Circle November 17 on your calendar. It's show time for nature's most tantalizing sky event, the long-awaited Leonid meteor storm. You just might see the greatest cosmic spectacle of your life: a sky set ablaze by a fusillade of shooting stars. Then again, you might observe no more than an occasional streak of light zooming by every few minutes.
The unpredictability of the Leonids illustrates how difficult it is to anticipate exactly where anything in space will be at any time in the future. Most people imagine that the heavens operate with clocklike precision. But centuries ago, Isaac Newton discovered that his neat orbital calculations only worked perfectly when figuring the movement of two celestial bodies. Add just one more, and the motions get so complicated we can only approximate them. Recent computer simulations show that our solar system is full of chaos.
The situation is especially confusing for small asteroids and comets, because they're so easily buffeted by the gravity of their planetary neighbors. A chunk of space rock that comfortably, but barely, misses Earth tomorrow could come back to strike us in our lifetime. We cannot foretell its precise path far in advance. That's why astronomers don't know whether this year's Leonid serving will be feast or famine.
Leonid meteors are mostly sand-grain-sized fragments of the comet Tempel-Tuttle, bits of debris that flake off as the comet slowly defrosts every time its roughly 33-year orbit guides it back around the sun. Each November, when Earth passes through Tempel-Tuttle's orbit, some of those pellets slam into our atmosphere. Especially stirring displays occur about every third of a century when we plow through the thickest part of the debris cloud, which clings close to the comet.
Before sunrise on November 13, 1833, the Leonid shower unleashed 100,000 shooting stars per hour. Many astonished Americans thought the world might be coming to an end. At Yale College, Dennison Olmstead studied the paths of the meteors and announced that they had arrived from deep space. In a neighboring office, Hubert Newton calculated that the swarm would return in 1866. And it did. But ever since then, gravitational Ping-Pong has kept everyone guessing about how mighty the appearances would be.
In 1899 and 1933 the Leonids were a bust. In 1966, however, 40 meteors per second were seen for more than an hour in southwestern skies. One of my former students remembers watching the shower from a train window while speeding across Texas on November 17 that year. She, too, was certain Armageddon had arrived. All the other passengers in the train car were asleep, and for a moment she wondered whether to wake them up and say: "Excuse me. It's the end of the world."
Two years ago, Tempel-Tuttle passed by the sun, bringing us a superb meteor shower last year but not an extraordinary storm. That display arrived a day ahead of schedule, so it's a good idea to start looking on the night of November 16. Don't bother with a telescope or binoculars, because it's better to take in as wide a view as possible. Check the sky every hour or so, scanning the heavens for at least five minutes. That should be enough time to catch a falling star, even if all you get is the ho-hum 20 meteors per hour typical of most years. But with a little luck, this year's celestial lottery will reward you with the greatest show off Earth.