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On Feb. 15, 2013, a meteoroid with a size of about 19 m plunged into the terrestrial atmosphere at 19 km s −1 and burst at an altitude of about 30 km over the city of Chelyabinsk, Russia. Here we present light curves for the bolide in the red, green, and blue color bands, derived from an analysis of a video that was recorded by a dashboard camera and released on the Internet. Our results demonstrate that the bolide was blue–green in color, which is inconsistent with the Planck spectrum before the meteoroid began to fragment. Fragmentation triggered a flare-up of the bolide and 90% of its radiation energy at optical wavelengths was released within a period of about 2 s after that. During the same period, the brightness ratios among the three bands became consistent with 4000 K blackbody radiation. Based on the peak luminosity, a surface area of several square kilometers would be required for a 4000 K blackbody. It is considered that the radiation source of the bolide was an elongated cloud of vapor and debris produced through severe fragmentation of the meteoroid. [ABSTRACT FROM AUTHOR]

Lunar impact flashes have been observed at collisions of meteoroids against the non-sunlit lunar surface at speeds exceeding 10 ​km ​s−1. We detected 13 flash candidates between 6.2 and 9.9 in R-magnitude on December 15, 2018 during the Geminids meteor activity. Two or three observatories confirmed eleven of them. We obtained their spectra in the wavelength range between 400 and 870 ​nm. They are continuous and red, with best-fitted single blackbody spectra indicating the temperatures of about 2000–4000 ​K. The temperatures for a few successive movie frames at 16 ​ms or 25 ​ms intervals decrease with time. Incandescent ejecta, consisting of melt droplets or dust, and the radiant floor of an impact crater could be the source of these flashes, except for the initial stages. At the beginning of some flashes, we found an excess of fluxes at short wavelengths of less than about 600 ​nm. The composites of two blackbody spectra may fit the spectra better where their temperatures are about 2000 K and 6000 ​K. The contribution of a high-temperature vapor plume, generated at the very beginnings of the impact phenomena, could be important. Image 1 • Visible and near-infrared spectra of lunar impact flashes were obtained for the first time. • The spectra of the lunar flashes due to the impacts of the Geminids' meteoroids are continuous and red. • Best-fitted single blackbody spectra show flash temperatures of about 2000–4000 ​K. • The composite of two blackbody spectra, where its high-temperature component would represent radiation from an impact-generated vapor plume, could explain the observed spectra better in the initial stage of the flashes. [ABSTRACT FROM AUTHOR]

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. • 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. [ABSTRACT FROM AUTHOR]