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The first confirmed lunar impact flash due to a non-Leonid meteoroid is reported. The observed Perseid meteoroid impact occurred at [18.sup.h][28.sup.m][27.sup.s] on August 11,2004 (UT). The selenographic coordinates of the lunar impact flash are 48 [+ or -] 1[degrees]N and 72 [+ or -]2[degrees]E, and the flash had a visual magnitude of ca. 9.5 with duration of about 1/30 s. The mass of the impactor is estimated to have been 12 g based on a nominal model with conversion efficiency from kinetic to optical energy of 2 x [10.sup.-3]. Extrapolation of a power law size-frequency distribution fitting the sub-centimeter Perseid meteoric particles to large meteoroids suggests that several flashes should have been observed at this optical efficiency. The detection of only one flash may indicate that the optical efficiency for Perseid lunar impact is much lower, or that the slope of the size distribution differs between large meteoroids and typical sub-centimeter meteoric particles. Keywords: Impact processes; Meteoroids; Meteors: Moon, surface

We investigate the spin up of nonspherical asteroids by successive random collisions with other smaller asteroids. The spin-up rates are calculated for ellipsoidal and spherical bodies of the same mass. Then, we derive the relative spin-up efficiency, that is, the ratio of the spin-up rate of the ellipsoidal body to that of the spherical body. We suppose a collisional process in our model different from that adopted in the previous works. The results show that ellipsoidal bodies can spin up more rapidly than spherical bodies. Key Words: asteroids, rotation; collisional physics; cratering; impact processes; rotation.

Optical flashes observed on the night side of the Moon during the 1999 Leonid meteor shower have attracted the interest of astronomers. These flashes are attributed to high-velocity impacts of Leonid meteoroids on the lunar surface. Here, we report five lunar flashes detected over a 5.8-h observation period centered at 11:25 UT on Nov. 18, 1999, in Japan. The flashes are characterized by an abrupt brightening. Three flashes exhibited afterglows that remained visible for at least 50 ms, which is longer than the duration predicted for radiation from an impact-generated plasma cloud. We show that thermal radiation from hot droplets ejected from the lunar surface during high-velocity impacts could be the cause of the afterglows. Key Words: cratering; impact processes; meteoroids; meteors; Moon, surface.

We calculate the momentum transfer efficiency for small cratering impacts from 59 high-velocity (0.76-4.4 km/s) oblique impact experiments using nylon projectiles and targets made of basalt, aluminum, mild steel, mortar, and nylon. High-speed video frames show an initial luminous stream downrange of impact point for all the target materials. For basaltic and mortar targets there follows axi-symmetric ejection of target material around surface normal at the impact point. Our results suggest that: (1) momentum carried away by the axi-symmetric ejecta would significantly contribute to the normal component of momentum transfer efficiency; and (2) the penetration depth of the projectile into target could determine the tangential momentum transfer efficiency. We calculated the efficiency of angular momentum transfer from the translational motion of projectiles to the rotation of ellipsoidal asteroids using the efficiencies for the linear momentum. The efficiency is more than four times that for spherical asteroid at their principal axis ratio of 2 : 1.4 : 1. If the experimental results hold for the impact that formed the largest crater on 253 Mathilde, the largest projectile estimated may have despun the asteroid to the present slow rotation by chance. Key Words: asteroids, rotation; collisional physics; cratering (asteroids); impact processes (asteroids); Mathilde; rotation (asteroids).