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August 5, 2009 > TechKnow Talk: When the Stars Fall from the Sky

TechKnow Talk: When the Stars Fall from the Sky

Each year in August, the Earth's orbit takes it through a massive cloud of dust left by the passage of a comet. The result is often a wonderful display of meteors streaking across the night sky, appearing to originate from the constellation Perseus. This Perseid meteor shower peaks on or about August 12. Before offering some viewing tips for this year's meteor shower, we'll take a closer look at meteors and what causes them.

To understand meteors, we must first understand comets. Unlike the orderly, nearly circular progression of the planets and moons of our solar system, comets exhibit wildly eccentric orbits. They range far out into space, often beyond even the farthest planets. But they remain prisoners of the sun's gravity, and are periodically pulled back to make a near pass around the sun before sailing back into the far reaches of the solar system.

These very elliptical orbits can have periods, or the time between near-sun passes, varying from a decade or two to many thousands of years. The handful of comets with which astronomers are most familiar and that produce noticeable meteor showers have periods of a few hundred years or less. There are probably millions of comets in our solar system, but until their orbit brings them near the sun (and the Earth), they are too distant to detect.

In fact comets are quite small, as celestial bodies go, ranging from the size of a large sports stadium to a large mountain. They are irregularly shaped, similar in appearance to asteroids. Like asteroids, they are composed of rock, but comets also contain a significant amount of water and silicon-based minerals, plus "volatile" organic materials such as methane, other carbon compounds, and ammonia.

All these components are frozen solid in the extreme cold of deep space, but as a comet passes within the orbits of the inner planets and approaches the sun, it warms. The heat melts or vaporizes some of the volatile materials from its surface which stream off into space carrying along bits of dust. This leaves a trail of debris in the path of the comet composed primarily of gases and very small dust particles.

By the way, the "tail" of a comet, sometimes visible from Earth, is not this debris field, but a plume of ionized gases produced by the solar wind, the stream of electrically charged sub-atomic particles constantly emanating from the sun. The tail always points directly away from the sun and moves with the comet; the debris is left behind in the comet's wake.

A meteor shower is the result of the Earth passing through this debris stream of gas and dust. As a bit of dust encounters Earth's atmosphere it burns, briefly glowing white hot against the night sky. The relative movement of the Earth intersecting the particle causes the meteor to leave a bright trail across the sky as it expires: the larger the particle, the longer the trail. Most meteors result from dust motes smaller than a grain of sand.

Each time a comet visits the inner solar system and crosses Earth's orbit it deposits more debris, and meteor showers are often more spectacular in ensuing years. However, since the comet never traverses exactly the same path, over the millennia it produces several streams or "filaments" of debris in space, and the Earth may intercept a richer or poorer source of meteors in any given year. Astronomers are working to map these filaments and provide better predictions of the intensity of meteor showers.

During the course of the year, Earth travels through the debris streams of many comets, though August has been noted by astronomers since the first century AD as a period of unusually high meteor activity. In 1862, a comet was discovered by U.S. astronomers Lewis Swift and Horace Parnell Tuttle as it approached the sun, and Comet Swift-Tuttle was ultimately identified as the source of the August Perseid meteor shower, one of the best and most consistent annual displays.

Swift-Tuttle returned in December of 1992, and is now believed to orbit the sun roughly every 133 years. Its next pass is estimated to be in 2126. Interestingly, Swift-Tuttle is on an orbital path that may eventually cause it to collide with Earth or the moon, though probably not in the next several hundred years.

Indeed, comets are far from immortal. Some may collide with another body, as Comet Shoemaker-Levy did when it impacted Jupiter in 1994. Others may pass so close to the sun or a large planet that the gravitational forces either break it apart or whip it out of the solar system forever. If a comet survives to orbit again and again, the sun will eventually burn off all the volatile materials and render it an inert rock, similar to an asteroid.

After the Perseids, the next most reliable meteor shower is the Leonids (radiating from the constellation Leo). Occurring in mid-November, the Leonids are seeded by Comet Tempel-Tuttle, which orbits the sun every 33 years (next appearance in 2031). Halley's Comet provides the less impressive Eta Aquariids shower each May. Halley, with an orbital period of 75 years, will return in 2061.

A note on terminology: the term "meteor" is often misused. Any small object in the solar system is called a meteoroid. When such an object enters the Earth's atmosphere and burns, it becomes a meteor. In the rare event a meteor is so large that a portion survives to impact the surface of the Earth, what remains is a meteorite.

The Perseid meteors should be noticeable August 10-14, with the best viewing the nights of the 11th and 12th. To view the meteor shower, find a dark area away from city lights. Douse all lights and move away from roads, so your eyes can adjust to the dark and viewing is not interrupted by headlights. Bring a comfortable chair or blanket (the TechKnow Guy prefers a reclining lawn chair), and face the northeast quadrant of the sky. Even August nights may get chilly, so bring a blanket too.

Assuming a clear sky, Perseid meteors should be visible beginning about 11 p.m. as Perseus rises. This year the moon, at more than half full, will rise around midnight. The bright moon will interfere to some degree with viewing the meteors, so the best viewing may be shortly before moonrise, even though the greatest number of meteors will occur in the hours before dawn.

At peak activity, about one meteor per minute should streak across the sky. As you sit back and enjoy the show, it's easy to imagine why our ancestors thought of them as falling stars.

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