Asteroid Bennu Surface 50 Times Weaker Than Ground Coffee

A team from the University of Colorado Boulder published a new model on September 4 in the journal Nature Communications that calculates how strong an asteroid is from the size and shape of its tiny particles. Applied to the asteroid Bennu, the model showed that the tensile strength of its surface sits below one pascal. The paper, DOI 10.1038/s41467-026-75169-4, is authored by Paul Sanchez, a senior research associate at the university, along with his collaborators. The point is a practical one: this kind of number feeds directly into programs that simulate an impact on an asteroid, the only tested method of defending Earth from such an object.

Scientists have been asking how small asteroids hold together at all for more than a decade. Space missions have shown that bodies like Bennu, Ryugu and Itokawa are not solid rock but granular asteroids, loosely bound piles of dust, rock and boulders held together by their own gravity and weak cohesive forces. Sanchez and his collaborators proposed back in 2010 that this extra strength comes from Van der Waals forces between the tiny grains of regolith, the surface dust and rock. The idea was tested in 2014 in simulations using perfectly round particles, but two things were still missing: the simulations could not handle the irregular shape of real grains, and the strength of the cohesion between particles was assumed rather than measured. The second gap closed in 2023, when NASA’s OSIRIS-REx probe brought samples from Bennu back to Earth and made direct measurement of those forces possible for the first time.

Rather than simulate an entire asteroid, the team modeled just a small piece of one. They built a matrix of tiny bound grains packed between two boulders roughly a meter across, forming a kind of granular bridge, then pulled the boulders apart until the bridge broke. The breaking point gives a measure of tensile strength. The team ran 78 such simulations using LMGC90, an open source contact dynamics program. Both round and polygonal grains were tested, ten different shapes ranging from nearly perfectly round to elongated, sized from two to five centimeters, in both pure and mixed batches. The results showed that two factors together determine strength: grain size and sphericity, meaning how far the shape departs from a sphere. Smaller, more angular grains create more contact points within the same space, which makes the bridge stronger.

When this framework was applied to Bennu, using the cohesive forces measured on the returned samples, the resulting surface strength came out below one pascal. That figure matches independent estimates from remote sensing and what was recorded during the sample collection itself in 2023.

Why Is Bennu’s Surface So Weak?

The comparison Sanchez offered in a statement accompanying the study is what makes the finding memorable. He first noted that a small cylindrical amount of freshly ground coffee has a strength of about 50 pascals and that a finger can still be pushed into it.

“The surface of Bennu is fifty times weaker than that.”

The cause, in his account, is a shortage of the finest dust. “If there aren’t enough small grains to fill the gaps between the larger ones, there aren’t enough cohesive contacts to strengthen the structure,” Sanchez explained.

The practical value of this work lies in planetary defense. How far an asteroid moves when something hits it depends on exactly these kinds of material parameters, which until now had to be assumed. NASA already tested such an impact in 2022, when its DART probe struck the asteroid Dimorphos. The study’s authors say their framework, tied to grain size, size distribution and porosity, supplies the basic parameters for programs that simulate impacts on asteroids, the only method tested so far for nudging such an object off its path.

The team says it plans next to extend the model to a wider range of grain sizes and to try to describe an asteroid’s interior, a part no mission has reached yet. That is also the limit of this finding: the model was derived from a simulation of one small slice of structure, a bridge between two boulders, not an entire body’s mass, and it was applied only to the surface layer of Bennu, for which cohesive forces have been measured from the returned sample. It says nothing directly about what the interior of that asteroid, or any other granular body, looks like, and it does not prove how a whole body would behave in an actual impact. It simply supplies one of the input numbers such calculations have been missing until now.