Bennu’s surface turned out to be 50 times weaker than ground coffee

Asteroid Bennu, the near-Earth object NASA’s OSIRIS-REx mission sampled in 2020, has long been treated by scientists as a loose pile of rock and dust rather than a solid boulder. A new laboratory-based model has now put a number on exactly how loose that material is, and the surface turns out to hold together with less strength than a cylinder of grounds sitting in a coffee filter. The result comes from a study built to predict the strength of any granular asteroid from the shape of its individual particles, not just Bennu’s. It reshapes how mission planners think about landing on, sampling from, or ever attempting to redirect a body like it.

A Framework for Predicting Granular Strength

Researchers built what they describe as a universal scaling framework, a mathematical model that predicts the tensile strength of a granular asteroid based on the size and shape of the particles that make it up. Rather than treating every rubble-pile asteroid as its own unsolved puzzle, the framework lets scientists estimate how strongly a surface clings together just by knowing basic properties of its grains, without needing a dedicated mission to test each one directly.

Applied to data gathered during NASA’s OSIRIS-REx mission, the model indicates that Bennu’s surface carries a tensile strength of under one pascal, according to the study published in Nature Communications and reported by Phys.org. Planetary scientist Paul Sanchez, one of the researchers behind the work, compared the material directly to ground coffee: a small cylinder pressed from fresh grounds holds together with roughly 50 pascals of strength, enough to survive a light poke, while Bennu’s surface gives way under a fraction of that pressure.

Why the Surface Barely Holds Together

The explanation comes down to what is missing rather than what is present. Granular material gains strength when small particles fill the gaps between larger ones, creating enough points of contact for weak cohesive forces, including static attraction and minor chemical bonding, to add up into something resembling structural integrity. Bennu appears to be short on that fine-grained filler.

Without enough dust-sized grains wedged between its larger fragments, the asteroid has far fewer of these cohesive contact points than a denser, better-packed surface would produce. The body is held together mostly by its own weak gravity and by boulders resting loosely against one another, rather than by any meaningful internal cohesion binding the grains themselves.

Bennu Joins a Known Family of Rubble Piles

Bennu is not the only asteroid a space mission has found to be built this way. Japan’s Hayabusa2 mission returned samples from the asteroid Ryugu, and its predecessor, Hayabusa, sampled Itokawa; both turned out to be loosely bound aggregates of rock and dust rather than single solid bodies, held together largely by gravity rather than rock strength. Ground-based radar and spacecraft imagery of other near-Earth objects have turned up similar boulder-strewn, low-density surfaces, suggesting that “rubble pile” is closer to the rule than the exception among asteroids of this size class.

The new framework gives researchers a way to compare these bodies on common terms, predicting how each would respond to impacts, sampling attempts, or close approaches once the size and shape distribution of the surface particles is known, without sending a new spacecraft to test every candidate individually. That matters for a field where missions take years to plan and a single spacecraft can only visit one target at a time; a model that generalizes from three visited asteroids to thousands of catalogued ones multiplies the value of every future flyby or sample-return effort.

Rethinking Deflection and Sampling Strategies

The finding carries weight beyond pure planetary science. Any spacecraft designed to touch, sample, or nudge an asteroid off course has to account for how the surface will respond to contact, and a surface this weak behaves very differently than a solid rock would under the same force. A kinetic impactor mission, of the kind NASA already tested against the asteroid moonlet Dimorphos, transfers momentum differently to a target that absorbs an impact by crumbling than to one that resists it like solid stone.

NASA’s OSIRIS-REx team learned a version of this lesson firsthand. When the spacecraft briefly touched down on Bennu in 2020 to collect its sample, the sampling arm sank further into the surface than engineers had expected and had to fire thrusters to back away before the site gave way beneath it, an early real-world preview of just how little resistance the material offered. Engineers had modeled the surface as something closer to coarse gravel; the actual encounter looked more like touching down in a loosely packed snowbank.

Ground Truth From a Returned Sample

OSIRIS-REx delivered its sample capsule to Earth in September 2023, giving laboratories physical material from Bennu to study directly instead of relying solely on remote sensing from orbit. That returned sample, combined with images and force measurements gathered during the mission, gave researchers the data needed to test the new strength framework against an asteroid that had already been visited, rather than against a simulation alone. NASA maintains an overview of the OSIRIS-REx mission and its sample-return timeline on its mission page.

The team’s next step is extending the model to other rubble-pile asteroids and, eventually, to the interiors of these bodies, since the current framework describes only the surface layer and leaves open the question of whether material deeper inside behaves the same way or holds together more tightly under its own weight.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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