The Leonids are debris shed into space by Comet Tempel-Tuttle, which swings through the inner solar system at intervals of 33 years. With each visit the comet leaves behind a trail of dust in its wake.

In the 2004 Observer's Handbook of the Royal Astronomical Society of Canada, meteor experts Margaret Campbell-Brown and Peter Brown indicate that this year's peak activity should occur on the morning of Nov. 17. They cite 09:00 GMT, which corresponds to 4 a.m. ET and 1 a.m. PT.

This time is highly favorable for North Americans, especially those in the eastern United States and eastern Canada.

Other meteor researchers, however, such as Jeremie Vaubaillon of France, David Asher of Ireland and Esko Lyytinen of Finland, have examined Leonid prospects for this year and also suggest watching for some meteor activity two days later, on Nov. 19.

A brief bevy of 30 to 60 Leonids per hour may seen, but the time frame when these meteors are predicted to be most numerous (approximately 19:00 to 22:00 GMT) favors observers in Asia and Australia (where it will be the early morning hours of Nov. 20).

The meteors will appear to emanate from out of the so-called "Sickle" of Leo, but prospective viewers should not concentrate on that area of the sky around Leo, but rather keep their eyes moving around to different parts of the sky.

The parent body, comet 55P/Tempel-Tuttle, passes its dust trail through the orbit of the Earth around November 18-22 every year, but the magnitude or peak rate of meteor encounters varies on a regular three-decade cycle.

Comets are an important source of meteoroids. After many visits near the sun, a comet's "dirty-snowball" nucleus of ice and dust decays and fragments, leaving a trail of meteoroids along its orbit.

Some "meteoroid streams" cross the Earth's orbital path, and when our planet passes through them some of these particles enter the atmosphere. The outcome is a meteor shower - the most famous being the "Perseids" in the month of August and the "Leonids" in November.

Most Leonid particles are tiny and will vaporize very high in the atmosphere due to their extreme speed (about 44 miles per second, or almost 71 km/sec), so they present no threat to people on the ground or even in airplanes.

It is only when these small rocks (or meteoroids) enter the Earth's atmosphere that they are called meteors.

They make a dramatic and beautiful entrance, burning up and producing a short glowing trail in the night sky that rarely lasts more than a second or two. The temperature of the meteor trail can reach about 4,600 degrees Celsius (8,312 Fahrenheit).

Most meteors are completely destroyed at altitudes between 80 and 110 kilometers, but those that make it to the ground are given yet another name: meteorites.

Public observers of this year's meteor shower and fireballs are invited to report their results online with various astronomical organizations.

Recent years (from 1998 to 2002) have seen some exceptional Leonid meteor showers and storms in some years. The last one occurred in 2002 but, from the results of computations, it appears that no other storm is expected for the coming decade. At present, no Leonid storm until at least 2033 has yet been identified.

Three years ago, "Leonid meteor storm yielded rich research results for NASA astrobiologists," said Dr. Peter Jenniskens, a NASA astronomer based at Ames Research Center and principal investigator for an airborne research mission to collect data unhampered by ground lights and most local weather conditions.

"Findings to date indicate that the chemical precursors to life - found in comet dust - may well have survived a plunge into early Earth's atmosphere."

Jenniskens and his international cadre of researchers think that much of the organic matter in comet dust somehow survived the rapid heating of Earth's atmospheric entry.

"Organic molecules in the meteoroid didn't seem to burn up in the atmosphere," he explained. They may have cooled rapidly before breaking apart, he concluded.

Another finding with potentially important implications for astrobiology is that meteors are not as hot as researchers had previously believed.

"We discovered that most of the visible light of meteors comes from a warm wake just behind the meteor, not from the hot meteoroid's head," said Jenniskens. This warm wake has just the right temperature for the creation of life's chemical precursors, he said.

Utah State University researchers found that, during the meteors' demise in the atmosphere, their rapid spinning caused small fragments to be ejected in all directions, quite far from the meteoroid's head. This is an important finding for astrobiology, because it means that meteors may be able to chemically alter large amounts of atmosphere.

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