This finding, led by Paul Sánchez of the University of Colorado Boulder, reveals that Bennu is not a solid rock but a loose aggregation of boulders, pebbles, and dust held together by weak gravitational forces and minimal cohesion.
Surface Fragility and Composition
Bennu’s surface strength was measured at less than one pascal, with some estimates suggesting it could be as low as 0.001 to 0.01 pascal. For comparison, freshly ground coffee has a tensile strength of about 50 pascals, meaning Bennu’s surface is 50 times weaker. This fragility was confirmed by analyzing samples returned by NASA’s OSIRIS-REx spacecraft in 2023, which showed the asteroid’s material ranges from submicron dust to rocks several centimeters across. The sample also contained angular and irregular particles, which scientists say contribute to its low cohesion.
The OSIRIS-REx mission encountered unexpected challenges during its 2020 touchdown, as the spacecraft’s sampling head plunged far more easily into Bennu’s surface than anticipated. A subsequent analysis in Science Advances described the subsurface as having near-zero cohesion,
highlighting the asteroid’s loose, rubble-like structure.
Rubble-Pile Structure and Spin Dynamics
Researchers modeled Bennu’s structure using simulations of 78 granular bridges
— masses of particles squeezed between boulders. The study found that smaller grains created stronger bridges by increasing points of contact, but Bennu lacks sufficient fine particles to bind its rubble tightly. This explains why the asteroid remains intact despite its weak cohesion. However, its rapid rotation — completing a full spin every 4.3 hours — raises questions about how it avoids disintegrating.
A 2000 study on asteroid rotation suggested that larger rubble-pile asteroids rarely spin faster than 2.2 hours, as their gravity would otherwise fail to hold them together. Bennu’s faster spin challenges this theory, indicating that factors beyond gravity, such as particle shape and packing, play a critical role in its stability. The OSIRIS-REx team’s analysis of Bennu’s equatorial ridge and surface composition further supports the idea that the asteroid formed from a fragmented parent body, possibly a primitive ocean world.