Word: dust
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Dates: during 1980-1989
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Scientists long ago recognized that every comet has not one but two tails, not always visually distinct, both extending millions of miles by the time the comet has moved close to the sun. They now know that the yellowish, often curved tail is composed of dust particles released during sublimation and swept away from the sun by the pressure of solar radiation. Sunlight reflecting off the tail produces the fiery effect. The second, bluish appendage is called the plasma or ion tail. It is formed when gases from the comet's nucleus become charged by solar radiation and then react...
...hundreds of these fragments. Most of them are sent outward into interstellar space, but some are hurled toward the sun as comets. Although the Oort Cloud has yet to be seen, most astronomers agree it exists and is the flotsam left over when a nebula, a massive cloud of dust and gas, collapsed and formed the solar system...
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...
...probe was launched from Kourou, French Guiana, last July; as of last week it was 21 million miles from earth and nearly three times as far from Halley's. The little ship and everything on it are built for survival, and with good reason. The dust particles around the nucleus are expected to strike Giotto with such great velocity that a speck weighing a tenth of a gram would penetrate an aluminum plate about 3 in. thick. To prevent damage, the side of the craft facing the comet is covered with a double shield, one made of aluminum...
...craft revolves on its axis, a solid-state optical camera extending from the bottom rim like a bent stovepipe will snap a photograph once every four seconds. The pictures will be instantly transmitted to earth and shown live on television. Mass spectrometers will analyze the composition of the dust from the nucleus, and other instruments will examine the properties of the ions in detail and measure the magnetic field girdling the comet's head. To prevent contamination of the surrounding space by the exhaust from Giotto's thrusters, controllers will turn off the engines at least 24 hours ahead...