Ancient And Modern Perspectives
Throughout history, comets were viewed as omens. Halley's comet is no exception, and almost every apparition is linked to a major world event: in 11 B.C. to Agrippa's
death; in A.D. 451 to Atilla the Hun's only defeat; in A.D. 1066 to William of Normandy's conquest of England. Even in 1910, people panicked, believing the comet's tail contained poisonous gas which would exterminate all life on Earth.
A different picture preceded Halley's 1986 apparition. Astronomers worldwide trained their telescopes on the heavens and the "International Halley Watch" became the largest international scientific cooperative ever. Ironically, the comet was first seen by a California Institute of Technology graduate student, David Jewitt, and staff astronomer, Edward Danielson, who "borrowed" a few hours' viewing time through the 200-in (508-cm) telescope on Palomar Mountain in California. Also, six spacecraft soared to probe Halley's secrets, collecting data which confirmed Fred Whipple's 1950 theory of a solid nucleus composed of ice and rocks and providing new information. Giotto came to within 370 mi (596 km) of Halley's nucleus, capturing for the first time fascinating images of a potato-shaped, 9 × 5 × 5 mi (15 × 8 × 8 km) core with an irregularly shaped, dark surface crust. Only about 4% of the ices were exposed, the vapors of which emit gas and dust which create the gigantic, glowing coma and tail.
Cometary dust particles consist primarily of silicates-silicon, magnesium, and iron; and CHON particles-carbon, hydrogen, oxygen, and nitrogen, which were undetected until the VEGA and Giotto space missions. CHON particles suggest organic matter in the nucleus and, although providing no proof, the discovery renewed speculation that cometary molecules may have provided the stimulus for living organisms on Earth.
Gas analysis suggests that about 78% of Halley's nucleus is ice from water; 13% from carbon monoxide; 2% carbon dioxide-undetected until VEGA 1; 1-2% ammonia and methane-undetected until Giotto; while hydrocyanic acid, sulfur, and other gases combine for less than 1%. Giotto may also have detected the unexpected presence of polymers, created by formaldehyde molecules. The comet's basic chemical composition is similar to other solar system bodies.
Asimov, Isaac. Asimov's Guide to Halley's Comet. New York: Walker and Company, 1985.
Bailey, M.E., S.V.M. Clube, and W.M. Napier. The Origin of Comets. Oxford: Pergamon Press, 1990.
Lancaster-Brown, Peter. Halley & His Comet. Poole, England: Blandford Press, 1985.
Yeomans, Donald K. Comets: A Chronological History of Observation, Science, Myth, and Folklore. New York: John Wiley & Sons, Inc., 1991.
Marie L. Thompson