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
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!
Scientists believe that Comet S-L 9 originated beyond Neptune. Over centuries of travel, various bodies perturbed its orbit towards Jupiter, so it became 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:
Just before reaching Jupiter, the Hubble Space Telescope took this view of some of the comet's fragments, enlarged enough to show cometary material ablating off in a direction away from the solar wind:
The world of planetary science quickly and efficiently mobilized for this collision 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, on July 16, 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):
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). A more detailed sequence of observations of event A was obtained through the telescope at the South Australian Astronomical Observatory
The Hubble Space Telescope obtained impact images at several wavelengths:
And HST obtained the first view of the impact site - a black scar, but no crater (any depression would be transient in the jovian atmosphere)
One of the biggest events was the strike of nucleus G on July 18. As imaged at the Keck Observatory:
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 HST imaged the scar (larger than Earth's diameter) left behind in the surficial layers, through a green and a methane-based filter:
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
The second and third show a color IR images, the first acquired by NASA's IR telescope on Mauna Kea, Hawaii and the second taken from the Calar Alto observatory in Spain:
Another 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 Earth instead, a catastrophe of a magnitude never before experienced by humanity (but certainly by the dinosaurs) would have ensued. We've been warned!
Shoemaker-Levy was thought to be a 'once-in-a-lifetime' event. It was somewhat surprising, at first, when another large impact occurred on Jupiter on July 20, 2009. A new transient spot appeared near the south pole. Here are two images:
The Hubble Space Telescope has taken a follow-up picture of the scar that shows it, like those of Shoemaker-Levy, has by then been smeared out:
The initial impact scar was the size of the Pacific Ocean. No one foresaw this impact. No object in space was observed to be closing on Jupiter. The object most likely was an asteroid or comet. The implications of this event are troubling: 1) large impacts may be more frequent than some have thought; 2) such impacts can occur in short succession; 3) impactors can be missed, even with watchful search programs; and 4) Earth is certainly not immune to being a target.
Shoemaker-Levy taught us what to expect from impacts into liquids and gases. Our concern with that mode is real: asteroids/comets could hit the Earth's oceans. But of greater danger could be impacts on land. NASA conducted a mission to comet Tempel-1 named Deep Impact, covered on the next page - that not only added information about impact on a solid surface but also gained knowledge about the nature of that surface
(*) 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. Read about his life on this Wikipedia biography.
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:
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.