Over the 3-day period from 12 to 14 October, 1990, the sodium abundance in the lunar atmosphere at 80° South increased by 60%, while interspersed measurements at the equator showed no substantial change. The source is suggested to be an unknown meteor shower with a radiant near the south ecliptic pole. A low relative velocity of ∼20 km/sec, combined with small particle masses, would keep the shower below the detectability threshold of radar. The stream could evolve from a reasonable asteroidal or cometary orbit with perihelion somewhat greater than 1 astronomical unit (AU) and a major axis of a few AU. The short residence time of lunar sodium makes it much more favorable than the terrestrial sodium layer for detection of such an event.
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