A 490-meter-wide asteroid whose surface strength is less than 1 pascal. What does 1 pascal even mean? The most vivid conversion comes from author Paul Sánchez in a phys.org interview: press freshly ground coffee into a small cylinder and its strength is about 50 pascals — Bennu's surface is 50 times fluffier than that. As he put it himself: "You could poke a hole in the coffee column with one finger."
First, a timeline note, exposing an old habit of science journalism: the paper itself was published in Nature Communications on July 8, and it was the September 4 phys.org interview that sparked this wave of coverage — a "new study" reported two months late. But the chain of numbers drawn out in that interview is worth retelling.
It All Started with a Botched Sampling
On October 20, 2020, the OSIRIS-REx spacecraft's robotic arm touched Bennu's surface at 10 cm/s, preparing to blast it with nitrogen gas to stir up samples. The planned surface was relatively firm rubble. In reality: the sampler head sank several centimeters, the gas blasted a crater about 68 cm deep, and the entire arm sank nearly half a meter. The University of Arizona described it as "a punch into a ball pit." The mission returned 121.6 grams of sample — double the 60-gram target. A lucky accident.
From Sample to Strength, Skipping No Steps
The paper's backbone connects three puzzle pieces: lab-measured inter-particle cohesion forces (0.5–4 nanonewtons, Jardine et al. 2025), the average grain size from imaging statistics (1.2 mm, Ballouz et al. 2026), and 78 contact-dynamics simulations performed by this study itself.
The simulations used the open-source platform LMGC90, developed at the University of Montpellier in France. The scenario is very specific: a "grain bridge" of 2–5 cm particles wedged between two one-meter boulders — how much force does it take to break it? 18 runs with spherical grains, 60 with polyhedral grains across ten aspect ratios, from perfectly round to elongated and flat.
The results fall between 0.001 and 0.01 Pa. The paper's own wording: "approaching effectively no strength." The prior upper bound inferred from remote sensing and the sampling event was under 1 pascal — two independent paths line up.
The Best Part: It's Weak Precisely Because the Grains Are Sticky
I originally assumed this kind of fluffiness meant the grains weren't sticky. Turns out that's half backwards. Bennu's grains, taken individually, are quite sticky — nanonewton-scale van der Waals forces were genuinely measured. The real reason it's weak is that the surface lacks fine dust: without enough small particles to fill the gaps, there are few contact points, and thus few adhesive bonds can form. Great glue, just spread too thin.
The simulations also reveal two counterintuitive knobs. Smaller grains make stronger bridges (strength drops from 0.005 Pa to 0.001 Pa as grain size grows); flatter grains are also stronger (strength rises from 0.0008 Pa to 0.0024 Pa as aspect ratio goes from 1 to 0.4), because flat grains interlock and offer richer contact types. The two effects can even cancel each other — large-and-flat can match small-and-round. That's where the "universal scaling framework" in the title comes from.
One honest caveat: the key parameter, contact number Zc, can't be measured on the sample — the paper itself admits the experimental means are insufficient. So it delivers a scaling law, paired with one measurable input.
Why Bother Calculating So Precisely
An asteroid's surface strength directly determines how humanity can push on it. When DART struck Dimorphos in 2022, momentum was amplified by ejecta — and the amplification factor depends on how loose the surface is. Rubble piles like Bennu make up the bulk of near-Earth asteroids; how they respond to a "shove" is a core parameter of planetary defense.
There's also a timing coincidence: ESA's Hera probe launched on October 7 and arrives at the Didymos binary system this December to examine DART's impact crater. At that point, this chain of numbers pulled from 121.6 grams of lab material will be checked against a real impact site.
References: Nature Communications paper | phys.org interview | Walsh et al. 2022 sampling event analysis | ESA Hera mission