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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! 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 the splat of a horse fly. From "That Gunk on your Car: A Unique Guide to Insects of North America." This disgusting smear reminds the author of a Leonid fireball he saw in 1998. Kids, especially young boys, will love the analogy -- and indeed it's a good one. Earth, like a speeding car, races around the Sun sweeping up everything in its path. As far as we know there are no insects in space, but there are plenty of small asteroids and bits of comet dust. They hit Earth's atmosphere -- splat! -- and disintegrate as fiery streaks of light called meteors.
Coincidentally, many of those dust specks will be about the size of tiny insects -- perhaps as small as a flea or a mite. Meteoroids, which is what scientists call the dust grains before they burn up in our atmosphere, make vivid streaks across the sky not because they're big, but because they are fast-moving. In this respect cosmic meteoroids differ from Earthling bugs. Bugs move slowly with respect to cars, but meteoroids travel through space about as fast as Earth does. (Our planet circles the Sun traveling 30 km/s or 67,000 mph.) Nevertheless, like slower-moving bugs, meteoroids accumulate mostly on the front windshield. You won't find meteoroids on the front windshield of your car, of course. Instead, they streak across the front windshield of our planet. That's right -- Earth has a windshield. It's our atmosphere. The atmosphere protects us from the solar wind and assorted debris of space much as a car's windshield deflects the elements from its passengers.
The front windshield of our planet is the early morning sky. Earth circles the Sun dawn-side first, scooping up whatever lies on that side of the planet. Because Earth rotates once a day everyone gets a daily turn looking out the front windshield. It's overhead around 6 a.m. local time. Those dark hours just before sunrise are usually the best for meteor watching. That's why most experts suggest looking for Perseids before dawn on Sunday, August 12th. No matter where you live, the shower will climax when Earth's "front windshield" is overhead. But Earth, like a car, has many windows -- the front isn't the only one. What about the others? Rear windows tend to be dull. Not many bugs accumulate on the rear pane of a car and, likewise, not many meteoroids catch up to Earth from behind. The early evening sky (Earth's "rear window") is not a good place to look for shooting stars. The July 23, 2001, Pennsylvania fireball, which streaked over the US east coast around 6:00 p.m. local time, was an exception. It was a spectacular "rear window" meteor. Side windows -- the ones to the left and right of passengers in cars -- can be more interesting. Zooming down a bug-infested country lane, side windows don't collect many insects. But the ones they do are worth examining. Bugs that strike side windows do so at a shallow angle, and they leave remarkable streaks -- long and colorful! Below: S. Kohle & B. Koch captured this Perseid meteor on film in 1993. It is not an Earthgrazer, but it is very colorful. [more]  This weekend you can watch for the celestial rendition of such streaky bugs when Perseid meteoroids strike our planet's "side window." Between 9 p.m. on Saturday, August 11th, and midnight on Sunday, August 12th, the Perseid radiant (the point in the sky from which meteors will appear to stream) will lie low on the north-eastern horizon. Meteoroids emerging from the radiant during that time will skim the atmosphere horizontally -- much like a bug skimming the side window of an automobile. Meteor watchers call such shooting stars "Earthgrazers." They leave colorful, long-lasting trails. "These meteors are extremely long," says Robert Lunsford, secretary general of the International Meteor Organization. "I have never been able to capture an Earthgrazer on film. Being shy, they tend to hug the horizon rather than shooting overhead where most cameras are aimed." "There are exceptions," he added. "The most spectacular Earthgrazer I ever saw occurred shortly after 11 p.m. on November 16, 1996. I was facing east, watching for Leonid meteors, when this orange streak crawled over the hill low in the east. It climbed high [and traversed the southern sky]. The event lasted at least 5 seconds -- an eternity for a meteor watcher. Its peak brightness was similar to Jupiter. The entire track was a vivid orange and the head of the meteor had a diffuse halo."
Right: "Ouch!" by artist Duane Hilton. You can find more of Duane's art, along with lesson plans and activities about the Perseid meteor shower, at Thursday's Classroom. Spotting Perseid Earthgrazers is a good thing to try this Saturday after 9 p.m., Cooke added. The Perseid shower, per se, won't climax until Sunday morning before dawn -- but by then a bright quarter Moon will lie high in the sky casting a strong glare across Earth's "front windshield." Saturday-night Earthgrazers, spotted before moonrise, could prove to be the best part of the 2001 Perseid shower. Perseid Earthgrazers -- long and colorful, shy, truly remarkable. And they won't leave a gooey residue! Catch some if you can. Editor's note: This story offers a casual definition of Earthgrazer -- i.e., any meteor that skims more or less horizontally through the atmosphere and leaves a long, colorful trail. Meteor scientists use a more precise definition: "We say that an Earthgrazer is a meteor that comes from a radiant that is below the horizon -- usually between 0 and 10 degrees below," notes Bill Cooke. No matter how you define them, however, they're something special. |
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"Earthgrazers
are rarely numerous," cautions Bill Cooke, a member of the
Space Environments team at the Marshall Space Flight Center.
"But even if you only see a few, you're likely to remember
them."
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