happened to celestial rocks when they collided at high speed. This helped them understand the evolution of asteroids and their danger to Earth.
Experiments with granular materials
Engineers at Johns Hopkins University have discovered new details about how granular materials such as sand and rocks behave during extreme collisions — a discovery that could help protect Earth from dangerous asteroids.
Using new experimental techniques and advanced computer simulations, the team has found that these materials can behave unexpectedly in high-speed collisions, and this discovery challenges traditional models. Their work is published in the Journal of the Mechanics and Physics of Solids.
“Our study shows that different parts of a material, and even different grains of sand, can behave in very different ways during the same impact event,” said team leader Ryan Hurley, an assistant professor of mechanical engineering at Johns Hopkins University’s Weiteng School of Engineering and a research fellow at the Hopkins Extreme Materials Institute (HEMI). “What we found has the potential to inform applications ranging from asteroid deflection to industrial processes like tablet manufacturing.”
The team fired projectiles from a gas cannon at speeds of up to 2 km/s into granular samples made of aluminum and soda-lime glass and observed the behavior of the samples in the first few microseconds after impact. Although such experiments are typically performed on-site at HEMI on the JHU campus in Baltimore, this experiment was performed at Advanced Photon Source (APS) in Chicago because it required the use of special X-ray facilities to visualize the impact.
Characteristics and results of the experiments
“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 that,” said Sohanjit Ghosh, a graduate student in the Department of Mechanical Engineering and lead author of the paper. “We combine X-ray images with numerical models that we’ve developed, and that makes the two-dimensional X-ray image into a three-dimensional process that gives us the full picture of what’s happening, both in time and space.”
The researchers found that in addition to other chemical reactions, the heat created by intense compression causes the grains to fracture, melt and re-consolidate. It was found that the grains interacted with each other differently at different impact velocities. As the velocity increases, so much thermal energy is transferred that the grains actually melt and then reform.
The team noticed that different metallic materials showed different ways of dissipating energy during high-speed impacts. Materials such as aluminum absorb energy through defect formation and plasticity, while brittle materials such as soda-lime glass dissipate energy through fracture and fragmentation.
Importance of experiments for future missions
Researchers say these findings could provide the basis for future missions like DART in 2022, which struck an asteroid, changing its trajectory.
“All asteroids have this layer of sand, called regolith, on top of them, and when you shoot them, it’s the regolith that dissipates a lot of the impact energy,” Ghosh says. “We can infer from the combination of such modeling and experiments how different materials in different environments and impact conditions will behave.
According to Ghosh, although the planning for the experiment lasted several months, the actual physical experience was completed literally in the blink of an eye.
“The timescales of the experiments are very short—several hundred nanoseconds,” he said. “We prepare an entire experiment for a month and then it’s over in a few microseconds.
According to phys.org
