What to be scared of

If Bill Bryson can be trusted (A Short History of Nearly Everything, 2003), we should all be afraid of asteroids. Here's why.

The last two hundred years have seen asteroid hunters working at a frantic pace to locate them in the heavens and write them up for future reference..  At the end of the nineteenth century about a thousand of them had been counted up and named, but it was the 20th-Century that improved the method and bookkeeping until by 2001 there were some 26,000 identified.  Which sounds good until you realize that there are about a billion more waiting to be discovered and catalogued, each one on a path that will bring it near to Earth at some point on its regular orbit.  It is inevitable that another impact, possibly of extinction magnitude, will occur.  Some experts think we are far overdue for just that sort of planetary cataclysm. It would be a Doomsday scenario.

Even if a relatively small asteroid, say the size of a house, were to hit, it would destroy a city, and these are much more common than the massive asteroids that would kill all life on the planet.  How many house-sized asteroids are floating around in "Earth-crossing orbits"?   The number "is almost certainly in the hundreds of thousands and possibly in the millions, and they are nearly impossible to track."

A couple of these house-sized asteroids zipped past Earth in 1991 and 1993, missing us by about a hundred thousand miles each, close calls when you talk about space dimensions.  We didn't see either one until it had passed us, which led one expert Timothy Ferris to say, in the words of Bryson, that "such near misses probably happen two or three times a week and go unnoticed."

The Doomsday Asteroid, Meteor, or Comet:

"An asteroid or comet traveling traveling at cosmic velocities would enter the Earths's atmosphere at such a speed that the air beneath it couldn't get out of the way. . . .In this instant of its arrival in our atmosphere, everything in the meteor's path--people, houses, factories, cars, would crinkle and vanish like cellophane in a flame.

"One second after entering the atmosphere, the meteorite would slam into the Earth's surface. . . . The meteorite itself would vaporize instantly, but the blast would blow out a thousand cubic kilometers of rock, earth, and superheated gases.  Every living thing within 150 miles that hadn't been killed by the heat of entry would now be killed by the blast. . . .

"For those outside the zone of immediate devastation, the first inkling of catastrophe would be a flash of blinding light, the brightest ever seen by human eyes--followed an instant to a minute or two later by an apocalyptic sight of unimaginable grandeur:  a roiling wall of darkness reaching high into the heavens, filling an entire field of view and traveling at thousands of miles an hour.  Its approach would be eerily silent since it would be moving far beyond the speed of sound. . . .

"But that's just the initial shockwave.  No one can do more than guess what the associative damage would be, other than that it would be brisk and global.  The impact would almost certainly set off a chain of devastating earthquakes.  Volcanoes across the globe would begin to rumble and spew.  Tsunamis would rise up and head devastatingly for distant shores.  Within an hour, a cloud of blackness would cover the planet, and burning rock and other debris would be pelting down everywhere, setting much of the planet ablaze.  It is estimated that at least a billion and a half people would be dead by the end of the first day. . . .

"And in all likelihood, remember, this would come without warning, out of a clear sky."