Hidden Cracks Explain Why Asteroid Bennu Loses Heat

A boulder-covered asteroid should hold its warmth the way a stone holds the heat of a campfire. Yet the rocks brought back from asteroid Bennu Loses Heat almost as quickly as fine sand — a contradiction that has puzzled planetary scientists for years. Now, laboratory tests on the returned samples have found the missing piece: the boulders are riddled with hidden cracks.

According to a write-up of the study published by Technology.org, the research was led by Andrew Ryan of the University of Arizona's Lunar and Planetary Laboratory. His team discovered that the sample particles fall into two distinct types. Rough, "hummocky" grains have lower thermal inertia, fine pores, and winding networks of tiny cracks that resist breaking apart. Smoother, "angular" grains are harder, hold heat better, and carry fewer but longer fractures that split them more easily. When the lab results were scaled up to boulder size, they matched the measurements the spacecraft had recorded at the asteroid.

Getting there took an unusual toolkit. Researchers at Nagoya University in Japan applied a technique called lock-in thermography to measure how heat moves through the grains, while scientists at NASA's Johnson Space Center in Houston used X-ray computed tomography to build three-dimensional images of the samples' interiors — all inside nitrogen-filled gloveboxes that kept the pristine material sealed off from Earth's atmosphere. Those scans were then fed into computer simulations tracking heat flow through the rock. Porosity alone could not reproduce the spacecraft's readings; the widespread internal fractures turned out to be equally important, as reported by RocketNews.

Why Bennu Loses Heat faster than solid rock should

The puzzle dates back to before any spacecraft arrived. Early measurements from NASA's Spitzer Space Telescope showed Bennu shedding heat quickly, a trait associated with fine materials like sand that warm and cool fast. But when OSIRIS-REx reached the asteroid in 2018, it found a landscape dominated by large boulders rather than smooth terrain. Solid rock typically retains heat far longer, so a boulder-covered world cooling like a beach made no sense — until the cracks were found.

"It turns out that they're really cracked too, and that was the missing piece of the puzzle," Ryan said. The cracks may date back to geological activity inside the parent body that Bennu broke away from, or they may have formed more recently from micrometeoroid impacts and the daily stress of heating and cooling in sunlight.

The finding also resolved a mismatch between the lab and the spacecraft. The returned grains showed higher thermal inertia in the laboratory than the orbiter had recorded, echoing similar results from Japan's Hayabusa2 mission to the asteroid Ryugu. The difference points to particle size and internal structure — not just composition — as the deciding factors, and samples from Ryugu show crack networks much like those in Bennu's rougher grains, suggesting the same mechanism explains low heat readings on other carbon-rich asteroids.

What colder rocks mean for planetary defense

This is not just a geology curiosity. As Bennu rotates, its surface releases heat unevenly, and that tiny push — known as the Yarkovsky effect — shifts the asteroid's orbit over decades. A 2021 analysis built on OSIRIS-REx tracking data put Bennu's chance of striking Earth on September 24, 2182, at 1 in 2,700, with the cumulative odds through the year 2300 at about 1 in 1,750. Forecasts like that live or die on thermal properties, so pinning down exactly how Bennu's boulders absorb and release heat makes long-range predictions more reliable.

The study's second author, Ron Ballouz of the Johns Hopkins University Applied Physics Laboratory, said the work changes how scientists interpret telescope data from afar. "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. Until now, astronomers had to guess at the internal structure behind the heat signatures they measured remotely.

The timeline shows how long the answer took. Spitzer's early readings flagged the anomaly, the 2018 arrival revealed the boulder fields, and only after samples reached Earth could laboratories dissect the rocks grain by grain. The analysis was published in Nature Communications in October 2026, and the spacecraft itself has moved on — it is now flying a follow-on mission to the asteroid Apophis. As researchers keep working through the Bennu material, they expect the cracked, porous rocks to keep refining how heat, orbits, and asteroid surfaces are understood.

Space rocks keep making news on the site: NASA recently spotted new ghost nebulas in a deep-sky portrait of Cassiopeia, and two newfound asteroids slipped inside the Moon's orbit. For a look at what asteroid impacts can do on Earth, see how an asteroid crater under the North Sea unleashed a monster tsunami.