From: Marshall Space Flight Center
Posted: Monday, December 3, 2001
During the 2001 Leonid meteor storm, astronomers observed a curious flash on the Moon -- a telltale sign of meteoroids hitting the lunar surface and exploding.
November 30, 2001: Vivid, colorful streaks of light. A ghostly flash. Strange crackling noises and twisting smoky trails. Add to those a cup of hot cocoa, and you have all the ingredients for a delightful meteor shower ... on Earth.
The recent Leonids were a good example. On Nov. 18th our planet plunged into a debris cloud shed by comet Tempel-Tuttle. Sky watchers saw thousands of meteors -- each streak of light a tiny bit of comet dust disintegrating in the atmosphere.
A quarter of a million miles away, another Leonid shower was happening. But the recipe was different: Blinding flashes of light. Flying debris and molten rock. Sizzling craters. And certainly no hot cocoa! That's what the Leonids were like ... on the Moon.
"Like Earth, the Moon also plowed through comet Tempel-Tuttle's debris field on Nov. 18th," says Bill Cooke of the NASA Marshall Space Flight Center. But, unlike Earth, the Moon doesn't have an atmosphere where meteoroids harmlessly disintegrate." Instead, lunar Leonids hit the ground and explode.
David Palmer, an astrophysicist at the Los Alamos National Laboratory, saw just such an explosion from his backyard in White Rock, New Mexico. The 2001 Leonids were well underway when Palmer trained his 5-inch Celestron telescope and a low light video camera on the crescent Moon. "It was twilight," says Palmer. "Even so, the flash was bright enough to detect." He had spotted a Leonid crashing down near Sinus Media -- a lava plain on the lunar equator.
Far from New Mexico, observers on the east coast of the United States saw it, too. Using 8 inch telescopes equipped with video cameras, David Dunham in Maryland and Tony Cook in Virginia independently recorded the flash -- a double confirmation. "We estimate it was as least as bright as a 4th magnitude star," says Dunham, director of the International Occultation Timing Association.
This marks the second year Dunham and Palmer have seen lunar Leonids. They and others video-recorded six meteoroid impacts on the Moon during the 1999 Leonid meteor storm. The brightness of those flashes ranged from 7th to 3rd magnitude.
"Actually, we've known for many years that Leonids hit the Moon," notes Cooke. "Between 1970 and 1977, Apollo seismic stations detected impacts during the Leonids and several other annual meteor showers. What's new since 1999 is that we're actually seeing the explosions from Earth."
The first reports of bright lunar Leonids two years ago puzzled many scientists. Their calculations suggested that a Leonid hitting the Moon would need to mass hundreds of kilograms to produce an explosion visible through backyard telescopes. Yet there was little evidence for such massive fragments in the Leonid debris stream. Hundred-kilogram meteoroids hitting Earth's atmosphere would produce sensational fireballs, brighter than any sky watchers actually saw. Furthermore, lunar seismic stations operating for years had detected nothing larger than 50 kg.
To solve the mystery, Jay Melosh, a planetary scientist at the University of Arizona's Lunar and Planetary Lab and an expert on planetary impact cratering, teamed up with Ivan Nemtchinov, a Russian physicist skilled in computer simulations of nuclear explosions.
Experience with bombs came in handy solving this problem, says Melosh: "Leonid impacts aren't as potent as a nuclear warhead, but they are powerful. They hit the Moon traveling 72 km/s or 160,000 mph -- that's 240 times faster than a rifle bullet. In fact, the energy per unit mass in a Leonid strike is 10,000 times greater than a blast of TNT."
Using computer programs designed to study bomb blasts, Melosh and Nemtchinov discovered that Leonids didn't have to be so massive to produce flashes as bright as those detected by Dunham and Palmer. Impactors massing only 1 to 10 kg could do the job.
"That's more like it," says Cooke. "We occasionally see kg-sized fragments burning up in Earth's atmosphere. They appear as very bright fireballs that disintegrate completely before hitting the ground." On the Moon, of course, there's nothing to stop them from reaching the surface.
According to Melosh, here's what happens when the Moon and a 10 kg Leonid collide:
Much of the ground within a few meters of the impact point would be vaporized, and a cloud of molten rock would billow out of a growing crater. "At first the cloud would be opaque and very hot, between 50,000 K and 100,000 K," explains Melosh. "But the temperature would drop rapidly. Milliseconds after the initial blast, the cloud would expand to a few meters in diameter and cool to 13,000 K. That's the critical moment," he says, "when the vapor becomes optically thin (transparent); then, all the photons rush out and we can see a flash of light from Earth."
An astronaut watching the event on the Moon, perhaps a hundred meters or so from the impact, would be momentarily blinded by the Sun-bright explosion. There wouldn't be a deafening report, however, and onlookers wouldn't be knocked down. "There's no air on the Moon to carry shock waves," explains Melosh. "Even so, you might have to pry some nasty bits of molten rock out of your space suit."
Fortunately for future Moon colonists, there's little chance of being hit. Cooke explains: "During an intense Leonid meteor storm like the one Earth experienced in 1966, the lunar flux of meteoroids more massive than 10-5 gm would be 1 per square-km per hour. If we assume really chubby or bulky astronauts with a cross-sectional area of 0.5 square-meters, then the probability of being hit by a 10-5 gm Leonid is only 0.00025." Such tiny particles carry enough energy to puncture a spacesuit, but the astronaut inside would remain mostly intact, says Cooke. "The probability of being hit by something that might totally vaporize you -- like a 10 kg fragment -- is a billion times less."
So ... lunar Leonid meteoroid showers might not be as scary as they sound. Future denizens of the Moon might even take up a new astronomical hobby: ground watching. "I saw a hundred puffs of moondust every hour," they might say after a good spate of Leonids. "And, ooh that fireball ... what a blast!"
Editor's Note: After this article was published, a second lunar flash was confirmed for the 2001 Leonid shower. Video tapes recorded by Roger Venable from eastern Georgia (USA) and David Dunham of Maryland reveal a lunar Leonid on Nov. 18, 2001, at 23:19:16 UT near Tranquillitatis.
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