 In mid-July, major telescopes on Earth and in space will be trained on planet Jupiter for an encounter that could be one of the most significant astronomical events in history. A string of over 20 ice and dust fragments from an ancient comet will plummet into Jupiter, causing massive explosions. Geologist Eugene Shoemaker, astronomer Carolyn Shoemaker and amateur astronomer David Levy found the comet in March 1993 during a survey of the sky at Mount Palomar Observatory. No one is quite sure what effect the comet's impact will have on the planet, an immense globe of gas a thousand times larger than the Earth. Well, the $64 question is how big are the fragments? At one point some months ago, we thought the largest might be about three, maybe even four kilometers across. If they were that large, then altogether these comets would deliver about 100 million megatons of energy. Computer simulations of the impact's effect on Jupiter have been made by astronomers at the Massachusetts Institute of Technology. During the six days of bombardment, major observatories of the world hope to study the event. The impact sites will occur on the opposite side of Jupiter, away from Earth's view. The Galileo spacecraft now en route to Jupiter will view the event head on, eventually sending back images. Whether the event goes off with a bang or a whimper, few astronomers will miss this cosmic event of a lifetime. In mid-July, major telescopes on Earth and in space will be trained on planet Jupiter for an encounter that could be one of the most significant astronomical events in history. A string of over 20 ice and dust fragments from an ancient comet will plummet into Jupiter, causing massive explosions. It's the first time ever that we've seen a comet captured in orbit around Jupiter, and it's the first time ever we'll actually be able to predict an impact. Geologist Eugene Shoemaker, astronomer Carolyn Shoemaker, an amateur astronomer David Levy, found the comet at the end of March 1993. Oh gee, it's a lovely day. From their perch atop Mount Palomar in Southern California, they monitor the night sky using a 60-year-old 18-inch telescope. The vintage instrument has perfect properties for spotting fast-moving objects. With it, the Shoemakers have discovered dozens of comets. It is actually a camera. Exposures of eight minutes are used for each picture. The field of view takes in the wider than normal portion of the sky, making it an ideal workhorse for spotting new objects. Okay, does that look right? Yes, and minus 13, 18. The evening of the discovery, high clouds threatened to cancel their efforts. Prospects didn't look good, but David Levy suggested that they continue using damaged film that would have been thrown away. That decision paid off. We knew he had something very unusual. It was so unusual we were a little unsure just what we were dealing with. So I started to scan it and Jupiter was a big overexposed looking blob on one side. And I started at the top and across and down. And then as I moved across about a little more than halfway down, I suddenly thought I saw something. Caroline used a stereo microscope to look at two pictures taken 40 minutes apart of the exact same location. As she viewed the images simultaneously, everything looked flat except fast-moving objects like asteroids or comets. They appeared to float. I turned to the others and I said, I don't know what I've got. It looks like a squashed comet. The squashed comet turned out to be a chain of fragments. They were formed when Jupiter's immense gravitational force pulled the comet apart as it passed by the giant planet. NASA's newly repaired Hubble Space Telescope took these images of the comet, its powerful mirrors bringing each fragment into sharp focus. Well, the $64 question is how big are the fragments? At one point some months ago, we thought the largest fragments might be about three, maybe even four kilometers across, about two and a half, up to two and a half miles in diameter. If they were that large, then altogether these comets would deliver about a hundred million megatons of energy. But no one is quite sure what effect the impacts will have on the planet. An immense globe of gas a thousand times larger than the Earth. Timothy Dowling and Joe Herrington of the Massachusetts Institute of Technology have developed a computer marvel that shows the effect of the comet's impact on Jupiter's atmosphere. The comet pieces will plunge down underneath the clouds, these beautiful ammonia clouds that are on the surface, and finally just give up and explode and all of that hot heated gas will rise up and then it'll fall back down and you'll get just like throwing a rock into a pond, you'll get this big splash and then the slower ripple. During the six days of bombardment, major observatories of the world hope to study the event. The impact sites will occur on the opposite side of Jupiter, away from Earth's view. Scientists hope to see after effects of the impacts or flashes of light reflected onto Jupiter's nearby moons. The Galileo spacecraft now en route to Jupiter will view the event head on, eventually sending back images. It will be an unprecedented opportunity to learn about the planet's weather patterns. Joe Herrington believes the explosions could trigger storm systems in normally stable areas. If the pieces are big enough to produce any kind of observable effect at all, anything that makes the planet look different from the way it looks today will learn something. And the more changes, the more we'll learn. In fact, there are valuable lessons in store for our own planet. There are a lot of people that believe that an impact about the size of the combined comet train is what killed off the dinosaurs. Gene Shoemaker has long supported the theory that comet material landed some 4.5 billion years ago and delivered life's essential ingredients to Earth. In essence, I think we're the progeny of comets. Comets arriving early in the history of the solar system delivered the volatile material. Come July 16th, amateur and professional astronomers alike will train their telescopes on Jupiter. Whether the event goes off with a bang or a whimper, few astronomers will miss this cosmic event of a lifetime. We knew we had something very unusual. It was so unusual we were a little unsure just what we were dealing with. We were pretty... Backed up and when I looked at it I realized that what I saw was a bar and this bar had coma and tails. And that was unspeakably weird. You just don't find something like that when you're looking for either asteroids or comets and nothing else. Yeah, the comet pieces will plunge down underneath the clouds, these beautiful ammonia clouds that are on the surface, plunge down underneath and finally just give up and explode and all of that hot heated gas will rise up and then it'll fall back down and you'll get, just like throwing a rock into a pond, you'll get this big splash and then the slower ripple.