By Don Henry
Around August 10–13 the Perseid meteors will streak through the night sky. If you have never seen a meteor shower, now is your chance. But what is a meteor shower? Why is this shower called “Perseid?” First, let’s talk about meteors.
Outer space is mostly empty, but whizzing around within our solar system there are rocks of many sizes, from wee dust particles to very large objects called asteroids. Some of these objects plow into the Earth’s atmosphere and cause the air to glow brightly. The flash of light from these particles is called a meteor; sometimes we call them shooting stars or falling stars. Meteors rarely make it through the atmosphere and reach the ground. When they do, they are called meteorites, and they can be spectacular.
In 2013 an asteroid entered the atmosphere over Russia at a speed of 20 kilometers per second (40,000 miles per hour). When the asteroid exploded in the sky, the flash of light was brighter than the Sun, and about 5,000 people were injured, mostly by broken glass from shattered windows. You can watch a video of the fireball at www.youtube.com/watch?v=svzB0QYNIWI.
Fortunately most of the meteors we see in the sky are short streaks caused by small particles that originate from comets and last less than a second or so. Comets, sometimes described as dirty snowballs, are comprised of ices (various hydrogen compounds) and rocky material. They orbit the Sun, going far beyond the orbit of Neptune.
A comet develops a tail if it gets near the Sun. Its surface warms and the solid ices change directly into gas (the ice sublimates) that escapes the comet’s weak gravity. The gas can drag dust particles from the surface to form a cloud (called a coma) around the comet. As the comet approaches nearer to the Sun, two tails form: one of ionized gas (a plasma) driven by solar wind directly away from Sun and the other of dust particles driven into a slight curve back in the direction from which the comet came.
At the same time pieces of rock the size of sand grains to pebbles too large to be affected by solar wind or sunlight drift slowly away and spread along the comet’s orbital path. When Earth crosses the path, these particles, traveling at speeds up to 70 kilometers per second (156,000 mph), pass through the atmosphere and heat it, causing the air to glow. That’s the streak in the sky we see. Because of the numerous particles, we can see many meteors in the sky as the Earth moves across the orbit of the comet. That’s a meteor shower.
If you watch a meteor shower and trace the paths of the meteors back through the sky, the paths will all seem to point back to the same place (called the radiant point) in the sky. The shower is given the name of the constellation nearest to the radiant point.
One of the best meteor showers is the Perseids; all the meteor streaks seem to be coming from the constellation Perseus the Hero, often reaching a peak of 50 to 100 meteors per hour (roughly one to two every minute). The Perseid particles originated from the comet Swift-Tuttle. The peak is reached around August 10–13 each year, and this year the best time of night is from late night until dawn. Unfortunately the moon will be fairly bright, so you might not see the dimmest meteors.
To view the shower, go somewhere away from bright lights, sit in a comfortable chair or lie down on a blanket, use bug spray, take along some snacks and drinks, and stare up at the sky. Don’t blink! You will see short flashes, and just maybe an exceptionally bright streak will race across the sky. Binoculars will not be needed; the meteors whiz by too quickly.
As we sit and watch the show, we can marvel at the universe in which we live.