The planet Jupiter is likely to be struck by a comet in the last week of July next year.
The release of energy is estimated to be thousands of millions of megatons of TNT _ greater than the impact thought to have brought the reign of dinosaurs to a cataclysmic end 65-million years ago.
This is the first time humanity will witness such a cosmic collision.
Calculations suggest the comet will hit the night side of Jupiter _ facing away from the Earth _ about July 20, 1994. If it hits the Earth-facing side of the giant planet, the conflagration will be visible in daylight and rank as the most spectacular astronomical event in human history.
Even so, the consequences may be spectacular. The face of Jupiter could be changed as the force of the explosion rips through the thick atmosphere of the biggest planet in the solar system. Debris could form a ring around the planet to rival that of its neighbor, Saturn.
Jupiter is almost certain to acquire new moons to add to its huge retinue. It already has four planet-sized moons and dozens of smaller ones. The existing moons may be showered with comet fragments and acquire fresh impact craters.
The comet was discovered March 24 with the 18-inch telescope on Mount Palomar, Calif., by Dr. Carolyn Shoemaker and amateur comet-hunter David Levy.
Shoemaker, who recently discovered her 30th comet, is married to Dr. Eugene Shoemaker, who watches for Earth-grazing asteroids using the same 18-inch telescope.
But the comet in question _ now called Comet Shoemaker-Levy 9 _ is a bit out of the ordinary.
Rather than just a single comet, Shoemaker-Levy 9 is a string of at least 17 mountain-sized chunks arranged in a line like the cars of a train. Astronomers think it was once a single comet that strayed too close to Jupiter and was shattered by the planet's enormous gravitational field.
Next July it will approach close enough to Jupiter for at least some of the fragments to score direct hits.
Just one fragment, hitting Jupiter at 60 kilometers per second, will release energy equivalent to more than a thousand trillion tons of TNT. The fragments that miss may well become moons of Jupiter or shatter into even smaller fragments to make a new planetary ring.
Although the actual impact is likely to happen on the wrong side of Jupiter _ with respect to observers on Earth _ the effects of the blast should be visible in the form of reflected light from Jupiter's moons.
Even amateur astronomers should have an excellent view. If the calculations are any guide, the aftershocks of a cosmic blast of this scale should be astonishing.