How Deep-Space Destruction Led to a ‘Rubble Pile’

If scientists could only peek inside an asteroid, they’d understand its origin and evolution and even garner important clues about how our solar system formed. While such internal probes are still in the realm of science fiction, a University of Maryland aerospace engineering researcher has used the mighty computing muscle of UMD’s Deepthought2 cluster to, in effect, “reverse engineer” how the asteroid Bennu came together as it sped through space.

The project led by Yun Zhang, a postdoctoral aerospace engineering researcher in Associate Professor Christine Hartzell’s Planetary Surfaces and Spacecraft Lab, used a variety of simulations based on numerical models; determining which one best matches Bennu’s observable features helped explain how it formed. Her findings were published on Aug. 6 by Nature Communications.

Zhang’s findings suggest that Bennu, a type of asteroid frequently referred to as a “rubble pile” because of its loose, low-density composition, came about as a result of a cataclysmic event. Most likely, a collision that destroyed a larger planetary body birthed Bennu.

“Asteroids of Bennu’s size—namely, with a radius of about 200 meters—can be monolithic rocks rather than rubble piles,” she said. “Our findings show that Bennu definitively falls into the rubble-pile category–its interior is very loose in its composition. Essentially, it’s a hodgepodge of rocky chunks and fragments that have become held together by gravitational forces.”

When the parent body broke apart, thousands of fragments eventually led to the formation not only of Bennu, but also many other rubble-pile objects, possibly including the nearby asteroid Ryugu, she said.

Both Bennu and Ryugu were the target of space missions: Ryugu by the Japanese space agency’s Hayabusa2 satellite, and Bennu by NASA’s OSIRIS-Rex. The latter, which launched in 2016 and touched down on the asteroid in 2020 to collect a surface sample, observed several aspects of Bennu that would prove crucial to Zhang’s work. It confirmed, for instance, that Bennu does not have a moon, which would have necessitated “a more cohesive interior.

Zhang’s models also factored in gravitational measurements obtained by the NASA spacecraft, as well as observations of surface slopes along with evidence of avalanches on Bennu's surface.

Her use of simulations in combination with OSIRIS-REx data makes her work pioneering, said Hartzell, the Planetary Surfaces and Spacecraft Lab director and a co-author of the study. “Others have run similar simulations before, but this she is the first to comprehensively compare the modeling outcomes to the detailed geophysical data from an actual spacecraft mission to a specific asteroid.”

The new research yields an additional take-home: Even minor variations in an asteroid’s internal composition can have far-reaching effects.

“This leads to very diverse evolutionary histories, and it explains why we can see so many different asteroid structures and shapes today,” Zhang said.