EXPERIMENTAL INVESTIGATIONS OF THE IMPACT FLASH

Experimental impacts into non-volatile, particulate targets produce long-duration impact flashes dominated by hot thermal sources. The evolution of the flash is highly dependent on the initial impact conditions. Understanding how different impact variables alter the evolution of the flash provides a means to examine early-time impact processes, offering insights into the projectile-target coupling, the partitioning of energy, and the generation of melt.

We performed experiments at the NASA Ames Vertical Gun Range to investigate the photometric and spatial evolution of the impact flash for impacts of Pyrex projectiles into particulate pumice targets. We varied two impact conditions: velocity (1.6 to 6.0 km/s); and impact angle (15 º to 90º, with respect to the horizontal). The impacts were observed with VIS-NIR photodiodes capable of 100 ns resolution and high-speed cameras capable of frame rates as high as 10⁶ fps.

In these experiments, the impact flash results primarily from hot material located below the pre-impact surface within the transient crater cavity. Time-resolved, spatially integrated light curves recorded by the photodiodes are characterized by an early-time spike, a rapid but delayed rise to a broad intensity peak, and a long-lasting decay. The maximum temperature occurs at the time of impact, so the rise to the delayed intensity peak is primarily caused by the growth of the radiating source area. An increase in the overall flash brightness with decreasing impact angle is controlled by the increasing exposure of the radiating material within the transient cavity.

At higher velocities, the Pyrex projectile is broken into fragments that radiate and scour the crater floor. As impact velocity decreases, the projectile is less damaged, the projectile and target interact for a longer length of time, and friction becomes more important.