In many cases, they saw two flashes before the main event. The first was believed to be either the fragment itself, heated by friction as it entered the highest atmosphere of Jupiter, or gas and dust in its wake being lit by the fragment. The second flash was believed to be the actual fireball, when the object had penetrated 200 miles or so into the atmosphere. This raised a plume thousands of miles above the cloudtops-and by this time Jupiter had rotated enough so we could study the plumes from Earth. The best was yet to be.
Plumes spread out sideways as well as rose.
And then, pulled by the enormous gravity of the giant planet, the plumes slammed back down into the cloudtops, creating shock waves that instantly heated those clouds to 10,000 degrees F. Spellbound scientists marveled at flares the diameter of Earth and as bright as the Sun's surface. As spectacular as it was, this kind of thing happens all the time on Jupiter. A poll taken at the meeting showed that 90 percent of those responding thought such bombardments occurred every 100 years or even every 10 years. And it is not unknown for Jupiter to break up a comet or asteroid before it hits the planet. Astronomers at the meeting estimated it happens every 150 years or so.