Lunar Leonids Strike Again
Leonid meteor shower, while the Earth passed through the debris stream shed by Comet Temple-Tuttle, it brought with it a straggler — the Moon. A few hours after the pebble-size bits hit the Earth and burned up in the atmosphere, the Moon underwent a similar bombardment. The difference was that Luna had no atmospheric armor to protect her from the incoming rocky rubble. Leonids hit the Moon fast and hard. And two of those impacts were spotted here on Earth.
Back in the 1999 Leonid shower, astronomers made their first-ever conclusive observations of meteors hitting the Moon. That year, conditions were ideal to see impact-created blinks of light: a first-quarter phase kept the target area in darkness while allowing a prolonged good view from Earth. Seven brief flashes were recorded on video cameras, the brightest reaching 3rd magnitude. In 2000 the situation wasn't nearly as favorable. The last-quarter phase of the Moon meant the Leonids impacted on the lit side.
But last month's spectacular shower during a crescent Moon gave astronomers another chance to look for lunar impacts. Although the Moon was visible only briefly from any given location before setting, observers videotaped two impacts: one at 23:19:16 Universal Time (11:19:16 Eastern Standard Time) on November 18th, the second about an hour later at 00:18:58 UT on the 19th. The latter was the brightest and reached 4th magnitude.
Like other observers, Anthony Cook trained his video camera on the Moon until it set, but didn't find the flash until he reviewed his tape later. "The 00:18:58 flash jumped out immediately and I [reported it]," says Cook.
Seeing these flashes of light no longer comes as a surprise to scientists. In the past few years, researchers have found that even small, low-density impactors, such as the Leonids, coming in at ultra-high velocities (72 kilometers per second) should make brilliant flashes. According to a study conducted by Natalya A. Artem'eva and her two colleagues (Institute for Dynamics of the Geospheres, Moscow), meteoroids need only be between 4 and 20 centimeters across to make flashes visible from Earth.