Fine Structure Of The Impact Flash And Ejecta During Hypervelocity Impact
An experimental study explores the visible impact flash created by high velocity impacts. 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, which contains information about both the target and the impactor.
Gary Simpson, K.T. Ramesh, and colleagues explored the impact flash by shooting 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.
The authors conclude that these impact flashes are created by the fragmentation of an ultra-fast jet of material ejected from the colliding bodies. 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 (over 22,000 miles per hour). The material making up the target and the size of the jetted particles can be inferred from the flash, according to the authors.
Cross sections of impact flash at early, transition, and late stages (not to scale). The initial stages of impact and flash formation by means of impact jetting are illustrated in A). The characteristic time, τ=ri/V0 , represents the limiting timescale over which jet formation can occur. After initiation and during initial penetration, the impact jet sweeps out from the target creating an expanding annulus of radiating material. The collision point centered (CP) frame of reference used in jet initiation calculations is illustrated. The post-jetting ejection behavior begins to transition B) to excavation of the impact crater. As the vapor cloud expands, cools, and decelerates relative to jetted particles, condensed phase material visibly separates from the vapor cloud. At later times, cavity excavation dominates the generation of fragments and the familiar ejecta cone develops fully C). The target is nearly fully perforated, as shown. The radiating vapor cloud dims and begins to dissipate.
Fine structure of the impact flash and ejecta during hypervelocity impact, PNAS Nexus (open access)
