What are the objects that strike the earth from space to form craters, and how is it that they can have such devastating effect, not only to dent the solid earth, but, as claimed by the Alvarezes, to play the starring role in dinosaur extinction? Astronomers have discovered that two types of cosmic objects are in orbits that sometimes intersect that of the earth: comets and asteroids.
Comets are "dirty snowballs"—mixtures of mineral dust and ices that evaporate under the heat of the sun to produce the visible tails that follow behind them for thousands of miles. The great comets of the 1990s, like Hayukatake and the spectacular Hale-Bopp, have been seen by hundreds of millions of people. Comets come from much farther away in the solar system than asteroids—from a vast cloud that surrounds the sun at an average distance about 40,000 times the earth-to-sun distance. Edmond Halley was the first to recognize that some comets are periodic, returning to our region of space with predictable regularity. Great comets had appeared over Europe in 1531, 1601, and 1682, and Halley figured out that these sightings were of one and the same comet. In another fine example of prediction, he claimed that the comet would reappear in 1758, and at definite intervals thereafter. Though he was not around to see it, Halley's Comet reappeared precisely on schedule and has continued to do so since, most recently in 1986. We can be certain that in 2061, this cosmic traveler will reappear, right on schedule.
In 1994, a rival to Halley for cometary fame appeared and, almost as suddenly, disappeared. Gene Shoemaker, his wife, Carolyn, and their colleague David Levy, had been searching the sky for comets and asteroids, carefully tracking the orbits of those they found, in order to determine whether the object might someday represent a threat to the earth. In 1993, through persistence and good luck, they spotted the comet that became known as Shoemaker-Levy 9 (SL-9; the ninth the trio had found together). Shortly after its discovery, SL-9 broke into pieces to present the image of a "string of pearls" to those who viewed it through telescopes. When astronomers plotted the comet's path, they realized to their surprise and delight that in July 1994 it was going to crash into Jupiter. It did so right on schedule, making planetary impact a reality seen by millions. Several of the fragments left dark spots on Jupiter that were larger than the earth. How fitting that Shoemaker, after a lifetime of studying craters, was not only the discoverer of the comet that was to produce the first planetary impact ever seen by human eyes, but was able to witness it.17
Asteroids are made of stone or iron; many are in orbits that cross that of the earth, meaning that a collision with our planet is theoretically possible. In March 1989, a previously undetected asteroid passed only 690,000 km from the earth, less than twice the distance to the moon. Calculations show that most earth-crossing asteroids cannot have been in their present orbits since the beginning of the solar system, or they would long since have collided with earth or been ejected into other regions of space. Some as yet unknown process must channel them into our region.
Because over recorded human history no recognizable impact crater has been formed (nor has impact cost a single life), it is reasonable to ask why scientists are confident that impacting asteroids and comets have caused great damage on the earth. The first point,
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