On summer nights 140 years ago, skywatchers were admiring a beautiful new comet that has been discovered in mid-July 1862 by Americans Lewis Swift and Horace Tuttle. The comet became favorably positioned high in the northern sky through the remainder of the summer.

During the last week of August, the comet was at its best, shining at second magnitude and displaying a long, bright tail. In telescopes, luminous jets of nebulosity blossomed outward from a bright, highly condensed nucleus. French astronomer Camille Flammarion ranked comet Swift-Tuttle among the ten "really fine and striking comets" of the 19th century.

Others would simply refer to it as the Great Comet of 1862.

Schiaparelli postulated that the comet had discarded the tiny fragments that produce the Perseids as it swept through the inner solar system. These swift streaks of light appear to dart across our northern skies from their namesake constellation from late July through most of August.

These cometary fragments -- countless bits of metal and stone -- are called meteoroids while they exist in and move through space.

But a meteor is not a particle of matter itself. It is merely the short-lived streak of light produced by the meteoroid as it is heated to incandescence by its plunge through the Earths atmosphere. (The handful of objects that hit the ground are called meteorites.) The kinetic energy released per gram of the meteoroids weight far exceeds the energy efficiency of the most powerful man-made explosives.

In 2002, Earth is expected to encounter the core of the Perseid swarm, where meteoroid concentration is densest, on Monday, Aug. 12. This is when the Perseids put on their best show as the meteors appear to diverge from a patch of sky near the Double Cluster in Perseus. This is actually an illusion of perspective, since that is the direction toward which the Earths orbital motion carries us at this time of the year.

At the same time, the meteoroids are traveling on parallel paths nearly perpendicular to the Earths orbit. The combined speeds of the Earth and the meteoroids cause the Perseids to rush into our atmosphere at average speeds of 37 miles per second (60 kilometers per second).

The bane of meteor observers is bright light, natural or artificial. The Moon will be a slender crescent and will set during the evening hours, and will be of no hindrance to Perseid viewing this year.

For an observer with access to a wide-open view of a clear, dark sky, meteors should appear at an average rate of about one every minute or two. Veteran observers, however, say that the Perseids tend to appear in bunches: several over an interval of minute or two, followed by a lull of several more minutes before the sky again "bears fruit."

Comet Swift-Tuttle made its most recent pass through the inner solar system nearly ten years ago, in December 1992. Its orbit is highly elongated and it takes roughly 130 years to make one trip around the Sun.

For several years before and after its 1992 return, the Perseids were a far more prolific display, appearing to produce brief bursts of as many as several hundred meteors per hour, many of which were dazzlingly bright and spectacular. The most likely reason was that the Perseids parent comet was itself passing through the inner solar system and that the streams of Perseid meteoroids in the comets vicinity were larger and more thickly clumped together.

In recent years, with the comet now far back out in space, Perseid activity has apparently returned to normal.

However, looking ahead into the future, a well-known meteor astronomer suggests that the Perseids may yet provide some surprises. Esko Lyytinen of Finland has made calculations concerning extra-dense filaments of material trailing well behind comet. He concludes that the Perseids may put on unusually strong displays in the year 2004 and especially again in 2028.

In 2004, Lyytinen believes Earth will pass through a trail of debris shed by comet Swift-Tuttle during its 1862 visit. The closest that Earth will come to the center of this debris trail will be 112,000 miles.

"I would expect a short peak of a few hundred meteors per hour, though they should be mostly quite faint," Lyytinen said. Most of Europe and western Asia will have the best chance of viewing this unusual Perseid shower.

In 2028, Lyytinen expects an even more dramatic Perseid display as the Earth passes to within 37,000 miles from a stream of debris that comet Swift-Tuttle released into space back in the year 1479.

"I expect this to produce a real storm [defined as 1,000 meteors per hour or more] over the United States, although it will come under rather unfavorable moonlight conditions," he said. The Moon will be near Last Quarter, outshining some of the dimmer shooting stars.