About the same time, their colleague and member of their asteroid search team, David Levy, made similar observations. After reporting their find to other observatories, word came back that the odd arrangement of dots was actually due to a string of individual cometary bodies, each with small coma and tail, stretched out for more than 700,000 km (434,000 miles) along an orbit tied into Jupiter. Then, in May of 1994, word was flashed that this comet, named Shoemaker-Levy 9 (S-L 9), was on a direct collision course that would end in a succession of hits onto Jupiter in July of that year. This would be a unique event - the first time humans would be witness to a major planetary impact!

Comet S-L 9 is believed to have originated beyond Neptune. Over centuries of travel it was eventually perturbed towards Jupiter to become a Short-Period comet whose very eccentric orbit carried it remarkably close to that planet. Calculations showed that S-L 9 had earlier passed as close as 20,000 km to Jupiter's surface, well within the Roche Limit (a distance out from a planet's center within which its gravitational forces are likely to disrupt a passing body), and had broken apart then into 21 individual fragments. Each nucleus was less than 2 km (1.25 miles) in diameter. The HST made this dramatic image:

The world of astronomical science quickly and efficiently mobilized for this upcoming event. As more precise estimates of just when and where the collisions would occur, it was realized that both HST and Galileo would be in position to observe at least some of the impacts, and telescopes over parts of the world should also see something. Guesses as to just what would happen ranged from penetration into the jovian atmosphere without much occurring to massive fireballs. Anticipation ran high among the network of watchers.

First to hit was the small nucleus at the left end of the string in the image above. Labelled A (with succeeding ones designated B, C, D.... through W [with I and O omitted]), this body struck Jupiter in its southern hemisphere, producing a detectable flash and subsequent plume picked up by observatories in Spain and Chile. This is favorably displayed in a sequence of color IR images made through a telescope fitted with a methane filter (which absorbs at most wavelengths but passes radiation in a narrow band around 2.3 µm):

The changing bright spot on the lower left surface is the thermal flash of the impact (the other bright oval above it is the Great Red Spot and Io is visible off planet as it moves past). Most of the subsequent impacts over the next 6 days were monitored either directly or as the impact scars moved into sight during Jupiter's rapid rotation. Galileo's camera caught several on the night side limb; here is W's hit on July 22, sequenced 7 seconds apart, showing the flash during and shortly after impact (it looks like a satellite off the planet but is really a momentary response set against the dark surface within the shadowed limb):

One of the biggest events was the strike of nucleus G on July 18. The rapidly rising plume from this, imaged in a time sequence by HST, shows it to have reached a height of 3000 km (~2000 miles) in about 6 minutes:

The scar in the surficial layers left behind, larger than the Earth's diameter, was later imaged by HST through a green and a methane-based filter:

This, and other scars, lead to the interpretation that each nucleus punches a tunnel into the jovian outer atmosphere, shocking, compressing, and flashing its gases. Materials both from the incoming comet and this atmosphere are carried into the hemispherical plume and also sidewards (influenced by the angle of impact) as crescent-shaped clouds, in much the same manner as associated with terrestrial impacts on land. The dark matter making up the scar is presumably color-altered constituents from the jovian atmosphere - perhaps HCN or sulphur derivatives. Unlike solid ground craters, those on Jupiter gradually dissipated, although their visible traces endured for several years.

Three views of Jupiter after all impacts had occurred demonstrate this persistence. The first is a natural color view made by HST of one side displaying impact sites

The second shows a color IR image taken from the Calar Alto observatory in Spain:

The third image, taken by HST's UV camera, brings scars H, Q, R, D, G and L (the round object in the upper hemisphere is the shadow of one of the moons) into sharp contrast:

Had any of the fragments from S-L 9 struck the Earth instead, a catastrophe of a magnitude never before experienced by humanity (but certainly by the dinosaurs) would have ensued. We've been warned!

Thus endeth this Great Voyage of Exploration through the Solar System! The previous 18 Sections should have convinced you that our Earth is an extraordinary, yet very livable place. But, this Section hopefully has opened your eyes to the fact that the rest of the solar system, while inhospitable, is clearly fascinating in its own right. Recently, several new planetary systems elsewhere in our galaxy have been discovered. Who knows what incredible worlds lie well beyond our own - perhaps some day to be remotely sensed from our planet and then, when the technology develops, visited by probes or even our species.

* It is with deep sorrow that the writer (NMS) must report here the death of his good friend and teacher of impact principles, when Eugene M. Shoemaker, who may rightly be called "the father of Astrogeology" was killed on July 18, 1997 in an auto collision (impact, ironically) in the interior of Australia while pursuing his further explorations of meteor craters that are so well displayed in that continent. It is fitting that part of Gene's ashes were onboard Lunar Prospector and will eventually, when that probe finally descends to the lunar surface, in a sense "land" on the Moon - a long time goal of his (to be a lunar astronaut) that was thwarted by health problems just prior to the Apollo program.

---

[an error occurred while processing this directive] [an error occurred while processing this directive]