Good heavens: Meteors may not shoot up the skies, after all

August 07, 1993|By Douglas Birch | Douglas Birch,Staff Writer

Some astronomers are sounding the all-clear signal. That torrent of shooting stars forecast for next week may be just a trickle after all.

In June, two scientists at the University of Western Ontario in Canada predicted a rare meteor storm for the night of Aug. 11-12. Astronomy magazines reported that the event might rival some of the most intense meteor bombardments in history.

Astronomers at the Space Telescope Science Institute in Baltimore decided to swivel the $2.5 billion Hubble Space Telescope in orbit to protect its mirror and solar panels. NASA postponed a shuttle flight until after the storm.

Then, several days ago, two astronomers at Queen Mary and Westfield College, part of the University of London, declared that the really big storm is coming next August, not next week.

Now along comes David Hughes, a reader in astronomy at the University of Sheffield in England and an authority on meteors.

What will it be, 1993 or 1994?

"Probably neither, actually," said Dr. Hughes. "It might be this year, it might be next year, but it's got an equal chance of being a high activity any year in the future. My advice is: Keep looking, folks."

The shuttle, he said, was never in danger. "NASA postponed that space shuttle for their own reasons," he said. "I don't know why. There was absolutely no reason to postpone it for the Perseids."

Don Kessler, a spokesman for NASA at the Johnson Space Flight Center, disagreed.

"The whole reason [for the launch delay] was just the possibility of a meteor storm," he said. "They would have preferred to launch earlier. There wasn't any other reason. We felt the possibility was high enough that we didn't want to risk it."

He said NASA recognized that there is only a slim possibility of an historic barrage. "But the possibilities included something that we've never experienced before," he said. "We fully recognize that it may be a full dud."

Dr. Hughes also said that Hubble is not in peril.

"In essence, it's going through a collection of dust particles that are very small and very well separated," he said. "Nothing is going to happen to it, I wouldn't have thought."

The worst that might happen to the orbiting telescope, he said, would be for a dust grain to gouge a pit in its main mirror.

That pit would probably be too small to see, he added.

Every year, Earth passes through a number of meteor showers -- all caused by active or extinct comets. The most reliable of

these, the Perseid shower, is produced by the comet Swift-Tuttle.

Swift-Tuttle is a sizable piece of real estate: about 11 miles across at its widest point, Dr. Hughes said, weighing perhaps a trillion tons. Mostly, it is made of icy material and rocky dust.

When the comet passes close to the sun once every 130 years or so, its mantle heats up and begins to evaporate, shooting particles off the comet's surface.

Most of particles are tiny, about the size of dust motes or sand grains. They are scattered throughout the comet's orbital path.

Each August, Earth passes through Swift-Tuttle's stream of debris. When it does, the planet gets bombarded with some of this cosmic gravel traveling at about 151,000 mph. The peak of the bombardment comes when part of Earth's atmosphere collides with the thickest portion of the stream.

Other astronomers, Dr. Hughes said, have wrongly assumed that Perseid storms are more likely to occur in the years after Swift-Tuttle passes close to Earth. (The comet whizzed by just last Dec. 31.)

Those astronomers are also "simply assuming the best possible case, a massive dust injection put into orbit so that it will intercept the Earth," he said.

A typical Perseid shower produces, at its peak, about 100 meteors each hour -- with some generating as many as 400 an hour and others as few as 25 an hour. A storm, if it occurs, could produce hundreds of meteors per minute.

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