A Spectacular Memory

Monday, August 10th. Midnight. Perseus rises in the northeast. One or two "stars" shoot out just above Perseus, in the northern sky. The annual Perseid meteor shower has begun. It will peak over the next few days, from the direction of Perseus. What are these "shooting stars," and why does the shower happen every year on the same date?

All the time, we're being bombarded by stones falling from the sky. Most are pretty small, about the size of a grain of sand. We know they're so small because when we see a flash of light showing that they're burning up in our atmosphere and we say to a friend, " Look, a shooting star," and our friend asks " Where?," the answer is always, " Too late. You missed it." In that moment, the grain of sand has burned up. That flash of light is called a meteor. Only rarely is one of these rocks large enough to survive the burning and land on the earth. Then it's called a meteorite.

Most of these " shooting stars" are random and can be seen most any night, usually from any part of the sky.

However, meteor showers are predictable because they're caused by the debris left by a comet. Comets are " dirty snowballs," balls of rock and ice a few miles across. When comets enter the warm inner part of the solar system, some of the ice evaporates. The sunšs light, reflected from this mist, gives us the spectacular displays.

But the ice is also the glue that holds the comet together. As the ice begins to evaporate, some of the rock and dust of the comet falls off. The more spectacular the comet, the shorter is its life.

A comet passed the Earth in 1862. Known as comet Swift-Tuttle, it returned in 1992, a hundred thirty years later. It is now past Uranus, and will not return until 2122. However, every year, as the Earth passes through the dust it left in its trail, some of this dust vaporizes and burns up as it enters our atmosphere. This is the annual Perseid meteor shower. A shower of lights in the sky -- what a memory to leave behind!

(08/07/09)