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Imagine you are a famed planetary geologist who played the key role in bringing the study of impact craters into the mainstream of astrogeological science. You are among the few who truly believed in the catastrophic dangers of inevitable major collisions between the Earth and large enough asteroids and comets. Your experiences in studying nuclear explosion craters and the much bigger impact craters had you firmly convinced that disasters of that magnitude were genuine threats to mankindís safety and even survival. So, in your later professional years you dedicated much of your time to a task shunned by all but a very few in the know: making an inventory of all detectable asteroids following potential Earth-crossing orbits. Then one night at the Observatory your wife, Carolyn, whom you had trained to help in this search, noticed a shift in a small bright object as she studied photographic plates exposed through the telescope on different nights. This surely was a comet. You contacted your colleague, David Levy, for confirmation - soon obtained. You let out the word to other observatories. Soon thereafter, one of these reported that the comet had split into pieces. Calculations then led to the astounding prediction that the pieces were on a collision course with Jupiterís gravity field and would likely be drawn to it in a series of collisions. No one on Earth had ever witnessed such an event, on this planet or any other. You, the expert on impacts, would realize a lifeís dream of actually observing and participating in a real time collision. Then, in July of 1994 it happened and the multiple collisions were of such magnitude as to be easily detected by the Hubble Space Telescope in space and many others on Earth. The world at last had come to know of the likelihood that big impacts are part of the Solar Systemís past and future and that humans had better start thinking of ways to protect our planet.

This is the saga of Gene Shoemaker, probably the most influential planetary geologist since the Space Age opened. The comet, of course, has immortalized his name, that of his wife, and their friend, by being called Shoemaker-Levy. On this page, we will recount this once-in-a-lifetime happening, which captivated millions as it happened and has pushed scientists and even the military into serious thinking about the genuine threat impacts pose to life on Earth.


Comet Shoemaker-Levy


In March of 1993, Carolyn Shoemaker, wife of Dr. Eugene M. Shoemaker* (the Dean of Astrogeologists and chief guru of the Impact School), and an accomplished teacher turned scientist, known particularly for her contributions in the search for asteroids and comets, inspected photos taken days apart using surplus film, from a damaged batch having high sensitivity. A small bright elongate object had moved relative to star reference points. As it was imaged later at higher magnification, it was revealed that the short streak displayed a strange but distinct pattern of separated dots

An early photograph through a telescope of the string of comas that made up Comet Shoemaker-Levy (SL-9).

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 a small coma and tail, stretched out for more than 700,000 km (435,000 miles) along an orbit tied into Jupiter. Then, in May of 1994, word came that this comet, by then named Shoemaker-Levy 9 (S-L 9), was on a 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 witness a major planetary impact!

Scientist believe that Comet S-L 9 originated beyond Neptune. Over centuries of travel, various bodies perturbed its orbit towards Jupiter, so it 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 (12,428 mi) to Jupiter's surface, well within the Roche Limit (a distance from a planet's center, within which its gravitational forces are likely to disrupt a passing body), and had broken apart then into 21 fragments. Each nucleus was less than 2 km (1.25 mi) in diameter. The HST made this dramatic image:

Hubble Wide Field Planetary Camera 2 image of Comet Shoemaker-Levy 9, May 1994.

The world of astronomical science quickly and efficiently mobilized for this event. Calculations pointed to a high probability that it would actually hit Jupiter in 1994. As more precise estimates of just when and where the collisions would occur, scientists realized that HST and Galileo would be in position to observe at least some of the impacts, and Earth-based telescopes should also see something. Guesses as to 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. Labeled 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. Below, we display the sequence of color IR images made through a telescope fitted with a methane filter (which absorbs radiation at most wavelengths but passes radiation in a narrow band around 2.3 µm):

 

A time series of color photos of Jupiter showing in the second and third ones a flash in the lower left (at

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 to the right). Astronomers monitored most of the subsequent impacts over the next six days 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. Next, we show W's hit on July 22, sequenced seven 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):

 

Time-series of telephotos documenting the impact of nucleus W on the dark side of Jupiter; appears as a bright flash of light.

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 reached a height of 3,000 km (about 1,864 mi) in about six minutes:

 

The glowing plume of gases rising above the jovian surface on the dark side, as nucleus G plowed into Jupiter, setting off an explosion-like reaction; shown here at different wavelengths (colors) using HSTís optical camera.

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

 

Hubble image pair showing the G impact scar through green and methane-based filters.

19-76: How do you explain that the impact of the G fragment, probably only a few kilometers in size, which on Earth would make a crater several hundred kilometers wide, actually made a visible scar on Jupiter that was at least as big as the Earth's diameter itself? ANSWER

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

The following 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

 

Natural color image made by HST showing several brown spots which are the scars of multiple impacts by SL-9 nuclei.

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

 

Color IR image showing impact scars from SL-9 cometary fragments hitting Jupiter,  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:

 

HST UV image of impact scars.

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

Two good Web sites that cover the Shoemaker-Levy event are: (1) and (2)

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, astronomers have discovered several new planetary systems elsewhere in our galaxy. Who knows what incredible worlds lie beyond our own? Perhaps someday we’ll remotely sense a sister planet and then, when the technology develops, visit it by probes or even by our species.

In the meantime, what should be the priorities in future exploration of the Solar System. NASA, ESA, the Russians and other space programs will make these decisions. But input from the public - especially space connoiseurs - who "foot the bill" is very much desired. The writer (NMS) has his own favorite 'top of the list': Mars! We now know enough to realize that if that planet had only been larger (thus holding more of its atmosphere) and its potential biogenic development could adjust to the decreased solar energy it receives then it may well have been a favored host for life to develop. As it is, the possibility has yet to be ruled out. Rovers and orbiters will help in the search. Man's visit would be decisive.

Some of the "flavor" of future exploration plans is covered in a JPL von Karman lecture. Access this at von Karman lectures, choosing the topic "Space and Earth Exploration:2020, November 21, 2002.

Now it is time to expand our remote sensing horizons towards Infinity to show how this technology lies at the heart of the exploration of the whole Universe. The next Section, 20, may be the most provocative of all.


(*) With deep sorrow, the writer (NMS) must report here the death of a good friend and teacher of impact principles, Eugene M. Shoemaker, who may rightly be called "the father of Astrogeology." He was killed on July 18, 1997 (three years to the day after the main SL-9 event; and also on my birhday), in an auto collision (impact, ironically) in the interior of Australia, while pursuing meteor craters that are so well displayed on that continent.

This photo captures very well the Gene Shoemaker I so respect. He is laughing, something that came easy for him. He is in formal garb (receiving an honor) but with his ubiquitous western lanyard instead of a tie.

It is fitting that part of Gene's ashes were onboard Lunar Prospector so that when that probe finally descended and crashed onto the lunar surface in search of water, in a sense he too "landed" on the Moon–a long time goal of his (to be a lunar astronaut) that health problems thwarted just prior to the Apollo program. A second honor was posthumously awarded him by renaming the NEAR probe to the asteroid 433Eros as the NEAR-Shoemaker spacecraft. And, to memorialize him on Earth, the meteoritical community, including the Australian contingent, has renamed the 30 km (18 mile) wide Teague crater to the Shoemaker crater, as seen below:

The Shoemaker crater (once, the Teague structure), in western Australia.

I have since visited his widow, Carolyn. She is carrying on the work that so preoccupied him in his last years: looking for asteroids and comets, particular those that might be a threat to Earth.

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net