An experimental study of the impact flash: The relationship between luminous efficiency and vacuum level

When a meteoroid impacts on the Moon, a brief flash phenomenon-called a "lunar impact flash"-can be observed by ground-based telescopes. The statistics of these events can efficiently determine the meteoroid size-frequency distribution in cis-lunar space. The luminous efficiency-that is, the ratio of luminous energy emitted to the impactor's kinetic energy-is the most important parameter for precise meteoroid-size estimation. Thus, it is necessary to understand the impact-flash mechanism and determine the luminous efficiency under various conditions. We have, therefore, performed hypervelocity-impact experiments at JAXA, and measured the relation between luminous efficiency and ambient pressure at various vacuum levels. Based on a high-speed spectroscopy, we find that the flash consists of blackbody radiation from hot ejecta and blackbody radiation from a vapor cloud together with emission lines and molecular bands. Spectrum analysis reveals that the initial luminous efficiency within the first few μs does depend on the vacuum level, whereas the luminous efficiency over several ms does not depend on the vacuum level.