No. 85: Jan-Feb 1993
That region of outer space near the earth carries a heavier load of flotsam and jetsam than scientists expected. The devastation of the 1908 Tunguska impact in Siberia warns us that this space debris -- meteors, comets, asteroids -- is an active threat. A recent spate of articles paints an ominous future.
The earth's retinue of mini-asteroids.
"Asteroids as big as houses pass near the Earth 100 times more often than anyone suspected. On an average day, about 50 asteroids measuring at least 10 metres across come closer to the Earth than the Moon, and each year about five such objects may hit the planet." (Second reference below.)
These startling data come from D. Rabinowitz and coworkers at the University of Arizona, who have been scanning nearby space with a telescope fitted with supersensitive charge-coupled devices (CCDs). They have picked up astronomical objects that have escaped conventional instruments. Several sources have been suggested for this unexpected, threateningly large population of small asteroids: (1) debris hurled earthward from collisions within the asteroid belt located between Mars and Jupiter; (2) the breakup of a large object formerly in orbit about the earth; and (3) fragments blasted off the moon by impacts of large asteroids there. (Kerr, Richard A.; "Earth Gains a Retinue of Mini-Asteroids," Science, 258: 403, 1992. Also: Mitton, Simon; "HouseSized Asteroids Home In on the Earth," New Scientist, p. 16, October 31, 1992.)
Comet Smith-Tuttle. At a conference in Sydney last October, astronomer D. Steele announced that comet SmithTuttle is heading towards a possible impact with earth on August 14, 2116. This 3.1-mile-diameter chunk of ice would have the destructive power of 20 million megatons (1.6 million Hiroshima bombs). (Anonymous; "Astronomer Predicts Comet Collision," Baltimore Sun, October 26, 1992.)
Some recent meteorite impacts. Turning from the dire consequences discussed above, just what sort of astronomical debris actually does hit the earth on a day-to-day basis? Fist-sized meteorites strike our planet about every two hours. These are the ones we read about in the newspapers; and they have left a surprisingly large legacy of damage to human structures. C. Spratt and S. Stephens, in a survey published in Mercury in 1992, listed 61 verified meteorite strikes since 1790 in which buildings and other human works were damaged. (Of course most fell harmlessly in the sea and unpopulated areas.) Spratt and Stephens also provide a table of 26 nearmisses of humans plus one confirmed human impact. At least one horse and a dog have been killed by meteorites. These lists make engrossing reading, but we cannot take the space to reproduce them here. (Spratt, Christopher, and Stephens, Sally; "Against All Odds," Mercury, 21: 50, March/April 1992.)