Word: comets
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...next two centuries of cometary science were relatively uneventful. In 1823 German Astronomer Johann Franz Encke, who calculated the orbit of a periodic comet that bears his name (it reappears every 3.3 years), insisted that the orbit of "his" comet could not be explained solely by gravity. He proposed that "ether," an invisible theoretical substance that at the time was believed to pervade space, exerted drag on the nucleus, slowing it down. After observing flares streaming from Comet Halley's surface in 1836, another German astronomer, Friedrich Wilhelm Bessel, conceived a more plausible concept, the fountain theory. Bessel proposed that...
...next big breakthrough came in 1950, when Fred Whipple, a Harvard astronomer, proffered a detailed model for the anatomy of a comet. In a delightfully evocative phrase, Whipple declared that comets are "dirty snowballs," dark conglomerates of mostly frozen water stippled with rocky fragments, dust particles and trace elements. As one of these snowballs swoops toward the sun, said Whipple, solar radiation begins to vaporize ice and frozen gases on the comet's sunward surface by a process called sublimation. The gases, carrying dust with them, form a light-reflecting coma that makes the comet visible from earth...
Like the heated gases bursting from a jet engine, the departing cometary ! molecules exert a force on the icy nucleus, giving the comet its independent thrust. But because the ice does not vaporize uniformly and the nucleus rotates, the jet action propels the comet in many directions and can either accelerate or decelerate it. Apparently this accounts for the nongravitational motions that astronomers had previously observed. After 35 years of scrutiny, Whipple's model of comet properties is still accepted today. "When I first realized about the jet action of comets," says the 79-year-old astronomer, "Boy! That...
Many other aspects of cometary theory have since been refined or expanded. By studying the spectra of light emitted from molecules broken down in the gaseous coma, scientists have estimated that a comet's nucleus consists of two-thirds water, one-fifth dust (particles averaging one-thousandth the width of a pinhead) and the rest a mixture of methane, ammonia, carbon dioxide and trace elements...
...When a comet is still as far away as 5 AU from the sun, about the distance of Jupiter, its most volatile material begins forming a coma that reflects light. By the time the most powerful telescopes first catch a glimpse of a comet, the coma already obscures the nucleus beneath. For every revolution a typical comet makes around the sun, its diameter is estimated to shrink about 6 ft. Hence the original size of the comet, the length of its orbit and how close it gets to the sun will determine its life-span. Astronomers estimate that Halley...