[meteorite-list] Why Meteorites Are Unlikely To Cause Fires

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:36 2004
Message-ID: <200310291530.HAA02082_at_zagami.jpl.nasa.gov>

http://techrepublic.com.com/5100-22-5089047.html

Today's Geek Trivia Answer
Fires in the skies
October 28, 2003

We asked why so-called scorching-hot, high-velocity
meteorites are an unlikely cause of earthbound fires
and explosions, despite numerous unconfirmed
reports to the contrary.

As opposed to what you may have seen in
the movies, meteorites are not terribly
warm when they hit the ground, and some
are even frost-coated. Untrained
observers assume that since meteors in
the upper atmosphere are visible due to
combustion, earth-impacting meteorites
must be hot as well. The reality is quite
the opposite.

First of all, friction causes the combustion
visible from meteors, but it's friction from
air rubbing against air-not air rubbing
against the meteorite. Meteorites strike
the atmosphere at roughly 30,000 miles
per hour. At that rate, the air can't move
out of the meteorite's way fast enough,
creating compression in front of it.

This compression heats the air to the
point of combustion, creating the familiar image
of a shooting star. But the combustion never actually
touches the meteorite. The compression happens so
fast that it creates a pocket of relatively cool,
slow air between the meteorite and the compressed air,
a phenomenon called standoff shock.

The transferred heat literally peels layers off the
space rock as part of a process called ablation. With
the hot layers peeled away, the core of the meteorite
remains cool, thanks to exposure to the vacuum of space.
High-altitude air may chill it further once it slows
down enough to shed its combustive fireball.

By the time meteorites hit earth, they're moving at
perhaps only 200 mph, and they can be as cold as hail or
snow. When meteorites have caused documented damage,
it's been because of their speed and mass, not
their temperature. Even then, reports have usually
proven unreliable.

Perhaps the most widely cited example of the first
confirmed meteorite injury is the case of Ann Hodges of
Sylacauga, AL. On Nov. 30, 1954, Hodges suffered a
bruised hip after a meteorite blasted a hole in her roof
and ricocheted into her.
Received on Wed 29 Oct 2003 10:30:44 AM PST