Lunar Impact Flashes by Geminid Meteoroids in 2007

IN 2007 M. Yanagisawa, H. Ikegami, M. Ishida, H. Karasaki, J. Takahashi, K. Kinoshita and K. Ohnishi. Univ. ElectroCommunications, Tokyo 182-8585, Japan. E-mail: yanagi@ice.uec.ac.jp. Moriyama-shi, Shiga 524-0104, Japan. Nerima-ku, Tokyo 176-0002, Japan. Kobe University, Department of Earth and Planetary Sciences, Kobe 657-8501, Japan. Nagano National College of Technology, Nagano 381-8550, Japan. Introduction: High-velocity impacts of large meteoric particles (meteoroids) onto the lunar surface generate optical flashes (Lunar Impact Flashes). The frequency of bright flashes is usually low, ie once in several tens of hours for 7th in brightness magnitude. The flashes are thus difficult to observe. However, the impact velocities of meteoroids are several tens of km/s, and we can study the hyper-velocity impact phenomena the velocity of which is not available in laboratory impact experiments, by means of Lunar Impact Flash observations. Further, we may be able to know the composition of meteoroid and lunar material by their spectroscopic studies in future. The flashes have been confirmed by video observations for the Leonid [1-3] and Perseid [4] meteoric activities. Though the flashes have been reported during the Geminid activities [e.g. 5], none had been confirmed by independent observers separated far enough from each other to discard the possibility of satellite glints. Here, we report the first confirmed observation of Geminid Lunar Impact Flashes. Observations: During the Geminid activity on 15 December 2007, observers in Japan confirmed four lunar flashes (B, C, D, E) by small telescopes with apertures of about 20 cm in diameter. A flash (A) was also recorded with two independent video cameras attached to a 45 cm telescope in an observatory. Brightness magnitude (mag.) and duration are summarized in Table 1. They must be caused by the impacts of Geminid meteoroids. The velocity vector is then known for them (33 km/s). Assuming that the optical energy is 0.2 % of the kinetic energy of meteoroid, masses of meteoroids are estimated. They are listed in Table 1 with the impact angles measured from lunar local horizon.