THE LEONIDS, ONE MORE TIME!
Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
November 2002

It seems to have become an annual event: predictions of a meteor storm in mid-November. Starting in 1999 experts in the emerging field of meteor shower forecasting have told us to look up, it'll be a good show. This year is no exception, except that it will be the last time for many years to come.

The Leonid meteor shower occurs annually around November 17 or 18 when the Earth passes through the orbit of Comet Tempel-Tuttle. The comet, a ball of icy material embedded with bits of dust and rock, travels around the sun once every 33.3 years. During this trip, the warmth of the sun causes part of the ice to sublime (convert directly from solid to gas, just like dry ice), which in turn releases some of the dusty material. The comet leaves swarms of dust and rock in its trail. This material then continues to circle the sun in the same orbit as the comet with nearly the same period.

When the Earth passes through the comet's orbit, it encounters this debris. The pieces of dust and rock (mostly the size of sand grains, some as large as marbles) enter the Earth's atmosphere at speeds of 160,000 miles per hour. This high speed causes the particles to heat rapidly to a temperature of several thousand degrees and glow for a very brief time before disintegrating about 60 miles above the Earth's surface. The glow they produce is what we see and call a meteor. Meteors can actually be seen any night of the year, but when the Earth encounters the dust in a comet orbit, the activity rate (average number of meteors seen per hour) increases and the event is called a meteor shower. The "typical" Leonid meteor shower produces rates of 10-15 meteors per hour–nice, but not always worth getting out of bed at 4 a.m. and standing out in 30 degree weather to see. However, in the few years following Comet Tempel-Tuttle's trip around the sun, Earth is much more likely to encounter tight swarms of particles released in "recent" trips. If the Earth passes through the outskirts of one of these swarms, rates can rise to 50 or 100 meteors per hour, and if the Earth passes through the center of a swarm, the rates will go even higher. If the rate exceeds 1000 meteors per hours (or about 17 per minute) for at least 15 minutes, the shower is classified as a meteor storm.

Comet Tempel-Tuttle's last pass around the sun occurred in 1999, and the strong showers and storms of the past few years have resulted from Earth's encounters and near-encounters with the trailing dust swarms. This year we can hope to see more of the same. Various experts in the field are forecasting peak rates of 2,000 to 6,000 meteors per hour around 4:30 a.m. (CST) on Tuesday, November 19, with the activity concentrated to a time between 4 a.m. and 5 a.m. Unfortunately, a full moon will interfere with the view. Bright moonlight makes it impossible to see the dimmer meteors, so observed rates will actually be reduced to about 500 to 1,000 meteors per hour (or about 10 to 20 per minute). So why bother? Because this is still significantly better than the average Leonid show, and because it won't happen again for several decades. Earth is not predicted to encounter another dust swarm until 2033, when Tempel-Tuttle makes its next visit to the sun.

So how should you plan to watch the shower? A few suggestions include

Remember, the peak is predicted to occur around 4:30 a.m. (or between 4 a.m. and 5 a.m.) in the pre-dawn hours of Tuesday, November 19 (or Monday night, the way some of us reckon). For the enthusiasts, the Leonid activity has already begun. You can see meteors (but not several per minute) from now until around November 20. The best time to look is between midnight and dawn.

Great information is available at www.space.com, www.spaceweather.com, and science.nasa.gov/headlines. Hopefully the weather will cooperate this year (wish for clear skies)! Get out and enjoy!

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1999 Leonids
2000 Leonids
2001 Leonids

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Last update:  November 15, 2002