THE LEONIDS OF NOVEMBER
Dr. Terry Goforth
Associate Professor of Physics, SWOSU
November 2001

November’s annual meteor shower, the Leonids, is once again preparing a nighttime light show for our delight. Each year for a few days before and after November 17, the Earth passes through the dust trail of Comet Tempel-Tuttle. Mostly dust-sized debris left in the wake of the comet enters Earth’s atmosphere at speeds around 25,000 mph. At these speeds, the atmospheric drag on these dust particles heats them to very high temperatures, causing them to glow brightly for a brief second before they disintegrate. The resulting bright streaks across the sky are meteors, more commonly referred to as falling stars. Space is full of these small grains, and Earth’s skies are brightened by meteors all the time, but when Earth passes through a comet’s path, the predictable increase in meteor rates is known as a meteor shower.

Earth passes through a comet's orbit while orbiting the sun. The Leonids have put on a pretty good show for Earth in the last three years. 1998 produced a series of fireballs–meteors that parallel the horizon and burn long and bright. While the number of fireballs wasn’t all that high, their brightness and endurance made it a memorable shower for all who saw it. In 1999, observers in Europe and the Middle East were treated to a full-fledged meteor storm (defined as more than a 1000 meteors per hour or about 17 per minute), but North America was left out. Last year, Europe and the Eastern U.S. witnessed rates around 40-50 per hour, with short bursts as high as 450 per hour. So what will this year hold?

The predictions for 2001 range from a strong shower to a mild storm. Various researchers are forecasting peak meteor rates from 800 to 4000 per hour occurring around 4 a.m. in the Central Time Zone. (Allow a 30-minute window on either side.) With no moon in the sky, this may be the best opportunity most of us will have in our lifetime to observed a bona fide meteor storm.

Meteor showers are annual events–Earth passes through several comet paths each year. Astronomers know when these passages take place, and as a general rule they can predict about how many extra meteors will be visible during these times. But until recently the prediction of meteor storms in which a dozen or more meteors may be visible each minute was based purely on past statistics and was not much more accurate than predicting the weather a year in advance. In the last few years, observations and increased computer power have combined to give astronomers an unprecedented understanding of the cause of meteor storms. New models not only allow them to predict meteor rates with greater precision, but also to narrow the window of time during which the increased activity is most likely to occur.

Meteor storms occur when Earth passes through a dense swarm or cluster of cometary debris. Comets move around the sun in highly elongated orbits which stretch from the warm regions near the sun to very far from the sun where the temperatures are unbelievably cold. Most of the time a comet is just a “dirty snowball”–a large ball of frozen water and other icy material embedded with bits of dust and rock. But when the comet comes closer to the sun, some of the icy material sublimes, turning from a solid directly to the gaseous state. (It is impossible for a liquid to exist in the vacuum of space.) This gas swirls around the solid comet nucleus and is pushed away from the comet by the solar wind, producing the familiar bright head and extended tail that most of us identify as a comet. Of course, as the icy material is released from the nucleus, so are the rocky particles that were embedded in it. These particles continue to orbit the sun more or less in the comet’s orbit, spreading out along the orbit as time passes. The comet’s orbit is not uniformly “polluted” by these particles. A small number are scattered throughout the orbit, but others are clustered to form regions with a high density of particles. If the Earth misses these clusters, an ordinary meteor shower takes place. If the Earth skirts one of the clusters, a brief period of high activity will occur. And if the Earth passes through a cluster (as is predicted this year), a full-fledged meteor storm is possible.

How do you go about observing the meteor shower? Telescopes and binoculars are useless. Your eyes are your best tool because you want to see as much of the sky as possible at one time. Here are some suggestions:

Once again, the forecast is for the peak activity to occur around 4 a.m. on Sunday, November 18. A good bet would be to get to your observation spot around 3 a.m. This will allow your eyes time to acclimate to the dark and still be in position just in case the shower (storm?) starts a little early. Keep your fingers crossed that we have clear skies, and enjoy!

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Last update:  November 15, 2002