As meteor showers go, the Leonids are sort of so-so. Compared to the consistent activity of the Geminids or the Perseids, with 90 to 100 meteors per hour at peak, the Leonids clearly don't measure up. Their typical peak is about 12 to 15 meteors per hour, though most observers usually see less than that.
But then sometimes -- very rarely -- the Leonid shower shows some attitude and produces hundreds, even thousands, of meteors per hour. At intervals of 33 years or so, the display is inclined to be more than a mere spritzing of meteors. It tends to be a veritable storm. Such a storm could come this year, in mid-November.
The anatomy of a meteor storm
Meteor storms are uncommon phenomena. The reason doesn't have as much to do with where meteors are in space as much as it has to do with where comets are, because comets are the sources of meteoroids.
When a comet comes out of the deep freeze of space and enters the inner solar system, its surface is warmed by the sun. Part of its icy crust suddenly turns to vapor, which solar radiation blows off the comet's surface. Like snow-encrusted cars barreling down the highway, comets leave in their wake a tenuous trail of ice, dust and grit. When Earth sweeps through this detritus, the particles burn up in our atmosphere, creating a brief shower of meteors.
The Leonid stream is composed of debris from periodic Comet Tempel-Tuttle, which has a period of precisely 33.18 years. When the comet makes a close approach to Earth, as it did January 17, 1998, it leaves behind a more concentrated wake of debris. Hence, when Earth passes through this part of the debris stream, we see a greater display of meteors than usual. And if conditions are just right -- if we pass through a dense swarm of debris -- we see a meteor storm.
Meteor showers each radiate from a specific area in the sky called the radiant, which is why they have the derivative names of the constellations. Thus, the Geminids emanate out of Gemini, the Perseids from Perseus, the Orionids from Orion and so on.
The Leonid meteors appear to radiate from a particular area within the sickle-shaped head of Leo the Lion. Observers see the most meteors when the shower's maximum occurs while the radiant is high overhead. Unfortunately, these circumstances do not favor observers in the United States this year. The last optimal U.S. apparition of Leonid meteors occurred in November 1966, when observers across the central part of the country saw an estimated 150,000 meteors per hour.
Prospects in 1999
Last year, the predicted storm didn't happen. True, the 1998 Leonids were more intense than usual, but there were no official counts of thousands of meteors. Most meteor observers put the number at peak closer to 200 to 250 an hour at best. Moreover, the display peaked some 16 hours earlier than predicted.
From my location in east Texas, I counted, at most, 25 meteors per hour. That's still not a bad number, because to see more than one meteor in an hour's time is a thrill, especially when they are fireballs, as many of these were. So while disappointed that I didn't see a storm, I was happy to see a shower that was obviously very active.
Astronomers comparing past Leonid storms to the Leonid display in 1998 have since developed what they think are more accurate models to predict how the Leonids will behave in 1999. According to several independent predictions, the Leonids should peak on November 18, at 2h 08m UT (2:08 a.m. in England). This time favors morning observers in western Asia, Africa and Europe. In the United States, the time corresponds to 9:08 p.m. EST and 6:08 PST November 17. Obviously, these times aren't ideal, but neither do they occur in full daylight. Observers in North America would do well to look for meteors between those times and dawn on the 18th.
As we saw last year, the Leonids are notoriously unpredictable. The distribution of meteoroids within the stream is irregular. Following Tempel-Tuttle's closest approach to Earth in January 1998, the debris cloud is more concentrated than usual -- about 300 to 10,000 times its normal volume. Therefore, it is possible that observers on either side of the November 18 UT peak may see brief, sudden bursts of activity.
Where to look, what to do
Look for the Leonids to emerge from the "sickle" asterism of Leo -- the Lion's head -- which clears the eastern horizon by midnight this time of year. By 1:30 a.m., Leo will have fully risen, just as the night side of Earth begins facing into its orbit. Peak or no, a great number of meteors may be seen at this time, particularly when the radiant is halfway up in the eastern sky. In the U.S., that occurs around 4 a.m. This year, too, the waxing gibbous Moon sets before the radiant rises, so it will not interfere with seeing meteors.
Even though the meteors appear to emerge from the radiant in the Lion's head, don't focus all of your attention on this region. Leonid meteors may be seen anywhere in the sky. The only constraining factor is that the meteor's path, from beginning to end, must point back toward Leo. If it doesn't, you've seen a sporadic meteor, not a true Leonid.
And don't worry about being struck by a Leonid meteor during the storm. There has never been a recorded meteor fall associated with this shower because, given the insubstantiality of the particles, none could survive their fiery plunge through the atmosphere. The only possible hazards posed to earthbound observers are getting a stiff neck from looking up too long or of exposure to the cold. Warm blankets, lounge chairs or sleeping bags and the proverbial thermos of warm cocoa or coffee are highly recommended.
What is the future of the Leonids? Some experts predict higher-than-usual activity for the next four or five years. In 2000, the Leonids peak is favorable for us observers in the Western Hemisphere. Unfortunately, the Moon will be a waning gibbous and located in one of the worst locations: just west of the Lion's head. Still, it would be unwise to assume that, for the next few years the Leonids will not be worth watching.
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