Light Shed on Strange Flashes Seen on Moon

Ultra-brief flashes of light seen on the dark side of the moon last fall were the first impacts of meteoroids ever seen as they occurred on the lunar surface, astronomers reported Wednesday.

The five impacts, from a stream of cometary debris known as the Leonid meteor showers, were detected Nov. 19 via a telescope in Monterrey, Mexico. A team formally announced the find Wednesday in the journal Nature.

“The flux of meteoroids associated with the Leonid meteor shower of 18 November, 1999, was predicted to produce observable flashes on the night side of the moon,” the seven astronomers reported. “We report the unambiguous detection of five such flashes, three of which were observed simultaneously by other observers.”

They said that all five flashes “were of very brief duration, as expected for high-speed impacts.”

Astronomer Brian Marsden, director of the International Astronomical Union’s central telegram bureau in Cambridge, Mass., said: “I’m still a little surprised that it happened. But it does seem to be true. It shows that we can predict these things rather well, both for the moon and the Earth.”

The observation shows that “yes, there are things hitting the moon that we can actually see as flashes,” Marsden said.

The astronomers made the observations to help understand how many meteoroids are out there wandering around in space. Most estimates have been based on visual observations, radar detection and the counting of craters on the moon.

In a sense, the astronomers wrote in Nature, “the moon can be regarded as a high-exposure facility with a huge collecting area.” As a result, “measurements of direct impacts may be very valuable” for estimating how many largish objects come cruising through the Earth’s vicinity.

Marsden said the meteoroids were probably fairly large fragile objects, materials that boiled off Comet Tempel-Tuttle when it traveled toward the sun. He added that Tempel-Tuttle is in an orbital path that brings it close to the sun once every 33 years. During each encounter, the sun’s heat boils material off the comet’s frozen surface.

The impacts on the moon were first reported to the world’s astronomers by the IAU’s notification service Nov. 26.