Friday, August 10, 2007

Cosmic Fireworks

This weekend, one of the best annual displays of shooting stars will light up the night sky. The Perseid meteor shower has its peak activity during August 11-12 every year, but it promises to be a particularly good show this year since it coincides with new moon -- ensuring dark skies if you're away from the city lights.

Contrary to their name, "shooting stars" aren't stars at all -- they are tiny pieces of dust from outer space that run into the Earth's atmosphere and burn up from the friction of moving at thousands of miles per hour. Most of the streaks of light that we see are caused by particles no larger than a grain of sand. When they burn up in the atmosphere, somewhere between 30 and 80 miles above the surface of the Earth, scientists call them "meteoroids". Only the very rare chunks of rock that are larger than a car have any chance of striking the ground, and these so-called "meteorites" are typically no bigger than your fist by the time they hit the surface. On average, only about 8000 meteorites weighing more than a few ounces make it to the ground every year (with another 16000 falling into the oceans). That's a little more than 1 per day for an area the size of the continental United States.

Meteors are streaking through the sky all the time, but sometimes the Earth moves through a dense cloud of dust that was left behind by a comet and we have a "meteor shower". Meteor showers are named for the constellation where all of the streaks of light appear to originate, which is roughly the direction that the Earth is moving in its orbit around the Sun as we intersect a particular cloud of comet dust. During the Perseid meteor shower you can trace most of the shooting stars back to the constellation Perseus, which rises in the northeast sky after sunset. Comet Swift-Tuttle, named after the astronomers who discovered it in 1862, is the source of the Perseid meteor shower. It orbits the Sun every 130 years, and most recently crossed the orbit of the Earth in 1992 -- leaving behind fresh debris for our viewing pleasure every August.

So find a spot away from the city lights, spread out a blanket or set up your reclining lawn chairs facing the northeast, and just look up. Give your eyes about 30 minutes to adjust to the dark, and keep your flashlights off or covered in red plastic. You can expect to see a Perseid meteor shoot across the sky about once or twice minute, with the show getting better and better after midnight when you are on the side of the Earth that is blasting through the dust cloud. Happy skywatching!