Aug 9, 2004

Horseflies and Meteors




Like bugs streaking down the side window of a moving car, colorful Perseid Earthgrazers could put on a pleasing show after sunset on August 11th.




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August 9, 2004: Splat! There goes another bug on the windshield.

Anyone who's ever driven down a country lane has seen it happen. A fast moving car, a cloud of multiplying insects, and a big disgusting mess.


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The next time that happens to you, instead of feeling grossed out, try thinking of the experience as an astronomy lesson. Your car is Earth. The bugs are tiny flakes of comet dust. The carnage on your windshield ... it's a meteor shower!


Right: A fiery meteor? No. It's a horse fly. From "That Gunk on your Car: A Unique Guide to Insects of North America."

Kids love the analogy: Earth, like a speeding car, races around the Sun sweeping up everything in its path. There are no insects in space, but there are plenty of meteoroids, little flakes of dust from comets and asteroids. They hit Earth's atmosphere--splat!--and disintegrate as fiery streaks of light called meteors.

This week lots of meteors will appear over Earth's northern hemisphere when our planet plows through a dense swarm of dust shed by periodic comet Swift-Tuttle. It's the annual Perseid meteor shower, which peaks on August 11th and 12th.




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Coincidentally, many of the meteoroids hitting Earth will be about the size of tiny insects--as small as a flea or a mite. They make vivid streaks across the sky not because they're big, but because they are fast-moving. Perseid meteoroids hit our atmosphere traveling 59 km/s (132,000 mph).

Bugs tend to accumulate on a car's front windshield. Think about it: bugs rarely splat on the rear windshield. They can't fly fast enough to catch a car from behind. Likewise, meteoroids accumulate on the front windshield of Earth.

Earth has a windshield? It's our atmosphere, which protects us from solar wind and comet dust much as a car's windshield protects passengers from wind, rain and bugs. Earth's front windshield is the early morning sky. Earth circles the Sun dawn-side first, scooping up whatever lies on that side of the planet. That's why it's best to look for Perseids just before dawn.

To see the greatest number of Perseids this year, be outside before dawn on Thursday morning, August 12th, when Earth's front windshield is overhead.

Side windows, the ones to the left and right of passengers in cars, are good, too. Zooming down a bug-infested country lane, side windows don't collect many insects. But the ones they do collect are worth examining. Bugs that strike side windows do so at a shallow angle. They leave remarkable streaks, long and colorful.


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Above: S. Kohle & B. Koch captured this colorful Perseid meteor on film in 1993. [more]

This also happens to meteors. For example, when the constellation Perseus (the source of the Perseids) hangs low near the horizon, meteors streaming from Perseus will skim the atmosphere horizontally, much like a bug skimming the side window of an automobile. Astronomers call these meteors "Earthgrazers." They tend to be long, slow and colorful.

Look for Perseid Earthgrazers on Wednesday, Aug. 11th, between 8:30 and 10:00 pm.

At that time, around sunset, Perseus will be hanging low in the northeast, perfectly placed to shoot Earthgrazing meteors over your head. Earthgrazers are rare. You might see only one or two, but that may be enough. A breathtaking Earthgrazer is the sort of meteor you're likely to remember years from now. And best of all, there's no gooey residue.


more information

The 2004 Perseid Meteor Shower -- (Science@NASA) Meteoroids in space since the Civil War will spice up this summer's Perseid meteor shower.

The Extraordinary Geomagnetic Perseid Meteor Shower -- (Science@NASA) A geomagnetic storm sparked auroras during the peak of the 2000 Perseid meteor shower. Could it happen again? Check


That Gunk on your Car -- A Unique Guide to Insects of North America

Perseids Photo Gallery -- (


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