Study provides new insights about the surface and structure of asteroid Bennu

When NASA's OSIRIS-REx spacecraft collected samples from asteroid Bennu's surface in 2020, forces measured during the interaction provided scientists with a direct test of the poorly understood near-subsurface physical properties ...

"The low gravity of rubble-pile asteroids such as Bennu weakens its near-subsurface by not compressing the upper layers, minimizing the influence of particle cohesion," said SwRI's Dr. Kevin Walsh, lead author of a paper about this research published in the journal Science Advances . "We conclude that a low-density, weakly bound subsurface layer should be a global property of Bennu, not just localized to the contact point." Fitting its designation as a "rubble-pile asteroid ," Bennu is a spheroidal collection of rock fragments and debris 1,700 feet in diameter and held together by gravity. It is thought to have been formed after a collision involving a larger main-asteroid-belt object. Rocks are scattered across its heavily cratered surface, indicating that it has had a rough-and-tumble existence since being liberated from its much larger parent asteroid some millions or billions of years ago.

The goal of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission is to collect and return at least 60 grams of surface material from Bennu and deliver it to Earth in 2023. Sample collection activities provided additional insights. According to Walsh, researchers involved in the OSIRIS-REx mission have so far measured Bennu's thermal properties and craters to estimate the strength and porosity of discrete particles of rubble-pile asteroids. The ensemble of particles (or regolith) at an asteroid's surface controlling and influencing long-term evolution has not been probed directly until now.

An SwRI-led study found that the surface regolith of the asteroid Bennu is primarily loose rubble. Images taken before and after the touch-and-go sample collection indicate surface disturbances up to 15 inches away. Credit: NASA/Goddard/University of Arizona Read more

A SwRI-led study found that the rocky fragments dominating the surface asteroid Bennu are weakly bound, exhibiting near-zero cohesion, probably due to size and low gravity of the small body. Credit: NASA/Goddard/University of Arizona Read more

A recent SwRI-led study provided new insights into the surface and structure of asteroid Bennu. NASA's OSIRIS-REx spacecraft data indicate nearly twice the void space near its surface in comparison to the overall body. Credit: NASA/Goddard/University of Arizona Read more

The images immediately before and after contact with Bennu show that in the approximately 1 second that elapsed the sampler head disturbed an area nearly 3 feet across and tossed debris into the air. Bennu provided minimal resistance to the sampler head being pressed into the asteroid, which is seen partly by the widespread disturbance caused by contact, and this data helped deduce that the upper layers of the asteroid were very lightly packed with significant void spaces. The yellow envelope shows the mapped disturbed area in the post-contact image, and the image in the bottom-right shows shadows over the lip of the sampler head and lofted debris that both helped deduce the properties of the surface. Credit: NASA/Goddard/University of Arizona Read more