Geology Reference
In-Depth Information
A recent survey of how people are most likely to die rated asteroid impacts pretty
low—something like 1 in 100,000. That's statistically about the same probability as death
bylightningoratsunami.Butthere'sanobviousflawinthispredictive comparison.Light-
ning kills one person at a time about sixty times per year. Asteroid impacts, by contrast,
probably haven't killed anyone in thousands of years. But one really bad day, one little
thwack could kill almost everyone all at once.
Chances are excellent that you don't have to worry, nor most likely will any of the next
hundredgenerations.Butwecanbeabsolutelysurethatanotherbigimpactofthedinosaur-
killing variety is coming someday, somewhere. In the next fifty million years, Earth will
suffer at least one big hit, maybe more. It's all a matter of time and probability. The most
likely culprits are so-called Earth-crossing asteroids—objects with highly elliptical orbits
that cross the plane of Earth's more circular path around the Sun. At least three hundred of
these potential killers are known, and in the next few decades some of them will pass un-
comfortably close. On February 22, 1995, a just-discovered asteroid with the benign name
1995 CR whizzed by within a few Earth-Moon distances. On September 29, 2004, aster-
oid Toutatis, an elongated 1.5-by-3-mile object, passed even closer. And in 2029 asteroid
Apophis, a 900-foot diameter rock, is predicted to cross much closer still, well inside the
Moon's orbit. That unsettling encounter will irrevocably alter the Apophis orbit and pos-
sibly bring it even closer in the future.
For every known Earth-crossing asteroid, there are probably a dozen or more yet to be
spotted. And when one of these projectiles is finally observed, it will likely be much too
close for us to do much about it. If we're the bull's-eye, we may only have a few days'
warning to settle our affairs. Dry statistics tell the tale of probabilities. Earth is struck by a
twenty-five-foot rock almost every year. Thanks to the braking effects of our atmosphere,
most such missiles explode and fragment into little pieces before hitting the surface. But
objectsahundredfeetormoreacross,whicharriveaboutonceeverythousandyears,cause
significant local damage: in June 1908, such an impactor leveled a swath of forest near
the Tunguska River in Russia. Exceedingly dangerous half-mile stones impact on average
aboutonceeveryhalf-millionyears,whileasteroidsaslargeasthreemilesindiametermay
arrive about once every ten million years.
Theconsequencesofanimpactwillvaryaccordingtothesizeandlocationoftheimpact.
A ten-mile boulder would devastate the globe just about anywhere it hits. (By contrast, the
dinosaur-killing asteroid of 65 million years ago is estimated to have been about six miles
across.) If a ten-mile object hits the oceans—a 70 percent chance, given the distribution of
land and sea—then all but Earth's highest mountain peaks will be swept clean by immense
globe-destroying waves. Nothing will survive up to a few thousand feet above sea level.
Every coastal city will utterly disappear.
