Author: Staff Report
Two views of an impact flash, and crater and ejecta development. A ring of yellow and orange against a blue background.
Stills from an ultra-high-speed movie showing two views of an impact flash, and crater and ejecta development during the first few microseconds, for a stainless-steel sphere impacting an aluminum alloy plate at 3 kilometers per second.

Impacts by debris and meteoroids pose a significant threat to satellites, space probes, and hypersonic craft. Such high-velocity impacts create a brief, intense burst of light, known as an impact flash. These flashes contain information about both the target and the object that caused the impact. For example, scientists have studied the impact of meteoroids on the moon by monitoring and analyzing impact flashes on its dark side.

A team of Johns Hopkins Whiting School of Engineering researchers, including Engineering for Professionals (EP) faculty members K.T. Ramesh, Justin Moreno, and Matthew Shaeffer, as well as mechanical engineering doctoral student Gary Simpson, have discovered that impact flashes are created by the fragmentation of an ultra-fast jet of material ejected from the colliding bodies. Their results appear in PNAS Nexus.

Working in the Hopkins Extreme Materials Institute‘s HyFire lab, the team shot stainless steel spheres into an aluminum alloy plate at a speed of three kilometers per second—about 6,700 miles per hour, or more than nine times the speed of sound. The resulting impact flashes were photographed using ultra-high-speed cameras and high-speed spectroscopy, which measures the color and brightness of the light.

Immediately after impact, a luminous disc is seen expanding around the impacting sphere. Only a few millionths of a second later, the disc takes on an almost floral shape, as fragments ejected from the impact crater form an ejecta cone, with petal-like projections at the outer edge.

“We found that minuscule, condensed fragments from the jet interact with the atmosphere to create an extremely bright radiating cloud of vapor, which expands at a speed of over ten kilometers per second, or more than 22,000 miles per hour,” said Simpson. “The material making up the target and the size of the jetted particles can be inferred from the flash.”

In addition to his role as a member of the EP faculty in mechanical engineering, K.T. Ramesh is the Alonzo G. Decker Jr. Professor of Science and Engineering, the senior advisor to President Ron Daniels for AI, and the executive director of the AI-X Foundry. He is a professor of mechanical engineering with joint appointments in the Departments of Materials Science and Engineering and Earth and Planetary Sciences.

Justin Moreno is an EP Biomedical Engineering faculty member and also serves as HEMI associate staff engineer.

Matthew Shaeffer is an EP Biomedical Engineering faculty member and also serves as HEMI senior staff engineer.