Editor's Note: This story appeared prior to the 2002 Leonids. The science involved remains relevant, but specifics regarding the hourly rates and dust stream encounters predicted for 2002 do not apply to other years.

When comet debris peppers Earths atmosphere during this years Leonid meteor shower, the sky will be ablaze as tiny bits of cosmic mass moving at incredible speed are converted to heat and visible light.

But how much energy is involved in these shooting stars?

The faintest meteor that becomes visible to the average viewer on Earth is typically about 0.6 millimeters across (less than one-tenth of an inch or about the size of a sand grain). While such a speck is here and gone in a flash, the energy involved could light a 100-watt light bulb for about 2.5 seconds, Cooke said.

A slightly larger meteor, just 1 millimeter across and only moderately bright, packs the punch of a .22 caliber bullet.

Spectacularly bright fireballs, for which this annual event is known, dissipate far more energy during their plunge through the atmosphere. A typical fireball, which can briefly shine as bright as the planet Venus, is the size of a marble, about 9 millimeters in diameter.

"Such a critter has a striking power in excess of 1 million joules, or about the same punch as a VW moving at 60 mph," Cooke marveled, "from a particle just over one-third of an inch across!"

Behind this power is one simple fact: The Leonids scream.

The shooting stars of the Leonids are mere bits of dust left behind by a comet called Tempel-Tuttle, which orbits the Sun in the opposite direction as Earth travels. The comets dusty exhaust orbits the Sun, too.

Each November, Earth runs into this debris and scoops some of it up. Because of the opposing orbital directions, the Leonids are moving much faster than a particle thats travelling through space in roughly the same direction as Earth a typical meteor on any other night, for example.

"It's like two cars hitting head-on," Cooke says of a Leonid. "Greater speed equals greater energy to be dissipated, which equals a real short life in the atmosphere."

A Leonid crashes into Earths upper atmosphere at more than 160,000 mph (72 kilometers per second). A typical bullet from a rifle, moving at what seems like blinding speed, creeps along by comparison at just 2,240 mph (1,000 meters per second).

The comet grains become visible at around 60 miles up (100 kilometers) because they heat up as they plow through air, to the point that they glow. Most of them disintegrate high overhead.

Cooke uses basic physics to calculate the energy output of individual shooting stars. But with predictions for an historical flurry of activity in 2002, I pressed him to speculate on the total energy output of this years Leonids.

"You are putting me out on the limb," he said, but he attempted what he terms a crude answer anyway.

Most of the mass will come from big fireballs, Cooke reasons, because if you put 10,000 typical small Leonids together, they would weigh just 0.1 grams. A bright fireball, however, can weigh as much as 85 grams (3 ounces).

During the course of the peak hour over North America in the pre-dawn hours of Nov. 19, predictions of shower activity suggest that about 2.5 pounds (1 kilogram) of material will rain down over an area visible by a single observer. It will create about 650 kilowatts of energy, Cooke said, enough to run a small house for about a month if it could be harnessed.

"However, when one takes into account the entire hemisphere of Earth facing the Leonids, then we find that about 12-13 tons (12,500 kilograms) of Leonid stuff will hit our atmosphere at a screaming 45 miles per second," Cooke explained. "Which, taken in total, will produce around 4 kilotons of energy, or about 4.5 million kilowatt-hours, which could run about 7,000 houses for one month."

The conversion of a meteors mass and momentum to heat and light is often said to be caused by friction with air molecules. This sounds logical, but its not true.

In fact, a meteor is moving so fast that it compresses the air in front of it, something like how a boat pushes water out of the way and makes a wake. The compressed air heats up by a phenomenon called ram pressure. Its the same thing that causes a hand-held air pump to get warm when you work it to fill a basketball.

The heated air, in turn, scorches the meteor. Temperatures can exceed 3,000 degrees Fahrenheit (1,650 Celsius).

A Leonids demise can look a bit like a fiery exclamation point in the sky, a long streak with, sometimes, a bright dot at the end. It is fitting punctuation to a journey that typically spans a stretch of time equal to a good chunk of American history.

The comet responsible for the Leonids, Tempel-Tuttle, orbits the Sun every 33.3 years on an elliptical trajectory that crosses Earths orbit, rounds the Sun and then swings out beyond the planet Uranus. On each pass through the inner solar system, the comet is bathed in heavier doses of solar radiation. Some of its outer shell boils off. This action explains why a few comets can sometimes be seen from Earth to glow dramatically and sport tails -- sunlight reflects off this fresh material.

Its also why we have meteor showers.

While Tempel Tuttle does not make itself visible to us, every orbit lays down a new trail of debris in a slightly different location. These trails really more like streams spread out over time as the individual dust particles continue to orbit the Sun, each taking a minutely different path.

This year, Earth will pass through two primary streams of debris. One, deposited in 1767, will generate an outburst of shooting stars over Europe. The other, from 1866, will fuel the North American burst of activity. These bursts will last less than a few hours, peaking perhaps in a flurry of activity that goes on for no more than 30 minutes.

The burst is caused by the streams area of greatest density. You can think of it as wading through a real stream the edges are shallow, and most of the water is in the deep middle.

There are many of these Leonid streams, many of them so ancient theyre no longer detectable as individual entities. For that reason, the total run of the Leonid meteor shower is from about Nov. 14-20. During this whole stretch, Earth will pick up Tempel-Tuttle dust thats been spreading for millennia.

Centuries from now, in fact, the two streams that caused this years storm will have become largely indiscernible, having spread out into the broader, overall river of dust that makes the Leonids a surefire annual event.

Next year, and in every year until at least 2033, Earth will miss all of the primary streams. The Leonids will persist, but they will be relatively subdued events, nothing like the display expected between midnight and dawn on Nov. 19, 2002, a show of power that could light your house for a month and then some.