Collision Course

The story of the Leonid meteor shower, which reaches its peak tonight, begins with the story of a comet. The comet Temple-Tuttle was first discovered in 1865. It was calculated to return every 33 years, but has been dimmer each time, and by 1997 was visible only in large telescopes.

All comets carry the seeds of their own destruction. A comet is a mixture of rock and ice often only a few miles across. They orbit the Sun on paths that take from a few years to a few centuries to complete. When they enter the warm inner regions of the solar system, some of the ice surrounding them evaporates, and forms the million-mile wide spectacular halo we associate with comets. But this ice is the glue holding together this "dirty snowball." The comet begins to fall apart. The orbit becomes littered with parts of the comet's body.

Grains of rock, most no larger than a speck of sand are left in the comet's wake, destined to orbit the sun in the same orbit as the comet itself. Now the Earth is headed on a collision course with the millions of rock granules littering this orbit. The result will be a meteor shower in the early morning hours of November 17th -- actually it will begin a few days earlier and end a few days later.

Every year on about November 17th the Earth, which is at that time heading towards the constellation Leo, passes through this orbit. That's why the flashes of light we call meteors or shooting stars radiate from that constellation. Leo rises at about 11:00 P.M., and the best time to look for meteors will be in the east after midnight. (Regulus, the heart of Leo, is the bright star rising in the east at 11:00; the orange " star" next to it is the planet Saturn.)

As the Earth, circling the sun at over sixty thousand miles an hour passes through this orbit, some of this dust falls into the Earth's atmosphere and vaporizes. So small are these granules, and so swift is their destruction, that when we see one of these flashes and exclaim to a friend "Look, a shooting star,"and she asks "Where?," the answer always is "Too late." In that second it vaporized.

(11/15/06)