Comet Shoemaker-Levy 9

The spectacular collision of comet Shoemaker-Levy 9 with Jupiter in July 1994 provided valuable information about comets, Jupiter’s atmosphere, and the Jovian role in diminishing potentially cataclysmic Earth-damaging debris in the inner solar system.

Overview

On the night of March 24, 1993, the husband-and-wife team Eugene (“Gene”) and Carolyn Shoemaker and their colleague David Levy were using the Schmidt telescope at Mount Palomar Observatory in California to take photographs in connection with a project designed to discover near-Earth celestial objects. Gene Shoemaker, who had recently retired from the US Geological Survey’s Astrogeology Research Program, which he had established, was an expert in Earth-orbit-crossing asteroids and comets. In their five years together, the Shoemaker-Levy team had already discovered eight comets, and they were pleased when one of their photographs of the sky near Jupiter revealed what Levy called “the strangest comet” he had ever seen. Its several tails and bat-shaped wings of dust reminded him of the American stealth bomber. They quickly realized that this fragmented comet was an important discovery, and three days later, they made comet Shoemaker-Levy 9 (SL 9) public in a circular published by the International Astronomical Union (IAU). Following tradition, the comet was named after its discoverers, with the number indicating it was the ninth comet that this team had found. Its formal IAU name became D/1993F2, in which the prefix “D” stated it was a periodic comet that later “disappeared,” 1993 was the year of discovery, the suffix “F” represented the half-month of discovery (F = March 16-31), and “2” meant it was the second discovery in that half-month.

Other observers, stimulated by this announcement, returned to photographs they had taken before March 24 and confirmed the discovery of the Palomar team. Using these and other data, astronomers calculated the comet's orbit, which, unlike all earlier comets, orbited Jupiter rather than the Sun. Its highly elliptical orbit had an apojove (the point farthest away from Jupiter) of nearly fifty million kilometers and a period of almost two years. Later data helped refine this orbit and provided clues about the comet’s early history. Like most comets, it had orbited the Sun, but it had been captured several decades earlier by Jupiter’s gravity. In early July 1992, it traveled so close to Jupiter, about 20,000 kilometers from the Jovian cloud tops, that the giant planet’s powerful gravity broke the comet into twenty-one separate fragments, each of which collected a coma of dust.

Following the convention established for previously fragmented comets, the twenty-one discernible pieces were labeled with letters of the alphabet (excluding I and O), and so SL 9’s fragments, which averaged a few kilometers in diameter, ran from A to W, with the brightest (and presumably the largest) piece called Q. With the orbit and fragments identified, astronomers soon realized that this piecemeal comet was on a collision course with Jupiter. For SL 9, there would be no escape from Jovian orbit to return to the Kuiper Belt, believed to be the source of Jupiter Family (JF) comets; instead, SL 9 faced extinction within sixteen months of its discovery.

Because astronomers knew when the comet would collide with Jupiter, they had time to organize observatories worldwide, including in such remote locations as Antarctica, to prepare to collect data on this unprecedented event. Because the initial impact would take place on the side of Jupiter, hidden from the Earth, several spacecraft would play important roles, particularly in the earliest observations. These spacecraft included Galileo, well on its way to study Jupiter; the Hubble Space Telescope (HST); Ulysses, which had been studying solar poles; the Roentgen Satellite (ROSAT), which had been surveying the sky for X-ray sources; and Voyager 2, which had been exploring the outer planets of the solar system.

As predicted, the comet’s first fragments slammed into the Jovian atmosphere on July 16, 1994, at a speed of about sixty kilometers/second, or fast enough to traverse the US in about a minute. When Jupiter’s rotation made the crash site visible to terrestrial observers, thousands of telescopes could see the dark spots that were created. Fragments of SL 9 continued to collide with Jupiter over the next 5.6 days. Because of the great excitement created by this astronomical event and because the Earth had been interconnected by various computer networks, images, observations, scientific data, and personal impressions were rapidly transmitted all over the planet. Many others experienced the event through television or the many stories in magazines and newspapers. By the time the final fragment, W, struck Jupiter on July 22, many millions of Earthlings had shared, in some way or other, this unique interplanetary event.

Knowledge Gained

The prodigious wealth of information created by the SL 9 event had significant implications for understanding comets, the Jovian atmosphere, and the future history of Earth. Astronomers knew, of course, that comets could be destroyed by collisions with the Sun, planets, and satellites, but the data from the fragmentation and collision of SL 9 with Jupiter revealed that its nucleus had been neither a solid body nor a loose agglomeration of materials but something in between. When the pieces hit Jupiter, spectroscopic analysis detected the presence of several elements absent from the Jovian atmosphere. These elements, which came from the comet, included nonmetals like sulfur and silicon and metals like iron, aluminum, magnesium, and even lithium, hitherto undetected in comets.

As expected, when the high-speed fragments of SL 9 penetrated the Jovian atmosphere, gigantic explosions and massive Seismic waves resulted. Fireballs created temperatures above 10,000 kelvins, which rapidly diminished to 2,000 kelvins. Collision-zone temperatures remained elevated for two weeks, but astonishingly, smaller impact sites had higher temperatures than larger ones. A typical fireball spread from fifteen to one hundred kilometers in about forty seconds, and some plumes extended to an altitude of 3,000 kilometers. These explosions also produced waves that sped across the planet at about 450 kilometers/second. These waves, which weakened in about two hours, posed a problem for astronomers. Disagreements developed about where they occurred (in the Jovian stratosphere or troposphere) and how they traveled (guided by a stable layer or generated by interlayer complexities). Just as spectroscopic analysis revealed some surprises about SL 9’s chemical composition, so, too, certain elements and compounds were discovered for the first time on Jupiter. For example, diatomic sulfur and carbon disulfide. By contrast, astronomers had expected to find sulfur dioxide but did not.

Astronomers also used other parts of the electromagnetic spectrum to gather data on the collision. For example, radio emissions at a specific wavelength (20 centimeters) indicated synchrotron radiation, most likely caused by the collision’s injecting very high-speed electrons into the Jovian magnetosphere, which also experienced other changes after the impact. Since Jupiter and comets were known to have water in their makeup, astronomers were surprised by the minimal amounts of water detected. Perhaps the comet’s fragments lost most of their water before the collision, or maybe the comet’s fragments were destroyed before they reached the planet’s water layer.

For many, the event's highlight was creating a series of dark spots that scarred Jupiter’s southern hemisphere for several weeks. Similar to the Great Red Spot, if smaller in scale, these dark spots were the most enduring transient features ever seen on the planet, although some historians of science pointed out that, in 1790, Gian Domenico Cassini had reported unusual temporary marks on Jupiter’s disk. If these had been due to a cometary collision, then SL 9’s crash onto Jupiter would not have been the unique event many touted.

Context

Earth has experienced steady and numerous collisions with interplanetary objects throughout its history. The collision of the Shoemaker-Levy 9 comet with Jupiter provided astronomers with valuable insights into how the crash affected Jupiter and, by analogy, how such comets may have affected other planets, including Earth. Gene Shoemaker estimated that comets had most likely caused about a fifth of the enormous impact craters on Earth. Linear crater chains have been photographed on Ganymede and Callisto, two of Jupiter’s satellites, probably due to cometary collisions. Some scientists have speculated that if SL 9 had collided with Earth instead of Jupiter, life would have been destroyed. According to many scientists, sixty-five million years ago, an asteroid or comet smashed into Central America, creating massive amounts of atmospheric pollutants that helped to bring about the extinction of the dinosaurs and many other forms of life.

In the history of life on Earth, other mass extinctions have occurred, and some scientists associate these with periodic comet showers. Various theories have been put forward to explain these periodicities—for example, Nemesis, a companion star of our Sun, may create perturbations in the Oort Cloud that lead to these recurrent invasions of comets into the solar system. Jan Oort was the first to suggest that this enormous reservoir of icy bodies might be the source of very long-period comets. However, SL 9’s collision with Jupiter revealed something very significant. Because of Jupiter’s powerful gravitational field, it attracts many asteroids, comets, and other interplanetary debris, resulting in fewer collisions of these objects with the inner planets, especially Earth. Some have even called Jupiter a “cosmic vacuum cleaner.” On the other hand, estimates indicate that small comets collide with Jupiter about once a century, and comets comparable in size to SL 9 hit it about once per millennium. Comet Shoemaker-Levy 9 certainly expanded knowledge about the nature and properties of comets and their interactions with other members of the solar system. Still, astronomers also realize that they need to learn much more before they will be able to make reliable predictions about some future comet’s possibly devastating collision with Earth.

Bibliography

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