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Remember the Leonid meteor storm last November? Great meteors. Lousy weather. Outdoors at 3 a.m. in mid-November is just too cold for comfort. The Perseids are different. They come in August when the cool night air is refreshing, not bone chilling. I can shuffle outside at 3 a.m. in my pajamas and still enjoy the show. Above: Photographer Dautel Nathalie caught this Perseid meteor (upper right) streaking through the Milky Way on August 12, 2001. [more]
Perseid meteors come from comet Swift-Tuttle. Every 130 years, the comet swoops in from deep space (beyond Pluto) and plunges through the plane of the solar system not far from Earth's orbit. Astronomers once worried that Swift-Tuttle might hit our planet, but recent data and calculations show otherwise. There's no danger of a collision for at least a millenium and probably much longer. Even so, little pieces of Swift-Tuttle do hit Earth. The comet's orbit is littered with bits of dusty debris. They bubble away from the comet's icy nucleus (propelled by evaporating ice) when Swift-Tuttle nears the Sun. These grains form a cloud that we plow through once a year. We're entering the outskirts of that cloud now (late July). Every hour, one or two meteors are streaking across the sky. It's the slow beginning of the Perseids.
Perseid dust particles are tiny, most no bigger than grains of sand. Yet they travel very fast--about 132,000 mph (59 km/s). Even a tiny dust speck can become a brilliant meteor when it hits the atmosphere at that speed. There's no danger to sky watchers, though. The fragile grains disintegrate long before they reach the ground. Because of the way the comet's orbit is tilted, dust from Swift-Tuttle falls on Earth's northern hemisphere. Viewed from Earth's surface, the meteors appear to flow from the constellation Perseus (hence the name Perseids). Perseus is easy to spot from Europe and North America, but it barely peeps above the horizon of, e.g., Australia and New Zealand. Southern hemisphere sky watchers will see very few Perseids. The following is true no matter where you live: The best time to look for meteors is when Perseus is highest in the sky--between 2 a.m. and dawn. On August 12th, set your alarm for 2 o'clock in the morning. Go outside; lie down on a sleeping bag or a reclining lawn chair with your toes pointed northeast; and gaze upward. Soon you'll see shooting stars racing along the Milky Way. Repeat the procedure on August 13th. The shower's peak is long-lasting, and you're likely to count plenty of meteors on both days. (The shower is expected to crest on August 12th during a 12 hour period centered on 2200 Universal Time. Dawn on August 13th would therefore be best for Europeans. Dawn on either day could be good for North Americans.)  Above: The northeastern sky at 4 o'clock in the morning on August 12, 2002. Meteors will seem to flow from a point in Perseus called "the radiant" (red dot). Don't forget to look for Saturn! [large map] Can't wake up at 2 a.m.? Try looking for Perseids instead around 9 or 10 p.m. when Perseus is hanging low. (Note: it's always hanging low in the southern hemisphere.) You won't see many meteors then, but the ones you do see could be memorable. Shooting stars that emerge from the horizon and streak horizontally through the atmosphere are called Earthgrazers. They are remarkably slow, colorful and bright. Earthgrazers are a good target for city dwellers. Urban lights wipe out the rush of faint meteors that streak across darker skies. From a big city you can see only the brighter ones. Perhaps August would be a good time for a trip to the country. There you can see the faint Milky Way and hundreds of Perseids ... and maybe no one will notice if you shuffle out in your pajamas to watch the stars. |
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