Clear, cloudless skies should be perfect foil for yearly meteor shower No moon, fair weather spell colorful Perseids display

August 11, 1991|By Mary Knudson

Sparks will fly from dark to dawn tonight and tomorrow night as the Earth's atmosphere slams into an orbiting junkyard of meteroids, punctuating the vast black landscape with flashes of yellow, orange and green lights.

Astronomers predict the annual Perseids meteor shower will be one of the finest in years because there will be almost no moon visible and weather forecasters are calling for clear skies with no rain.

The meteor shower is named after a constellation called Perseus, located in the northeast sky near the Big and Little Dippers, because that's where it appears to come from.

The light show will peak tomorrow night, but meteors should also be visible tonight and Tuesday night, said Jim O'Leary, director of the Davis Planetarium at the Maryland Science Center.

Meteoroids are pieces of dust, pebbles and rocks floating in a vacuum of space, a trail of debris left behind from the orbit of the Comet Swift-Tuttle.

There are hundreds of millions, maybe billions of tons of meteoroids out there in space, Mr. O'Leary says. Todd Ullery of the Maryland Science Center says that's enough to put 7,000 pounds of meteoroids per square foot within an area covering a 100-square mile radius of Baltimore.

vTC Every August at about this time, the Earth, on its orbit around the sun, collides with the orbiting meteoroids and the friction set off by this rough encounter incinerates the meteroids, at which moment they become known as meteors.

Meteor showers usually do not produce pieces that actually fall down to Earth. That happens in rare, sporadic events, and the fallen rocks are called meteorites.

While meteorites are rare, meteor dust is much more common than rainfall.

"Dust is raining down every day on the Earth," says Karen Meech, assistant astronomer at the University of Hawaii's Institute for Astronomy.

The magnetic substance, made primarily of carbon, can be found on streets, sidewalks and parked cars.

About two hours before dawn and after twilight, if you look in the direction of the sunrise or sunset, you may be able to see a bright wedge-shaped cone that is light reflecting off the dust of the solar system, Dr. Meech said.

The Comet Swift-Tuttle, named after two scientists who observed it once, is thought to orbit the sun every 125 years and was was expected to swing into view between 1980 and 1984. But it was not seen.

Unlike meteors, which burn for a few seconds and then disappear, comets are visible through telescopes for a month or longer.

Astronomers say there could be several reasons why they could not see Swift-Tuttle.

The length of the orbit may have been miscalculated. Or, Dr. Meech says, the comet could have released gas and dust particles with a force that acts somewhat like a rocket engine, changing its orbit.

Comets can also accumulate a cloud of dust so thick they are hidden from view, and they sometimes simply disintegrate.

But whether Comet Swift-Tuttle is seen again, the fireworks from its trail of meteoroids will delight earthlings for a long time to come.

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