Bennu, a near-Earth asteroid, had been studied for years using powerful telescopes. From a distance, it seemed fairly simple. Researchers expected a mix of rocks and smoother areas, maybe even patches of loose sand. That expectation didn’t last long.
When NASA’s OSIRIS-REx spacecraft arrived at Bennu in 2018, the surface told a very different story.
Instead of smooth, easy-to-sample terrain, the asteroid looked rough and chaotic. Huge boulders covered nearly every inch. It was not what anyone had planned.
When expectations fall apart
The mismatch wasn’t just visual. Earlier observations showed Bennu’s surface heating up and cooling down fast, like sand on a beach.
Scientists call this thermal inertia. Low thermal inertia usually means the surface is made of loose, fine material.
But the spacecraft saw something else entirely. Massive rocks dominated the surface, and those should behave differently. Large, dense boulders tend to hold onto heat longer, more like concrete after sunset. The two sets of data didn’t line up.
“When OSIRIS-REx got to Bennu in 2018, we were surprised by what we saw,” said Andrew Ryan, a scientist with the University of Arizona Lunar and Planetary Laboratory, who led the mission’s sample physical and thermal analysis working group.
“We expected some boulders, but we anticipated at least some large regions with smoother, finer regolith that would be easy to collect. Instead, it looked like it was all boulders, and we were scratching our heads for a while.”
Clues hidden inside the rocks
The answer started to take shape only after the mission returned samples to Earth. Scientists could finally study real pieces of Bennu in the lab instead of relying only on remote measurements.
At first, the idea seemed simple. Maybe the rocks were more porous than expected, filled with tiny holes that let heat escape faster. That would help explain the strange readings.
Closer inspection showed that many of the rocks were packed with cracks. Not just a few surface fractures, but networks running through them.
“That’s when things became really interesting,” Ryan said. “The thermal inertia measured in the lab samples turned out to be much higher than what the spacecraft’s instruments had recorded, echoing similar findings obtained by the team of OSIRIS-REx’s partner mission, JAXA’s (Japan Aerospace Exploration Agency) Hayabusa-2.”
Testing heat, one laser pulse at a time
To figure out how heat moves through these cracked rocks, researchers used a technique called lock-in thermography. A laser heats a tiny spot on a sample, and instruments track how that heat spreads.
The results added another layer to the puzzle. Small samples behaved differently from what the spacecraft had observed on the asteroid itself.
That meant scientists needed a way to connect lab-scale measurements with real, full-size boulders. They turned to advanced imaging.
A closer look at the samples
At the NASA Johnson Space Center in Houston, scientists handled the samples with extreme care. Even exposure to Earth’s air could change them. Each piece was sealed in a controlled environment before being scanned.
Nicole Lunning is the lead OSIRIS-REx sample curator within the Astromaterials Research and Exploration Science division at the NASA Johnson Space Center.
“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 Lunning.
“We can image right through these airtight containers to visualize the shape and internal structure of the rock that’s inside.”
Study co-author and X-ray scientist Scott Eckley explained how the scans revealed the internal cracks in full detail.
“X-ray computed tomography allows us to look at the inside of an object in three dimensions, without damaging it,” said Eckley.
Scaling up the answer
Once researchers understood the internal structure, they used computer models to simulate how heat would move through larger rocks. This step was key. A tiny sample behaves differently than a boulder several feet across.
When they scaled up their models, everything finally clicked. The cracked interiors allowed heat to escape faster than expected, even in large rocks.
That explained why Bennu’s surface acted more like loose material, despite being covered in boulders.
“It turns out that they’re really cracked too, and that was the missing piece of the puzzle,” Ryan said.
Why this matters beyond Bennu
This discovery does more than explain one asteroid. It changes how scientists interpret data from space.
For years, thermal measurements have helped researchers guess what asteroid surfaces are like. Now they know those readings can be shaped by hidden cracks, not just surface texture.
Ron Ballouz, a scientist with the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said this work transforms how scientists interpret the structure of an asteroid based on its thermal properties seen from Earth.
“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.
The full study was published in the journal Nature Communications.
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