Shoemaker-Levy 9, Comet

▪ astronomy

 comet whose shattered nucleus crashed into Jupiter over the period of July 16–22, 1994. The cataclysmic event, the first collision between two solar system bodies ever observed, was monitored from Earth-based telescopes worldwide, the Hubble Space Telescope and other Earth-orbiting instruments, and the Galileo spacecraft, which was en route to Jupiter.

      On March 25, 1993, a previously unknown comet positioned close to Jupiter was noted by Eugene and Carolyn Shoemaker and David Levy in photographs taken at Palomar Observatory in California. Most unusual was its appearance—it comprised at least a dozen cometary nuclei lined up like glowing pearls on a string. Week after week the nuclei spread farther apart until a total of 21 fragments were visible. An analysis of their common orbit revealed that the original comet, which had been revolving about the Sun, had grazed Jupiter's atmosphere and nearly crashed into it on July 8, 1992. At that time, tidal gravitational forces from the giant planet had broken the nucleus into many pieces, which were captured by Jupiter's gravity and thrown into an elongated two-year orbit around the planet. Astronomers calculated that the new orbit would bring the pieces back to Jupiter in July 1994.

 The train of fragments from Shoemaker-Levy 9 smashed into Jupiter's atmosphere with a velocity of 216,000 km (134,000 miles) per hour beginning July 16, 1994. They all hit the unobservable side beyond the limb of Jupiter as seen from Earth, but the planet's 10-hour rotation quickly brought each impact site into view. Separated in time by an average of seven to eight hours, each fragment plunged deeply into the Jovian atmosphere, leaving conspicuous scars aligned in a zone near latitude 44° S. Astronomers labeled the individual fragments with capital letters in order of arrival. Fragment G, with an estimated diameter of 3–4 km (1.9–2.5 miles), was probably the largest and heaviest. It left a dark multiringed blemish twice as large as Earth's diameter. Its impact delivered energy equivalent to several trillion tons of TNT—hundreds of times the yield of the world's supply of nuclear weapons. Each impact transformed quickly into an immense bubble of hot gas that glowed warmly in infrared images of Jupiter as it expanded for a few days in the atmosphere. The planet-girdling string of dark brown bruises, assumed to be fine organic cometary dust, remained visible for weeks; it faded slowly into a new, narrow belt induced by Jupiter's strong winds.

Armand H. Delsemme

 

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