a shield, preventing many small meteoroids from forming craters. Instead, these impacts are more likely to break apart the boulders or chip and fracture them. Also, some impactors that do make it through the boulders make smaller craters than they would if Bennu’s surface was covered in smaller, more uniform particles.

The team also found that the number of small and large craters offered different estimates for Bennu’s surface age, revealing the asteroid’s long and winding road through space.

The result supports the idea that Bennu was formed in our solar system’s main asteroid belt, between Mars and Jupiter, but the pull of gravity from other objects in the solar system sent it to the region of space near Earth.

“The crater sizes found throughout Bennu have helped us understand the story and life of this small asteroid, Bennu; it’s like scars found on a body, which also tells its own story,” Trang said in the press release. “Importantly, these craters told us that Bennu didn’t always live near Earth space as it does today, but once lived in the main asteroid belt. Then, somehow, many of these craters were erased, but then accumulated a new set of craters, which are smaller than those found on the moon due to boulder armor.”

For more information, visit NASA News.

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