Engineers break up rocks to see what happens when the top layer of an asteroid-like object is hit by an extreme external force

Engineers at Johns Hopkins University have discovered new details about how granular materials such as sand and rock behave under extreme impacts, findings that could one day help protect Earth from dangerous asteroids.

Using new experimental techniques and advanced computer simulations, the team revealed that these materials can behave in unexpected ways when struck at high speeds, a discovery that challenges traditional models. Their work is published in Journal of mechanics and physics of solids.

“Our study shows that different parts of a material, and even different grains of sand, can behave very differently during the same impact event,” said team leader Ryan Hurley, associate professor of mechanical engineering at the Johns Hopkins University Whiting School of Engineering and a member of the Hopkins Extreme Materials Institute (HEMI).

“What we discovered has the potential to illuminate applications ranging from asteroid deflection to industrial processes like tablet manufacturing.”

The team fired projectiles from a gas gun at speeds of up to 2 km/s at granular samples made of aluminum and soda-lime glass and observed the behavior of the samples during the first microseconds after impact. Although such experiments are typically performed on-site at HEMI on JHU’s Baltimore campus, this one was conducted at the Advanced Photon Source (APS) in Chicago because it required the use of special X-ray facilities to visualize the impact.

“If you go to the beach, you can only see the sand on the surface, but an X-ray can see what’s going on underneath,” said Sohanjit Ghosh, a doctoral student in mechanical engineering and lead author of the study.

“We combine X-ray images with digital models that we have developed, which transforms the two-dimensional X-ray image into a three-dimensional process that gives us a complete picture of what is happening, both in time and space.”

The researchers found that in addition to other chemical reactions, the heat created by intense compression causes grains to fracture, melt and resolidify.

“It’s interesting to see how the grains interact differently with each other at different impact speeds,” Ghosh said. “We found that as the speeds get higher and higher, the thermal energy transferred is such that the grains melt and then reform.”

The team observed that different metallic materials exhibit different ways of dissipating energy during high-speed impacts. Materials like aluminum absorb energy through defect formation and plasticity, while brittle materials like soda-lime glass dissipate energy through fracture and fragmentation.

The researchers say these findings could inform future missions similar to the 2022 DART mission, which struck an asteroid, changing its trajectory.

“All asteroids are covered in a layer of sand, called regolith, which dissipates much of the impact energy when they are thrown,” Ghosh says. “The combination of these models and experiments allows us to infer how different materials will behave in different environments and impact conditions.”

Ghosh said that while planning the experiment took several months, the actual physical experiment was completed in the blink of an eye.

“The experiments take place over a very short period of time: a few hundred nanoseconds,” he explained. “We prepare an entire experiment for a month and it is completed in a few microseconds.”

Mohmad Thakur, a research assistant at HEMI, was also a member of the research team.