Deep Impact

One of my favorite sci-fi/doomsday movies is Deep Impact, a story about a giant asteroid on a collision course with the earth and the efforts of mankind to save itself, or to save at least a very large handful of scientists, brains, farmers etc.

Anyway, I've recently started getting a daily quote from Delancey Place dot com. Extracts from relevant and interesting non-fiction books and articles and lectures etc. Today's was about asteroids and how often we narrowly miss being smashed to pieces by a stray chunk of cosmic rock.

"As Steve Ostro of the Jet Propulsion Laboratory has put it, 'Suppose that there was a button you could push and you could light up all the Earth-crossing asteroids larger than about ten meters, there would be over 100 million of these objects in the sky.' In short, you would not see a couple of thousand distant twinkling stars, but millions upon millions of nearer, randomly moving objects--'all of which are capable of colliding with the Earth and all of which are moving on slightly different courses through the sky at different rates. It would be deeply unnerving.' Well, be unnerved because it is there. We just can't see it.

"Altogether it is thought--though it is only really a guess, based on cratering rates on the Moon--that some two thousand asteroids big enough to imperil civilized existence regularly cross our orbit. But even a small asteroid--the size of a house, say--could destroy a city. The number of relative tiddlers in Earth-crossing orbits is almost certainly in the hundreds of thousands and possibly in the millions, and they are nearly impossible to track.

"The first one [crossing near the Earth] wasn't spotted until 1991, and that was after it had already gone by. Named 1991 BA, it was noticed as it sailed past us at a distance of 106,000 miles--in cosmic terms the equivalent of a bullet passing through one's sleeve without touching the arm. Two years later, another, somewhat larger asteroid missed us by just 90,000 miles. ... It, too, was not seen until it had passed and would have arrived without warning. According to Timothy Ferris, writing in the New Yorker, such near misses probably happen two or three times a week and go unnoticed.

"An object a hundred yards across couldn't be picked up by any Earth-based telescope until it was within just a few days of us, and that is only if a telescope happened to be trained on it, which is unlikely because even now the number of people searching for such objects is modest. The arresting analogy that is always made is that the number of people in the world who are actively searching for asteroids is fewer than the staff of a typical McDonald's restaurant. (It is actually somewhat higher now. But not much.)"

Bill Bryson, A Short History of Nearly Everything, Broadway, Copyright 2003 by Bill Bryson, pp. 194-195.