Geography Reference
In-Depth Information
KILLER ASTEROIDS FROM SPACE ! sounds like the title of a bad science fiction movie from the 1950s. Yet a
planetary collision is not as outlandish as it may sound, even though some astronomers take it more seri-
ously than others. And many dismiss the likelihood as so remote as not to warrant any serious expenditure
of limited scientific resources or valuable research time. But it is an issue that the National Aeronautics
and Space Administration (NASA) and Congress have taken seriously enough to study at some length. In
congressional hearings in recent years, the results of those studies have been scary enough to make head-
lines and get editorial writers to scratching their heads.
First of all, look at precedent. We know that it happens. Although asteroids that make it into the atmo-
sphere in the form of small meteors and meteorites usually either burn up or land harmlessly in the ocean
or some unpopulated area. A falling stone occasionally punches a hole in someone's roof. But sometimes
meteorites are not so small. A huge crater in Arizona was once presumed to have been caused by volcanic
action, but it is now known to be the result of a meteor crashing to earth. There have been enough met-
eor crater discoveries elsewhere in the world to ascertain that it happens. A crater in southern Australia
measures a hundred miles across. This didn't just happen millions of years ago, either. Once in this cen-
tury—and possibly twice—the earth has had a “close encounter” with “incoming” from outer space.
The first of these recent events occurred in 1908 in the Tunguska region of Siberia. An explosion there
leveled a huge area and destroyed millions of trees in a 1,200-square-mile forest. The trees left standing
all bent away from a point of impact. But no traces of a fallen meteor were ever discovered—no physical
evidence that anything actually hit the earth. The Tunguska event is now widely presumed to have been an
asteriod or icy chunk of a comet glancing off the earth's atmosphere, at perhaps five miles above the earth's
surface, and then skipping away into space, like a stone skimming across a lake. The destructive power of
this cosmic fender-bender has been likened to the explosive force of twenty hydrogen bombs (without the
radiation released by nuclear weapons). The explosion also lofted an enormous load of dust into the air and
possibly destroyed a substantial portion of the protective ozone in the atmosphere. Scientists estimate the
object that caused this destruction was about 150 feet in diameter.
The second impact is less certain. But in 1978, a huge explosion was detected in the South Atlantic. At
the time, it was attributed to the detonation of an atomic bomb, perhaps a test by South Africa and/or Is-
rael, both of which predictably denied such an event. Today, astronomers are guessing that what exploded
was a rather small asteroid slamming into the earth with the equivalent of a hundred kilotons of TNT, far
more destructive than the bomb that leveled Hiroshima.
Then, in 1991, astronomers using a telescope in Arizona called Spacewatch saw an asteroid pass si-
lently by. At its nearest, the asteroid was about 106,000 miles away. That seems far, but not when you
are talking about space and the intersection of planets. Had that asteroid, measuring about twenty-six feet
across, hit the earth, it would have done so with the power of three Hiroshima bombs.
The grandest question of all is whether or not such an event had anything to do with the extinction of
the dinosaurs. Many scientists now accept the likelihood that the extinction of the dinosaurs 65 million
years ago was the result of the impact of an enormous asteroid crashing into the Caribbean basin near Yu-
catán in Mexico. This theory was first put forth by Walter and Luis Alvarez in the early 1980s. This killer
asteroid kicked up a huge cloud of debris that plunged the entire world into darkness lasting months. The
fireball produced a deadly chemical rain and would have altered the makeup of the atmosphere, perhaps
for centuries—the actual time frame for the dinosaurs' demise.
This is the theory of K-T extinction —so called because the event supposedly took place at the time
boundary between the Cretaceous (K in science shorthand) and Tertiary periods. It has been buttressed by
the geological evidence of other extraterrestrial impacts that occurred in different time periods and led to
similar mass extinctions.
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