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August
10, 2009: 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 grossing 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,"
by Mark E. Hostetler. [more]
Earth,
like a speeding car, races around the Sun sweeping up everything
in its path. There are no insects in space, at least none
that we know of, but there are plenty of meteoroids, little
flakes of dust from comets and asteroids. They hit Earth's
atmosphere and--splat!--they 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 swarm of dust shed by periodic
comet Swift-Tuttle. It's the annual Perseid
meteor shower, which peaks on August 11th and 12th.
Just
as bugs tend to accumulate on the front windshield of a car,
Perseids accumulate on the front windshield of Earth.
Earth
has a windshield? It's the 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 usually best to look for Perseids just before
dawn.
A
good time to see Perseids this year is before dawn on Wednesday
morning, August 12th, when Earth's front windshield is overhead.
You could see dozens of meteors despite the glare of a 66%
gibbous Moon.
Side
windows, the ones to the left and right of passengers in cars,
are good, too. Zooming down a bug-infested lane, side windows
don't intercept many insects, but the ones they do gather
are worth examining. Bugs that strike side windows do so at
a shallow angle, leaving long and colorful streaks.
Above: A Perseid meteor. Credit: S. Kohle
& B. Koch.
This also
happens to meteors. When the constellation Perseus (the source
of the Perseids) hangs low near the horizon, meteors streaming
from Perseus will skim the the top of Earth's atmosphere,
much like a bug skimming the side window of an automobile.
Astronomers call these meteors "Earthgrazers." They
tend to be long and colorful.
Look
for Perseid Earthgrazers on Tuesday night, Aug. 11th, between
about 9:00 and 11:00 pm local time.
Earthgrazers
don't come in large numbers. The special geometry required
to produce them keeps counts low, but even one or two is enough.
A breathtaking Earthgrazer is the sort of meteor you're likely
to remember for years.
Best
of all, there's no gooey residue.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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