NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft observed craters on Bennu's boulder-covered surface as protection against small meteoroid impacts. During its trip to the near-Earth asteroid Bennu, OSIRIS-REx collected a small sample for study on Earth. From Cape Canaveral Air Force Station, the mission launched on September 8, 2016. A sample will be returned to Earth in 2023 after the spacecraft reaches Bennu in 2018.

In this month's issue of Nature Geoscience, lead author Edward (Beau) Bierhaus of Lockheed Martin Space, Littleton, Colorado, said the observations shed light on how asteroids like Bennu respond to energetic impacts.

In other words, Bennu formed from debris left after an ancient impact destroyed a much larger asteroid. In the aftermath of the collision, the fragments coalesced under their weak gravity to form Bennu.

The team analyzed craters on Bennu using unprecedented, high-resolution data sets from OSIRIS-REx camera images and data from the Laser Altimeter (lidar) on the spacecraft (OSIRIS-REx), the spacecraft's laser ranging instrument.

The process of measuring craters on Bennu and their density was particularly exciting for David Trang, a co-author of the paper from the University of Hawaii at Mnoa, Honolulu. A discovery at Bennu expanded our knowledge of impacts and was unique to small and rocky bodies.

By measuring the size and abundance of craters, scientists can estimate how old a planet's surface is. Over time, impact craters accumulate, so a surface with a large number of craters is older than one with a few. A crater's size is also related to its impactor's size, with larger impactors making larger craters. Asteroids, which are celestial objects, tend to have many more small craters than large ones because small meteoroids are so abundant.

Similarly, Bennu's larger craters decrease in number as their size increases. However, the trend is backward for craters smaller than 6.6 to 9.8 feet (around 2 – 3 meters) in diameter, with the number of craters decreasing as their size decreases. Bennu's surface is showing some unusual behavior.

Many small meteoroids are prevented from forming craters by Bennu's abundance of boulders. As a result, these impacts more commonly fracture or chip boulders instead of breaking them apart. As a result, some impactors that do pass through the boulders create smaller craters than if Bennu's surface was covered in smaller, more uniform particles, like beach sand.

Bennu's surface changes differently when this activity is performed than objects with fine-grained or solid surfaces. One of the fastest-acting processes on an asteroid's rubble pile surface is the displacement of boulders caused by a small impact. Consequently, the surface of Bennu appears many times younger than the interior,' said Bierhaus.