During the 2001 Leonid meteor storm, astronomers observed a curious flash on
the Moon — a telltale sign of meteoroids hitting the lunar surface and

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

“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

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

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.