An experimental study of the impact flash: The relationship between luminous efficiency and vacuum level. (August 2020)

• Record Type: Journal Article
• Authors: Fuse, Ryota; Abe, Shinsuke; Yanagisawa, Masahisa; Hasegawa, Sunao
• Is Part Of: Planetary and space science. Volume 187(2020)
• Publication Date: 2020-08
• Subjects: Lunar impact flash -- Meteoroid -- Impact experiment -- Luminous efficiency; Space sciences -- Periodicals; Atmosphere, Upper -- Periodicals; Sciences spatiales -- Périodiques; Haute atmosphère -- Périodiques; 523
• DOI: 10.1016/j.pss.2020.104921
• Languages: English
• ISSNs: 0032-0633

Abstract

Abstract: 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. Highlights: Hypervelocity Impact flash consists of blackbody radiation from ejecta and vapor cloud, emission lines, and molecular bands. Luminous efficiency within a few μs is affected by vacuum level due to ablation between impact fragments and ambient atmosphere. Luminous efficiency over several ms does not depend on vacuum level. For lunar impact flash study, we can perform rough vacuum level (e.g., several tens of Pa) and frequent impact experiments.