Horseflies and Meteors
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 love the analogy; indeed, it's a good one.
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 small asteroids and bits of comet dust. They hit Earth's atmosphere--splat!--and disintegrate as fiery streaks of light called meteors.
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Coincidentally, many of those specks, which scientists call meteoroids, will be about the size of tiny insects--perhaps 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).
Like bugs, meteoroids accumulate mostly on the front windshield. In this case, we mean the front windshield of our planet. Earth's windshield is the atmosphere. The atmosphere protects us from the solar wind and assorted bits of space debris much as a car's windshield deflects the elements from its passengers.
Below: Tiny comet flakes like this are at the heart of fiery-looking Perseid meteors. This one is only 10 microns across. Some are as large as tiny insects. [more]
That's why most experts suggest looking for Perseids just before dawn. 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. Earth's "rear window," the early evening sky around 6 p.m., is not a good place to look for shooting stars.
Side windows (the ones to the left and right of passengers in cars) are more interesting. 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.
This happens to meteors, too. Go outside on Sunday, August 11th during the hours just after 9 p.m. The constellation Perseus--the source of the Perseids--will lie low in the northeastern sky. Meteors streaming from Perseus then will skim the atmosphere horizontally, much like a bug skimming the side window of an automobile. Astronomers 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've 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 was a Leonid. I was facing east when a vivid 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."
"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."
If you don't spot any on Sunday night, he added, try again after nightfall on Monday, August 12th. The shower will still be going strong and Earthgrazers will be possible then, too.
Perseid Earthgrazers: shy, remarkable, long, colorful. And no 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 point that is below the horizon -- usually between 0 and 10 degrees below," notes Bill Cooke.
more information
That Gunk on your Car -- A Unique Guide to Insects of North America\
Summer Meteor Shower -- (Science@NASA) The Perseid meteor shower peaks this year on Aug. 12th and 13th. The warmth of northern summer makes it one of the year's most inviting sky shows.
Perseids Photo Gallery -- (SpaceWeather.com) Find out what sky watchers saw last year.
The Extraordinary Geomagnetic Perseid Meteor Shower -- (Science@NASA) A geomagnetic storm triggered dazzling auroras during the peak of the 2000 Perseid meteor shower.
International Meteor Organization -- more information about meteor showers
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