Halley's comet, a periodic comet usually appearing every 76 years, is named after English astronomer Edmond Halley (1656-1742), the first person to accurately predict the return of a comet. This famous comet follows a retrograde (east-west), elliptical orbit , providing a magnificent, astronomical spectacle. In 1910, Earth passed through its brilliant, fan-shaped tail which soared 99 million mi (160 million km) into space . During its last apparition (appearance) in 1986, space probes and ground-based technology gathered valuable scientific data on its size, shape, and composition. In 2024, the comet will reach aphelion (furthest point from the Sun ) millions of miles outside Neptune's orbit, make a U-turn, and begin its thirty-first observed return to perihelion (point nearest the Sun) inside the orbit of Venus , arriving in 2061. Observed by Chinese astronomers in 240 b.c. and maybe even 466 b.c., Halley's Comet may make 3,000 more revolutions and live another 225,000, if recent estimates calculated from data collected by the space probe Giotto are correct.
Edmond Halley's prediction
In the late seventeenth century, comets were believed to follow parabolic (U-shaped) orbits and appear only once. The gregarious, outgoing Edmond Halley boldly suggested to his reclusive but genius friend, Isaac Newton, that comets may travel in an ellipse and appear more than once. Newton initially rejected the idea, even though his laws of motion and gravitation clearly allowed for such orbits. Later Newton accepted the possibility that comets can follow elliptical paths, orbiting the Sun repeatedly. In 1695, basing his work on Newton's laws of cometary motion , Halley computed the orbits of two-dozen comets, including the comet of 1682. He suggested the comets of 1531, 1607, and 1682 were one and the same, even venturing to predict its return in 1758. He was also the first to consider the perturbative (disruptive) effect of planets on a comet's orbit. Allowing for Jupiter's influence, he narrowed the comet's return to late 1758 or early 1759. Astronomers around the world anxiously watched the sky, aspiring to be the first to recover (find) the comet. On Christmas eve, 1758, German farmer and amateur astronomer, Johann Palitzch, spotted the comet which would forever bear Halley's name.
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
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—The point furthest from the Sun during orbit.
—Point closest to the Sun during orbit.
—Change in the orbit of an astronomical body by the gravitational influence of another body.
—Identical molecules which join together to create different, more complex molecular chains.
—First sighting of a returning comet.
- Retrograde orbit
—Opposite direction to the path of the planets.
Halley's comet or Comet Halley (hăl´ē, hā´lē), periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted. It reappeared in 1835 when it was carefully recorded by visual observers, and in 1910, when its long tail and outbursts of dust jets were observed photographically. For its most recent return in 1985 and 1986, astronomers observed it from the ground and from space. A massive observing effort (1982–89) including visual observations, photography, and studies of the area around the nucleus, was coordinated by the International Halley Watch. Japan, the European Space Agency, and the USSR sent spacecraft to study the comet; the Vega and Giotto probes revealed a darker-than-expected nucleus 8 km (5 mi) wide and 15 km (9 mi) long, and shaped like a potato.
See NASA Special Publications, Atlas of Comet Halley (1987); M. Grewing, ed., Exploration of Halley's Comet (1988).