13. Colliding worlds

A comet hits Jupiter

Discovery of Comet Shoemaker-Levy 9

Eugene Shoemaker, his wife Carolyn, and the amateur astronomer, David Levy, were involved in routine observations the evening of 23 March 1993, a dark and cloudy night with stormy weather on its way. They were continuing a ten-year search for comets and asteroids that might be headed toward the Earth using the small 0.46-meter (18-inch) wide-field photographic telescope at Palomar Observatory in California. Good and expensive film couldn’t be wasted during the poor weather conditions, so some fogged film, which had been partially exposed to light, was used to photograph a clear place in the night sky near Jupiter before the clouds covered it up. Two days later, when Carolyn Shoemaker examined the images taken on the flawed film, she saw an elongated feature that looked to her like a “squashed comet”. When the discovery was confirmed with better telescopes, the stretched-out blur of comet light was resolved into several objects aligned along a single straight line projected in the sky, like pearls on a string. In accordance with tradition, it was named Comet Shoemaker-Levy 9, after the discoverers, ninth in a series of objects the trio found traveling around the Sun in short-period orbits.

Fig. .. 

When a comet nears the Sun, the ices in the comet nucleus turn directly from solid to gas and release dust to form a round, fuzzy coma, sometimes accompanied by a tail that points away from the Sun. But instead of a single coma and tail, powerful telescopes revealed a train of baby comets, each with its own invisible nucleus, nearly spherical coma and elongated dust tail.

Breakup and Collision

Comet Shoemaker-Levy 9, abbreviated SL9, consisted of the pieces of a former, single comet that had been trapped in a two-year orbit around Jupiter for decades. But when it traveled too close to Jupiter in 1992, the comet was shredded apart and launched on a trip to oblivion. It passed within about 20 million meters of Jupiter’s cloud tops, and 90 million meters from the planet’s center. So there was a modest difference between the planet’s gravitational attraction on the near and far side of the comet, enough to rip the fragile comet into at least 20 observable pieces. Most of these fragments remained visible over the entire subsequent lifetime of the comet.

Fig. .. 

What made SL9 unique was that the broken comet was inexorably hurtling along a path to total destruction, doomed to collide with Jupiter. Orbital calculations indicated that the train of comet fragments would plunge into the giant planet in July 1994, two years after the former single comet’s disruption and more than one year after the discovery of its pieces.


The collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994 was perhaps the most widely witnessed event in astronomical history. Practically every telescope in the world was trained on Jupiter during impact week, between 16 and 22 July 1994. Infrared heat detectors were placed at the focal point of the Keck Observatory’s giant 10-meter telescope atop Mauna Kea in Hawaii, and the Hubble Space Telescope was poised to record the event at visual wavelengths. Every other major astronomical observatory participated, as did numerous amateur astronomers from their own backyard.

Detailed calculations indicated that the collisions would be on the dark “back” side of Jupiter, hidden from the Earth’s view by the body of the giant planet. So it might be something like watching a World Series ball game from a seat behind a stadium post. The comet fragments would nevertheless strike Jupiter close to the side facing Earth, so astronomers hoped that something would be seen when the planet’s rapid rotation, of once every 9 hours 55.5 minutes, brought the impact sites into view. Moreover, the Galileo spacecraft, on its way to Jupiter, had a direct view of the actual collisions from its unique position in space.

Instruments aboard the Galileo spacecraft measured temperatures that soared to 10 or 20 thousand degrees kelvin when the fragments plunged into the clouds of Jupiter. That’s at least twice as hot as the Sun’s visible disk, at 5.28 thousand degrees kelvin. Rising plumes of hot gas were hurled three thousand meters above Jupiter’s clouds, each generating a bright flash of infrared light.

What incredible luck, to have a comet break into pieces, hit Jupiter, and generate brilliant bursts of infrared light so close to edge of the planet’s backside that they could be seen from on or near our planet. And the good fortune didn’t end there, for the arching plumes of hot gas produced great dark scars when they cascaded back down into the giant planet.

Fallout Fig. .. 

The comet fragments dove into Jupiter, one after another, like the cars of a train when its locomotive is derailed. After generating a bright ball of light, each fragment disfigured Jupiter with a black scar that had never been seen before, twice as large as the Earth and spanning tens of millions of meters. Meanwhile waves swept across the impact site and reverberated deep within the planet, which seemed to shudder from the impacts.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University