for National Geographic News
Halley's comet won't return until 2061, but pieces of the celestial body are streaking across the sky. The heavenly show, known as the Orionids meteor shower, peaks Wednesday night, when sky-watchers may observe two dozen meteors per hour.
Though the comet remains distant, Earth is passing through the comet's ancient debris fieldwith dramatic results.
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"Over time comets leave a trail of debris along their orbits," explained Kelly Beatty, executive editor of Sky and Telescope and editor of Night Sky magazine.
Each time a comet orbits the sun, the star's heat strips comets of dust and ice. Scientists believe that Halley's comet sheds some 20 feet (6 meters) of dust and ice particles on each pass.
"For a select few [comets], the Earth goes through their orbits at the same time every year," Beatty said. "The analogy I like to use is a garbage truck full of sand. As it barrels down the road, the sand billows out the back end. And that's what Earth plows through."
Tiny Particles Light Up Night Sky
Earth passes near Halley's cigar-shaped orbit debris field twice each year: the Orionids shower fall in October, the Aquarids shower in May.
The tiny particles of ice and rock, some as small as a grain of sand, put on quite a show as shooting stars or meteor showers.
"What we see is not the particle burning up," Beatty said. "What we're really seeing is the particle transferring all that energy to the air molecules along its path and causing them to become superheated to the point that they are incandescently hot."
Meteor particles are among the smallest celestial objects that can be seen by the human eye.
"For anyone who has eaten a bowl of Grape Nuts, the little nuggets in there are a pretty good match in size, shape, density, and even color of what a typical meteor particle looks like from space," Beatty said.
The particles haven't yet had time to drift very far from their orbit. "Given enough time they will wander from the orbit," said Scott Sandford, an astrophysicist at NASA's Ames Research Center at Moffett Field, California. "When thousands of years have passed they will have spread out quite a bit, and after hundreds of thousands of years many particles won't be obviously associated with the orbit."
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