Cracked Boulders Explain Why Asteroid Bennu Loses Heat So Fast

Researchers at Nagoya University in Japan measured how heat moves through the samples using lock-in thermography, a technique in which a laser heats a tiny spot while instruments track how the warmth spreads outward, like ripples on a pond.

"That's when things became really interesting," Ryan said. The thermal inertia measured in the lab samples was much higher than what the spacecraft's instruments had recorded — a mismatch that echoed similar results from JAXA's partner mission, Hayabusa-2. The discrepancy pointed to the size and internal structure of Bennu's enormous boulders as the key.

How did NASA see inside the rocks?

To bridge the gap between tiny fragments and boulder-scale rocks, the team needed three-dimensional views of the samples' interiors.

At NASA's Johnson Space Center in Houston, curators sealed each particle in its own airtight container inside a nitrogen-filled glove box, then carried the sealed containers to an X-ray computed tomography scanner (XCT), which builds 3D images without cutting or damaging the material.

"The sample goes into its own 'spacesuit,' gets a CT scan, and then comes back to its pristine environment, all without having any exposure to the terrestrial environment," said Nicole Lunning, lead OSIRIS-REx sample curator at NASA Johnson and a co-author of the study. "We can image right through these airtight containers to visualize the shape and internal structure of the rock that's inside."

"X-ray computed tomography allows us to look at the inside of an object in three dimensions, without damaging it," said co-author Scott Eckley, an X-ray scientist at NASA Johnson.

Each scan produced a permanent 3D digital record of the particle, now stored in a publicly accessible database so researchers can study the rocks without handling the originals.

What does this change for asteroid science?

Ryan's team fed the X-ray data into computer simulations that tracked heat through asteroid rocks, scaling up from individual particles to the giant boulders on Bennu. The simulations reproduced the low thermal inertia the spacecraft had measured — closing the case.

Before this study, scientists suspected Bennu's boulders might be soft and spongy inside. The samples showed substantial pore space, but the widespread fractures proved equally important.

"It turns out that they're really cracked too, and that was the missing piece of the puzzle," Ryan said.

The implications reach beyond one asteroid. Astronomers routinely infer the physical properties of asteroid surfaces from telescopic measurements of how those bodies absorb and release heat — but interpreting such data without physical samples has been difficult.

The study's second author, Ron Ballouz of the Johns Hopkins University Applied Physics Laboratory, said the Bennu samples provide a direct bridge between remote measurements and real rock structure.

"We can finally ground our understanding of telescope observations of the thermal properties of an asteroid through analyzing these samples from that very same asteroid," Ballouz said.

For Bennu, at least, the heat is finally explained.