Horseflies and Meteors

Like bugs streaking down the side window of a moving car, colorful Perseid
Earthgrazers could put on a remarkable show before midnight on August 12th
and 13th.

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.

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.

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.

Beginning this Sunday evening, August 11th, 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. The disintegrating specks will
unleash a beautiful meteor shower called the Perseids, which peaks on August
12th and 13th.

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.

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.
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 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.