Geoscience Reference
In-Depth Information
seen by hundreds of millions of people. Comets come from much far-
ther 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 pre-
dictable 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 inter-
vals 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.
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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 theo-
retically 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 rea-
sonable to ask why scientists are confident that impacting asteroids
and comets have caused great damage on the earth. The first point,
