The amazing underground of asteroid Bennu

An analysis of data obtained by a spacecraft that investigated Bennu reveals surprising physical characteristics of the structure of this star.

Bennu is an asteroid about 500 meters in diameter, explored by NASA’s OSIRIS-REx space probe. The spacecraft left Earth in 2016 and arrived at Bennu in 2018. There it made an extensive series of observations, including a brief descent to the surface to collect soil samples. On May 10, 2021, he undertook the return trip to Earth. The objective of the return trip is to deposit a capsule with the collected samples on the Earth’s surface. However, an important part of the information obtained by OSIRIS-REx is already being studied and surprises are beginning to emerge.

Data collected during OSIRIS-REx’s sampling of asteroid Bennu show that the asteroid’s surface is essentially made of low-cohesive “rubble.” This can be deduced from the results of two recent investigations, one published in Science and the other in Science Advances, two academic journals.

In the study published in Science Advances by the team of Kevin Walsh, from the Southwest Research Institute (SwRI) in the United States, the forces experienced by the space probe were analyzed, finding that Bennu’s low gravity has given rise to a bed granular surface with weak cohesion between the particles.

Asteroid Bennu ejecting particles from its surface. (Photo: NASA Goddard/University of Arizona/Lockheed Martin)

The place where OSIRIS-REx made its descent to collect samples is inside a 20-meter crater nicknamed Nightingale (“nightingale”). As the spacecraft touched down, the spacecraft’s sample acquisition mechanism (TAGSAM) made contact and began to sink to the asteroid’s surface before releasing a jet of nitrogen gas that mobilized subsurface material and guided it to a collection chamber. . Through the analysis of images and spectral data taken during and after obtaining the sample, Dante Lauretta’s team, from the University of Arizona in the United States, has discovered that the underground material is darker and contains more fine particles than the Of the surface. This process generated a debris plume and a new elliptical crater 9 meters long.

Walsh and his colleagues investigated the physical properties of the material down to 10 centimeters below Bennu’s surface using images and accelerometer data. They reconstructed the forces exerted on the spacecraft in the short time between when it first made contact with Bennu’s surface and when the nitrogen gas was released. Walsh’s team has found that the near-surface material is loosely packed and less dense than the average for the asteroid as a whole, with very low cohesion. The material’s high porosity and low resistance allow dust and other small particles to move into the asteroid’s subsurface.

Spectral and thermal data collected during the mission suggest that these results apply to the entire asteroid and are not unique to the sampling site. (Source: AAAS)