Halley's comet
Halley's Comet, designated as Comet 1P/Halley, is the most renowned periodic comet, completing an orbit around the Sun approximately every 76 years. Its first recorded observations date back to 240 BCE, and it is named after Edmond Halley, who was the first to identify that sightings of the comet at regular intervals were of the same celestial object. The last appearance of Halley's Comet was between late 1985 and early 1986, a time when it could be observed with the naked eye under optimal conditions.
Despite challenging visibility for many observers, this return was significant for scientific research, leading to an extensive international collaboration to study the comet. Six spacecraft, including probes from the Soviet Union, Japan, Europe, and the United States, were launched to gather data on Halley's composition and structure. These missions provided groundbreaking insights, revealing that the comet's nucleus is an irregular, potato-shaped body approximately 9 miles long, with a surface that ejects substantial amounts of material, predominantly water. The next anticipated return of Halley's Comet will occur in 2061, although viewing conditions are expected to be less favorable than in its previous appearances.
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Halley's comet
The Event Astronomical body passes near Earth
Date 1985-1986
The return of Halley’s comet to the inner solar system during the mid-1980’s resulted in the first satellite fly-by explorations of a comet.
Halley’s comet (Comet 1P/Halley) is the brightest and most well known of all periodic comets, making a complete orbit of the Sun and returning to Earth’s vicinity every seventy-six years. Detailed observations of Halley’s comet have been recorded since 240 b.c.e. The comet is named for Edmond Halley (1656-1742), who was the first to recognize that the various objects witnessed in the sky at seventy-six-year intervals were actually the same object. The most recent return of Halley’s comet to the inner solar system was between autumn, 1985, and spring, 1986. During March and April, 1986, Halley’s comet could be viewed with the naked eye, if the observer was well south and far from urban lights. For most earthbound observers, however, viewing conditions were unfavorable. Despite the difficulty of seeing the comet from Earth, the comet’s return was extremely rewarding to scientists. In an excellent display of international cooperation, a massive effort was undertaken to study the comet.
Detailed earthbound observations of Halley’s comet on its return began in 1982. During 1985, an international fleet of six spacecraft were launched toward the comet. The spacecraft were to fly by the comet and send back immense quantities of data about its composition and structure, as well as its interaction with the solar wind. The spacecraft included the Soviet Vega 1 and 2 probes, Japanese Suisei and Sakigake probes, European Space Agency Giotto probe, and the United States’ International Cometary Explorer (ICE). Additional observations were made by spacecraft orbiting Earth and Venus. Returning comet data from all astronomical disciplines were coordinated by the International Halley Watch and archived at the Jet Propulsion Laboratory in Pasadena, California.
The six spacecraft encountered Halley’s comet between March 6 and March 25, 1986. The Vega and Giotto probes made the closest approaches to Halley’s comet. Acting as pathfinders, the Vega probes surveyed the comet first from distances of over 5,000 miles. Vega data were used to correct Giotto’s course as it closed to within 375 miles of the comet on March 14, 1986. Using remote sensing techniques, Giotto imaged the nucleus of Halley’s comet. The nucleus was found to be an irregular potato-shaped body 9 miles long and 5 miles wide with a density of 0.3 g/cm3. The nucleus’s surface was pockmarked by impact craters and covered with an almost-black crust. The comet was found to be ejecting three tons of material per second from its surface. The comet ejecta was 80 percent water, 10 percent carbon monoxide, and 2.5 percent carbon dioxide, with the remainder made up of ammonia and methane.
Impact
Science’s understanding of comets was greatly advanced by data obtained from the Halley’s comet spacecraft flybys. The results of these data have changed astronomers’ conceptual models of cometary structure and evolution. Halley’s comet will next return in 2061 and will offer even less favorable viewing conditions than those that existed in 1985-1986.
Bibliography
Brandt, John, Malcolm Niedner, and Jurgen Rahe. The International Halley Watch Atlas of Large-Scale Phenomena. Boulder: University of Colorado Press, 1992.
Ottewell, Guy, and Fred Schaaf. Mankind’s Comet. Greenville, S.C.: Astronomical Workshop, 1985.
Schaaf, Fred. Comet of the Century. New York: Copernicus, 1997.