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Lunar Leonids
For most stargazers, this year's quarter Moon during the Leonid meteor shower will be a blazing nuisance. Bright moonlight will overpower many faint shooting stars as the Earth passes through the outskirts of three cometary debris streams during a 36-hour period spanning Nov. 17 and 18, 2000.

by Dr Tony Phillips.

For some astronomers, the Moon itself will be the main event if a Leonid meteor storm erupts.

"On Nov. 17 [around 0500 UT] the Moon will pass approximately four Earth-diameters from the center of a dust trail left behind by comet Tempel-Tuttle in 1932," says David Asher of the Armagh Observatory, an expert on Leonid debris filaments. "The Moon will be considerably closer to the trail than Earth," raising the possibility of vigorous Leonid activity there.

When Leonid meteoroids rain down on the airless Moon, they won't cause "shooting stars" as they do on our planet. There's no atmosphere on the Moon where cosmic debris particles can incinerate as fiery streaks of light. Lunar Leonids will simply hit the ground with a head-spinning velocity of nearly 140,000 mph.

Prior to 1999, no one had ever recorded natural impacts of any sort on the Moon. But last year surprised Moon-watchers captured video footage of at least six exploding Leonid meteorites while the Moon passed through another debris stream from comet Tempel-Tuttle. The impact flashes were relatively faint - 3rd magnitude at most - but they were clearly visible in videos recorded by astronomer David Dunham and others.

Although the Moon will pass nearly as close to a debris stream in 2000 as it did in 1999, meteor watchers won't be able to spot the telltale flashes of lunar Leonids. This year the Moon will plunge into the stream farside first. All of the meteoroids that hit the Moon will land on terrain that's hidden from direct view.

Nevertheless, say scientists, there may be a way to monitor this year's activity indirectly.

"When a Leonid meteoroid hits the Moon it vaporizes some dust and rock," explains Jody Wilson of the Boston University Imaging Science Team. "Some of those vapors will contain sodium (a constituent of Moon rocks) which does a good job scattering sunlight. If any of the impact vapors drift over the lunar limb, we may be able to see them by means of resonant scattering. They will glow like a faint low-pressure sodium street lamp."

Even when there are no ongoing meteor showers, the Moon is surrounded by a gaseous halo called the lunar "exosphere," says Wilson. Consisting of a just a few hundred atoms per cubic centimeter, the exosphere is barely more than a vacuum. The solar wind blows it into a long tail much like a comet's. It points away from the Sun and extends for hundreds of thousands of kilometers. The giant tail is so rarefied that it's completely invisible to the unaided eye even when the Earth passes through it once a month around the time of the New Moon.

Wilson and his colleagues at Boston University, led by Prof. Michael Mendillo, routinely monitor the Moon's tail. They use extraordinarily sensitive cameras that can detect sunlight scattered from as few as 5 sodium atoms per cubic centimeter. For comparison, the density of Earth's atmosphere at sea level is 3 x 1019 molecules/cm3.


Crators left on the Moon's surface by the Leonid meteor storms.

"The primary goal of our research is to figure out what process is most responsible for producing the Moon's atmosphere," says Wilson. "Is it solar radiation, the solar wind, meteoroid impacts, or some combination? We're still not sure."

"Two years ago the density of the Moon's sodium tail tripled just after the 1998 Leonid fireball shower," he continued. "The meteoroids in '98 were larger than usual and they were really slamming the Moon when they hit. There's no doubt that the primary source of the exosphere at that time was the Leonids."

Mendillo's group was monitoring the Moon again in 1999 when a full-fledged Leonid meteor storm burst over Western Europe. "We know that the Moon was close to a debris trail in '99 because of all the impact flashes people saw," noted Wilson, "but the sodium concentration didn't seem to be much greater than normal."

It may be that 1998 was simply special. Leonid particles that hit the Moon (and the Earth) that year were relatively large ones (like marbles rather than snowflakes) that had accumulated into a coherent dust filament as the result of an orbital resonance with Jupiter. "I suspect the enhanced sodium tail in 1998 was related to the total mass of the impacting meteoroids rather than to their number," says Rob McNaught (Australian National University) who studies Leonid debris trails in collaboration with David Asher. "Of all the recent years we've studied, 1998 would probably have the greatest mass hitting the Moon and thus the greatest sodium production."

"I think we'll look back years from now and realize that 1998 was very special," agrees Wilson. "The fireballs on Earth were unique and we've never detected another meteor-related enhancement of the Moon's tail. That includes 1999 when the sheer number of Leonids hitting the Moon was probably much higher than the year before. Even in '98, when the sodium density tripled two days after the shower (that's how long it takes for sodium to travel down the tail the length of the Moon's orbit), the enhancement didn't last long. The sodium tail faded back to normal within 24 hours.

"All this could add up to solar processes, not meteoroid impacts, as the dominant day-to-day producers of lunar exospheric gas. But we need more data to be sure."


Dazzled observers saw brief scenes like this one, captured by A. Scott Murrell during the 1966 Leonid storm.

A better year for spotting lunar Leonids directly is just around the corner, say meteor experts. On Nov. 18, 2001, the Moon will be just 2 days past New when a furious Leonid storm is likely to erupt. David Asher and Rob McNaught predict as many as 10,000 meteors per hour on Earth and similar numbers of impacts on the Moon. (Leonid rates in 2000 are likely to be 100 times lower.)

Unlike this year, the 2001 "sub-Leonid point" (the spot where Leonids crash directly down onto lunar terrain) will be favorably positioned on the darkened nearside of the Moon, which will appear as a super-thin crescent. Camcorder shots of detonating Leonids may be possible for careful photographers filming just after the Sun sets on Nov. 18th next year.

Until then, Leonid impacts will be hidden from direct view. Moon-watchers must rely on ghostly vapors from the far side of the Moon, and not their own senses, to tell the tale of the Lunar Leonids 2000.

With this being a busy time for astronomers, we've trawled the web to bring you some links to help you through. Please take the time to visit Leonids Live, Leonids 1833 : The Night of Raining Fire and Leonids 2000.   

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First Science 2014