Bad Astronomy: Misconceptions and Misuses Revealed, from ...

138 SKIES AT NIGHT ARE BIG AND BRIGHT
outer parts very well. Not only do small meteorites not cause fires,
but many are actually covered in frost when found!
Large meteorites are a different story. If it’s big enough—like a
kilometer or more across—the atmosphere doesn’t slow it much.
To really big ones, the atmosphere might as well not exist. They
hit the ground at pretty much full speed, and their energy of
motion is converted to heat. A lot of heat. Even a relatively smallish
asteroid a hundred meters or so across can cause widespread
damage. In 1908, a rock about that size exploded in the air over a
remote, swampy region in Siberia. The Tunguska Event, as it’s now
called, caused unimaginable disaster, knocking down trees for hundreds
of kilometers and triggering seismographs across the planet.
The event was even responsible for a bright glow in the sky visible
at midnight in England, thousands of kilometers from the blast.
The fires it started must have been staggering.
Understandably, such events are a cause of concern. Even little
rocks—well, maybe the size of a football stadium—can have big
consequences. But it does take a fair-sized rock to do that kind of
damage. Little ones, and I mean really little ones, like the size of
an apple or so, usually don’t do more than put on a pretty show.
I remember seeing a bolide, as the brightest meteors are called, as
I walked home from a friend’s house when I was a teenager. It lit
up the sky, bright enough to cast shadows, and left a tremendous
train behind it. I can still picture it clearly in my mind, all these
years later. Sometime afterward I calculated that the meteoroid
itself was probably not much bigger than a grapefruit or a small
bowling ball.
But the big meteorites worry a lot of people, as well they
should. Very few scientists now doubt that a large impact wiped
out the dinosaurs, as well as most of the other species of animals
and plants on the Earth. That impactor was probably something
like 10 kilometers (6 miles) or so in diameter, and left a crater hundreds
of kilometers across. The explosion may have released an
unimaginable 400 million megatons of energy (compare that to the
largest nuclear bomb ever built, which had a yield of about 100
megatons). It’s no surprise that some astronomers stay up nights
(literally) thinking about them.