|A mote of comet debris produced this Perseid meteor. Stars appear as trails because of the long exposure time in this photo.|
Photograph courtesy of James W. Young/Table Mountain Observatory/JPL/NASA
Sometimes there are second acts in astronomy. Perhaps you missed last November's dazzling Leonid meteor blizzard—or you watched it and got hooked. Either way, you'll have another chance to see shooting stars this month. For the first time since 1999, the Perseid meteor shower will unfold under spectacularly dark, moonless skies. And if you're under clouds the first peak night, August 11, you can catch a repeat performance the following night.Like the Leonids, the Perseid meteors are minuscule bits of comet crashing into our planet's atmosphere—petite cousins of the giant impacts that may have wiped out entire species in the past. "Just one comet in a million can hit Earth," says Kevin Zahnle, a planetary scientist at NASA's Ames Research Center. But each time a comet passes close to the sun, it evaporates a little and leaves behind a trail of debris. Those comet crumbs spread out over millions of miles, greatly increasing the odds of contact. Each Perseid meteor still follows the same basic orbit as its parent, comet Swift-Tuttle, which raises a worrisome thought. If the Perseids can strike us, couldn't Swift-Tuttle do so as well?
Swift-Tuttle's nucleus is about 15 miles wide, large enough to cause global devastation if it hit. Fortunately, an impact can occur only if the timing is perfectly wrong. A few years ago, Brian Marsden of the Harvard-Smithsonian Center for Astrophysics forecast a possible collision with comet Swift-Tuttle in 2126. He refined his work and found that the comet will actually miss by a wide margin in 2126 but will make a closer approach in 3044, passing within 10 million miles of Earth. That is much nearer than any planet ever approaches, but it is still far enough that we can breathe easily—for now.
Each flash of the Perseids demonstrates the staggering power of an extraterrestrial impact. The meteors slam into Earth's atmosphere at 37 miles per second. The most brilliant ones are no larger than a grape, yet they arrive carrying more kinetic energy than an SUV barreling down the highway. These micro-missiles superheat the air, vaporize, and then excite the surrounding atoms in the atmosphere 60 miles up, creating a brilliant light. This atmospheric glow generates the bright meteor streak. Interactions of metal atoms from the meteor with energized oxygen and nitrogen atoms in the air can prolong the effect, giving rise to a luminous trail visible for a second or two.
Because of Earth's curvature, no meteor can be seen more than 700 miles away, but researchers can calculate the total quantity of strikes by extrapolating from the number observed at a single location. On a typical day, the planet is hit by 25 million meteors whose glow would be bright enough to be seen by the naked eye. During the peak of the Perseids, the rate increases more than tenfold. In all, Earth accumulates 100 tons of space debris every day—mostly in the form of fine, harmless dust. No Perseid has ever been known to survive its passage through the atmosphere.
Still, it's fun to watch them try. On August 11 the crescent moon sets well before midnight, just as the show starts cranking up. A dark, rural site with an open expanse of sky will deliver the maximum effect, roughly 60 streaks an hour. A city rooftop view might provide only one meteor every five or six minutes.
Don't worry about the supply running out. The Perseid reservoir, replenished every 130 years when comet Swift-Tuttle swings past the sun, is one of the most massive repositories of Earth-crossing material in the solar system, about a hundred billion tons. In fact, the best is yet to come, says meteor expert Peter Jenniskens of the SETI Institute: "The Perseids are slowly migrating toward the sun, and we're still just clipping the inner part of their swarm. Earth will run into increasingly dense Perseid meteoroids in the decades and centuries to come—the show is really just beginning."
Sky & Telescope magazine offers an observer's guide to meteors: skyandtelescope.com/observing/objects/meteors
. Also look at a detailed overview of the Perseid meteors compiled by Gary Kronk, a leading amateur astronomer: comets.amsmeteors.org/meteors/showers/perseids.html