Before 1992, no scientist had expected to witness an interplanetary collision in our lifetime. Remarkably, however, our generation was lucky enough to be at the right place at the right time. We describe how Gene and Carolyn Shoemaker, with amateur astronomer David Levy, discovered the comet that bore their name (Shoemaker-Levy 9, or S-L 9), how it was torn apart by the gravity of Jupiter, and how astronomers calculated that the 20 fragments of this object would crash into Jupiter during one exciting week in July 1994. From first-hand experience we describe the reaction of the astronomers, and tell how major space systems such as the Hubble Space Telescope and the Jupiter-bound Galileo spacecraft were brought into the world-wide campaign to record these impact events. Other astronomers fanned out across the world, to mount the most concentrated observing effort in history. Yet there remained many uncertainties, and as late as one week before the impacts, some astronomers predicted a cosmic fizzle.
CHAPTER 9: THE JUPITER COMET CRASH
The solar system no longer seems quite so far away as it did before July 1994. Here we are, close to the edge, protected from the true immensity of the universe by a thin blue line. A day will surely come when the sheltering sky is torn apart with a power that beggars the imagination. It has happened before. Ask any dinosaur, if you can find one. This is a dangerous place. -- NASA physicist Kevin Zahnle, quoted in Ferris: Is This the End? (New Yorker, Jan 27 1997) It was lunch‑hour in the dining hall at Kitt Peak National Observatory on Saturday, July 16, 1994. Normally, the southern Arizona observatory closes for the summer due to budgetary shortfalls, but an exception had been made for Comet Shoemaker-Levy 9. A group of scientists were comparing notes: MIT observers, several staff from the University of Arizona's Steward Observatory, plus other astronomers awarded time on the National Observatory's telescopes. They lingered after the meal to converse about the evening ahead, when the second comet fragment (named "B") was scheduled to hit Jupiter.
Despite mostly cloudy skies, the evening weather forecast provided a glimmer of hope. One of us (Chapman) had been assigned the 2.1‑m (84‑inch) telescope. During the previous evening, he and his assistants had ironed out several bugs in the observing procedures. Our hope was that when fragment B struck during the fading twilight, there might be a measurable enhancement in the brightness of Jupiter's inner moons, reflecting the brilliant meteor trail and subsequent explosion that would be hidden from our direct view behind Jupiter's horizon. Or, if we were very lucky, a plume from the impact might ascend high enough to be visible at the edge of Jupiter itself.
While some of the observers returned to their domes to give their instruments final check‑outs, several remained in the lunch room shortly before 2 pm when someone returned to report a rumor: the impact of fragment A had actually been seen! We rushed back to our own dome, fired up our lap‑top computer, and logged onto the Internet. An electronic‑mail "exploder" had been set up in a University of Maryland computer, and several hundred would‑be comet crash observers had signed up. Any one of us could send a message to the Maryland address and it would be instantly copied into the e‑mail boxes of all the other participants.
The historic first report on the comet crash came from the Calar Alto Observatory in Spain. An international team of observers at this Spanish‑German facility used a camera sensitive in the infrared (wavelength of 2.3 micrometers). Their electronic report was succinct: "Impact A was observed with the 3.5 m telescope at Calar Alto using the MAGIC camera. The plume appeared at about nominal position over the [edge] at around 20:18 UT. It was observed in 2.3 micron methane band filter brighter than Io."
What amazed the astronomers at Kitt Peak was the last sentence of the Calar Alto report, which said that the flash of impact A was brighter than Jupiter's moon Io. For astronomers, that is really bright! As the Sun dropped in the Arizona sky, additional reports of success were posted from observatories in Europe, the Canary Islands, South America, and even the South Pole ‑‑ all situated in the narrow strip of Earth where Jupiter had been above the horizon and observable in dark skies when fragment A struck Jupiter. As the Sun set in Arizona, half an hour before B's scheduled impact, we nervously inserted filters into our instrument and began tracking Jupiter, which ‑‑ we could see through the dome slit ‑‑ was fading in and out as clouds drifted past. Then we noticed that our telescope operator, who is responsible for moving our telescope, had posted a large, orange (colorized) image on his computer console. It was an infrared picture of Jupiter taken at the European Southern Observatory (ESO). Abutting the edge of Jupiter was an enormous bright "thing" ‑‑ the overexposed ejecta plume of fragment A. Soon he had retrieved from the World Wide Web a similar image posted by the South Pole observers. That was when it hit home to us that we were in for quite a show.
The Internet was the communications medium. Before comet crash week was over, literally millions of requests for images were made to accumulating archives of S-L 9 data, and the information superhighway slowed to a crawl. Yet this new system worked well enough to let hundreds of thousands of amateur astronomers, computer hobbyists, and media representatives look over the astronomers' shoulders as the drama unfolded.
We returned to monitoring our instruments but saw no hint of the impact of fragment B near its scheduled time, 7 hours after the excitement in Spain. Logging onto the Internet, we read similar negative reports from other observatories. Fragment B must have been a will-o'-the-wisp, a cloud of dust (making it visible from Earth) but lacking bigger chunks like those in Fragment A. We had missed our only chance to measure one of the S-L 9 impacts. But as we drove down the switchbacks on the mountain road, our vicarious exhilaration about the spectacular impact of fragment A crowded out any disappointment that the prime impact predicted for observers at our longitude had been a washout.
For most people, Comet crash week will be remembered for spectacular infrared pictures of glowing impact "flashes," for the televised cork‑popping of champagne at the Space Telescope Science Institute when the first Hubble image of plume A was posted, and for the excitement of astronomers and laypeople alike about the unexpectedly dazzling show from Jupiter. Many people all over the world could share the elation of Gene Shoemaker, Carolyn Shoemaker, and David Levy as "their" comet made history. In retrospect, the circumstances of the come crash could hardly have been better, unless it had happened a year later when the Galileo spacecraft was in orbit around Jupiter.
Seeing each impact in silhouette on the edge of Jupiter turned out to be a lucky break. As demonstrated by Hubble's portrait of plume A ballooning into view, the eruptions were so spectacular that they quickly soared above Jupiter's edge anyway. As debris was propelled upward and then arched ballistically back down toward Jupiter's stratosphere, the plume was viewed against the blackness of space. Had we instead been looking down on a plume projected against Jupiter's sunlit clouds, we would hardly have seen its shape ‑‑ indeed it might have been totally lost in Jupiter's glare.
We were also lucky that the impacts struck Jupiter's pre‑dawn longitudes, rather than Jupiter's post‑twilight side. By the time the ejecta were cascading back down into Jupiter's stratosphere, the planet's rapid (10‑hour period) rotation had carried the impact sites around into direct view, allowing astronomers to watch the awesome re-impact of the debris into Jupiter's stratosphere. As Jupiter's rotation carried the impact points further into sunlight and more directly across Jupiter's Earth‑facing side, observers were able to study how the dramatic dark impact scars evolved during the first five hours after each impact. Waves spreading out from ground‑zero looked like bulls‑eyes; their velocities could be measured for hours until they faded from view. The wave velocities may provide clues about Jupiter's atmosphere far below the visible clouds, where the deepest effects of the explosions set the planet's atmosphere reverberating. And spectroscopists could study the rapidly changing chemistry of Jupiter's stratosphere in the immediate aftermath of its contamination by both cometary debris and by gases dredged up from far beneath Jupiter's ammonia cloud deck.
We were lucky that S-L 9 hit where it did, but the geometry was confusing, nevertheless. It took many months for observers to sort out what it was that they had seen. The stunning pictures of what appeared to be huge explosions on Jupiter (misleadingly called "flashes"), shown on television and printed in news magazines, were not really the impact explosions at all. These infrared flares, which lasted for many minutes and peaked more than a quarter hour after each impact, were actually the re‑impacts of the erupted plumes back into Jupiter's atmosphere. The debris plummeted down, pulled by Jupiter's immense gravity, at about 10 km/s. Over regions larger than the entire planet Earth, stratospheric temperatures rose to more than 500oC. Such heat is unheard of in Jupiter's frigid part of the solar system, and the sensitive infrared detectors mounted on the Earth's largest telescopes literally went off scale as these regions came into direct view.
This phenomenon shouldn't have caught everyone by surprise, but it did. As we described in Chapter 6, several years earlier Jay Melosh had published this exact scenario for broiling the dinosaurs alive, by the heat of re-impacting K-T ejecta. But the physicists who used the world's most capable computers to predict what might happen on Jupiter were best equipped to study the first seconds and minutes of impact, so that's what they concentrated on, neglecting the development of the plume and its fall back into the jovian atmosphere. Those who did think about the later stages of the impacts failed to allow for the re-condensation of the hot plume gases into solid dust particles, which in turn were heated to incandescence during re-impact. We had the knowledge all along to have predicted late‑stage infrared flares, but instead we were surprised and, at first, confused.
Another, unexpected shock were those big, black bruises on Jupiter. We are used to whitish cirrus clouds and volcanic hazes in our own planet's stratosphere; perhaps that's why astronomers imagined that if S-L 9 induced any changes at all in Jupiter's appearance, it might be a whitish haze. Instead, observers ‑‑ including kids using small backyard telescopes ‑‑ were treated to giant, gaping black spots in Jupiter's mid‑south latitudes. Some of the spots were as large as Jupiter's famous Great Red Spot, larger than the planet Earth, and they were darker and more prominent than any atmospheric feature ever seen on Jupiter since the invention of the telescope. In hindsight, chemists realized that when you take a chemical stew of Jupiter's air and the carbon‑ and sulfur‑rich debris from a comet and heat it with multi‑million megaton explosions, you might expect a sooty residue.
The black spots, which were drawn out by stratospheric winds into a
globe‑encircling black belt, were composed of very fine dust floating at the top of Jupiter's stratosphere, were they persisted for several months. Initially black to Earth-based observers used to Jupiter's bland colors, the dark palls weren't really opaque or dense enough to block sunlight entirely. But calculations show that warming at high altitudes and cooling at depth were great enough to have affected Jupiter's climate, over Earth‑sized regions. A year after the impacts, the dark pall had faded, much of it spreading laterally around Jupiter's enormous girth and thinning out. On Earth, of course, there would be nowhere for a global pall to go, so it might well persist for more than a year. The persistence of S-L 9's atmospheric effects were noteworthy: two years after the impacts, the spectroscopic signatures of some rare chemical compounds formed in Jupiter's stratosphere were still strengthening.
With the impact of fragment W on July 22 ‑‑ its immense mushroom‑shaped plume duly captured by Hubble's camera ‑‑ Comet Shoemaker‑Levy 9 was history. Exhausted astronomers began to pack up their instruments and return from the far‑flung outposts to which they had travelled. However, for Galileo Project scientists, the fun was just about to begin. NASA's spacecraft, out beyond the asteroid belt, had apparently performed most of its computerized instructions correctly, or so the sparse engineering information that was radioed back indicated. The primary challenge was to locate and transmit down to Earth the most critical few percent of all the data now recorded on the spacecraft's tape recorder. That's all that could be dribbled back, at just 1 character per second, before work had to begin six months later preparing Galileo for its 1995 encounter with Jupiter.
The Galileo team, under the leadership of one of us (Chapman), made the difficult decisions required to estimate exactly when each fragment had struck and commanded the spacecraft to play back just those specific images. By August 8th, engineers had processed the first samples from the K impact, one of the most spectacular as observed from Earth. Just a few lines were returned, widely spaced across a frame that covered about two minutes of imaging data. It was like looking at a scene through cracks between the slats of a fence. But there it was: a brilliant spot of light, projected against Jupiter's night side. Even the preliminary data revealed that fragment K's explosion was visible to Galileo for more than half a minute. The impact of the final W fragment was obtained more simply: Galileo's shutter snapped pictures as fast as it could, once every 2.3 seconds, and the time‑lapse sequence was recorded on successive frames in arrays of 8‑by‑8 images, or 64 images per frame. One week after samples of K, Galileo radioed back the first sample data for W: just one row of 8 images, covering less than 20 seconds of time, but continuing the meteoric flash.
The first five images in the row showed nothing but a gibbous Jupiter, looking like the Moon between first‑quarter and full. But there, in the sixth, seventh, and eighth images was a brilliant point‑of‑light (actually saturating the central picture element in the seventh image), suspended against Jupiter's night side. The sequence revealed what a human observer aboard Galileo would have seen with a powerful pair of binoculars. First there was nothing . . . then, suddenly, a several‑second‑long flash right next to Jupiter's illuminated face, about as bright as one of the brightest stars in the sky. The flash was already fading in the final image on that row. Not until January 1995 were the remaining pictures of W returned from the spacecraft, but already on August 15th it was clear what had been seen: it was the last remaining fragment of the broken comet making its fiery plunge into Jupiter's atmosphere ‑‑ the mightiest meteoric bolide ever witnessed.
It is from the entire suite of data ‑‑ taken by Galileo, by Earth‑
orbiting satellites like Hubble, from airplanes high above the Pacific, and from mountain‑top observatories around the world ‑‑ that scientists have pieced together some understanding of what transpired on Jupiter in July 1994. One of the earliest efforts to compare and reconcile the various observations took place on August 18 at The Hague, about 3 weeks after the final impact. Once every three years the International Astronomical Union holds a General Assembly, attended by leading astronomers from around the world, and by coincidence the 1994 meeting directly followed the comet crash.
Observers from all over the planet converged in The Netherlands, many still suffering from a combination of sleep deprivation and excess adrenaline. It was immediately obvious that the various visible and infrared observations of "flashes", "fireballs", and "plumes" could not all be referring to the same phenomena -- not unless observatory clocks all over the world had substantial errors. Torrence Johnson of JPL, the Galileo Project's Chief Scientist, wandered from one person to another with a time-line of the W event sketched on a notepad, confirming that the observed phenomena spanned nearly half-an-hour of time. At The Hague, the scientists were like the proverbial blind men and the elephant, each trying to make sense of a different, narrow aspect of the comet crash. Only after months of analyzing their voluminous data and comparing notes at a series of scientific conferences, did astronomers reach a consensus about the major lessons learned from this remarkable natural experiment.
Just what happened when a comet fragment screamed into Jupiter at 60 km/s? Galileo's instruments, with their unobstructed view of the impact sites, best characterized the first minute, the period when the entire kinetic energy of an impactor's motion was converted into a multi‑million megaton explosion. What the spacecraft camera saw first was the meteor flash, before the disintegrating fragment plunged out of view beneath Jupiter's main cloud deck. Almost immediately the superheated tunnel of atmosphere just traversed by the meteor began to explode. Three Galileo instruments could observe simultaneously in three wavelength bands: ultraviolet, visible, and infrared. Together, they determined the temperature, altitude, and dimensions of the developing fireball. The G impact, perhaps the biggest of all, epitomized Galileo's results. A few seconds after the meteor, Galileo measurements reveal that there was a parcel of jovian atmosphere, located just above the main cloud deck, which was several kilometers across and radiating at a temperature hotter than the surface of the Sun. Within the next minute, this bubble of atmosphere was observed to rise, cool, and expand. Without a doubt, these were the early stages of the plume that Hubble a few minutes later watched soar into sunlight, more than 3,000 kilometers above the clouds.
Twelve minutes after G's impact, Peter McGregor, observing from Siding Springs Observatory in Australia, took perhaps the most memorable picture of the entire week, as the plume fountained back toward Jupiter, creating an infrared spectacle. Two hours later, Hubble took its most detailed picture of the bulls‑eye debris apron, and then visual observers around the world began to gape through their eyepieces at the most prominent feature ever to be seen on Jupiter.
That is what was seen, but it's not all that happened. Unquestionably, the more energetic comet fragments gave up most of their energy in explosions far below Jupiter's visible cloud deck. What observers watched was just the top of the explosions. Observers initially claimed that their data showed that the fragments had disintegrated above the clouds, and that the computer models predicting fragment penetration to hundreds of kilometers below the clouds were wrong. However, it remains unknown just how far the explosions penetrated into Jupiter's depths, for the deeper effects were swallowed up. What's clear is that the tops of the explosions were mighty enough to produce all the phenomena observed.
All, that is, except for the bulls-eye rings. Andy Ingersoll, a lanky professor at Caltech, is one of the world's experts on the atmospheres of the outer planets -- Jupiter, Saturn, Uranus, and Neptune. During the Voyager mission press conferences, he became a familiar face as he made his outer-planet "weather predictions," most of which proved true. Unlike our own Weather Bureau, which has to issue the daily forecast whether they feel confident or not, Ingersoll was free to issue only the predictions he was sure would come true. Five years after Voyager's last outer-planet encounter, S-L 9 enticed Ingersoll to make his most famous prediction of all. And this time he even put his money on it.
Ingersoll measured the bulls-eye rings radiating away from the largest impact scars. But their spreading velocities made no sense if the waves were generated at Jupiter's visible cloud surface. The rings, he concluded, must be the uppermost crests of waves generated far down below Jupiter's erstwhile water clouds. But even then, he couldn't make the velocities fit the theory . . . unless, he concluded, Jupiter has ten times as much oxygen (for every hydrogen atom) as the Sun does. His conclusion flew in the face of the widespread idea that Jupiter is pretty much a chunk of solar material and should have the same percentage of oxygen (which would make up the H2O) as the Sun. Cocky from his Voyager success and with reporters listening in, Ingersoll bet all comers $10 that the Galileo spacecraft would prove him right when its probe penetrated through Jupiter's water clouds in December 1995 and directly measured the humidity. Several dozen of his colleagues bet against him, and he lost every dollar. In fact, the Galileo Probe didn't find any water clouds at all, Ingersoll immediately paid up. Jupiter experts now think that the Galileo probe struck an unusually dry spot on Jupiter, and that overall the planet might be as wet as Ingersoll had expected. But even three years after the S-L 9 impact, Ingersoll still does not understand the nature of the expanding dark rings. He does have a graduate student working o the problem, however, as a possible thesis topic.
A major dispute among S-L 9 observers concerned the sizes of the fragments. Some thought they must have been very large; how else to explain the plumes rising 3,000 km high? Yet there were other indications that the fragments were small. All observers who tried to detect the meteor flashes by reflected light from Jupiter's moons were disappointed. One moon was even eclipsed from sunlight in Jupiter's shadow when the K fragment struck, yet no flash reflection was detected. No matter how pretty Galileo's images of the meteor flashes are, they were far fainter than some early predictions for large impacting fragments.
Apparently the fragments of the comet were not especially massive. Probably the whole original comet measured less than 2 km across. The individual fragments may have been tight swarms of smaller pieces, loosely held together by their own gravity during their final two‑year trajectories following break‑up. As the swarms plunged toward their demise, they stretched out a bit and might have been a kilometer long when they rammed into Jupiter's stratosphere, generating the visible plumes. However, the mass of each swarm would have made an object only a few hundred meters across, were it all compressed together. Research also shows that this modest remnant from the solar system's birth had virtually no strength at all, given the ease with which Jupiter's gravity pulled it apart. S-L 9 was little more than an aggregate of dirt and ice, with a bulk density of about 0.7 grams per cubic centimeter (less dense than an ice cube). It essentially fell apart in July 1992 as it sailed close to Jupiter.
In spite of remaining uncertainties about the nature of the comet, and of the jovian atmosphere it struck, there are some dramatic lessons that S-L 9 has taught us about planetary impacts. Each of the major observed phenomena on Jupiter -- the direct blast, the atmospheric waves, the plume blasting high above the atmosphere, the heat pulse from backfalling debris, and the long-lived dark clouds in the jovian stratosphere -- has a counterpart in terrestrial impacts. Environmental chnges previously inferred indirectly from the K-T boundary on Earth were seen unfolding before our eyes on Jupiter.
Perhaps most important, after the great comet crash, it was difficult for anyone to dismiss the idea of cosmic impacts as a fantasy. Jupiter's stratosphere was clearly devastated by the impact of a very modest‑sized comet, and there can hardly be any doubt that a similar event on Earth would have terrible consequences for human civilization. In the remaining chapters of this book, we will focus on the nature of this contemporary risk and on ways of developing a planetary defense against comets and asteroids.
3800 words (5/6/97)