From the frigid depths of the Oort Cloud that girdles planets a million million miles from the Sun, an icy interloper hurtles toward the inner solar system, gathering speed. Soon it crosses the orbits of Neptune and Pluto and plunges toward Saturn and Jupiter. By now it has sprouted a glowing tail of gases that stream behind it for millions of miles, making it barely visible to telescopes here.
At this point, astronomers would normally report the appearance of a new comet. It might have been so several years ago, when a celestial visitor named Shoemaker-Levy 9 was sighted athwart the orbit of Jupiter. But it turned out this was no ordinary comet. Instead of racing toward the Sun, Shoemaker-Levy 9 dawdled in Jupiter's powerful gravitational field. Working their calculations backward, astronomers estimated the asteroid-sized body may have been captured by Jupiter's embrace as early as 1970. Even more startling was the discovery that the comet's orbit around Jupiter was gradually decaying -- making a cataclysmic collision with the gas giant virtually inevitable.
Beginning Saturday, astronomers will have an unprecedented opportunity to witness this celestial Armageddon, when the remains of Comet Shoemaker-Levy 9 plunge into Jupiter's turbulent cloud tops and explode with the force of 250,000 H-bombs, releasing energy equivalent to 500 billion tons of TNT. On its last near-approach to the planet, in July 1992, the comet split into half a dozen irregularly shaped fragments; in a few weeks they will begin crashing into the Jovian atmosphere one by one over the span of several days.
The impacts won't be directly visible here because all of them will occur on the side of Jupiter facing away from the Earth. But astronomers won't have to wait long to observe the results, since Jupiter completes a spin on its axis every 10 hours. Scientists hope to glean important information about the physics of cosmic collisions from the debris visible hours after the impacts. Giant infrared telescopes in Hawaii will monitor the event, as will instruments aboard the Kuiper Airborne Observatory and the 200-inch reflecting telescope on Mount Palomar. The Hubble Space Telescope, the Galileo spacecraft and even the venerable Voyager 2, now more than 4 billion miles on the opposite side of Jupiter, also will join the hunt.
No one knows exactly what will happen when the comet strikes. This is a once-in-a-lifetime opportunity for planetary scientists, though, and they are determined to make the most of it.