Halley’s Comet

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Halley’s Comet

Halley’s comet (also called Comet Halley and, officially named, 1P/Halley) is a periodic comet appearing every 75 to 76 years. It is named after English astronomer Edmond Halley (1656–1742), the first person to accurately predict the return of the comet. This famous comet follows a retrograde (east to west), elliptical orbit, providing a magnificent, astronomical spectacle. In 1910, the Earth passed through its brilliant, fan-shaped tail that 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 of Neptune’s orbit, 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 BC and maybe even 466 BC, Halley’s comet may make 3,000 more revolutions and survive another 225,000 years, 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, English physicist and mathematician Sir Isaac Newton (1642–1727), 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 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 that 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 BC to Agrippa’s death; in AD 451 to Atilla the Hun’s only defeat; and in AD 1066 to William of Normandy’s conquest of England. Even in 1910, some people panicked, believing the comet’s tail contained poisonous gas that would exterminate all life on the Earth.

A different picture preceded Halley’s 1986 apparition. Astronomers worldwide trained their telescopes on the sky and the International Halley Watch became the largest international scientific cooperative ever

KEY TERMS

Aphelion— The point furthest from the Sun during orbit.

Perihelion— Point closest to the Sun during orbit.

Perturbation— Change in the orbit of an astronomical body by the gravitational influence of another body.

Polymers— Identical molecules which join together to create different, more complex molecular chains.

Recovery— First sighting of a returning comet.

Retrograde orbit— Opposite direction to the path of the planets.

planned. Ironically, the comet was first seen by 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.

In addition, six spacecraft, unofficially called the Halley Armada, soared to probe Halley’s secrets, collecting data that confirmed American astronomer Fred Lawrence Whipple’s (1906-2004) 1950 theory of a solid nucleus composed of ice and rocks and providing new information. The six spacecraft were: the Giotto space probe (European Space Agency), Vega 1 and Vega 2 (Soviet Union/France), and Suisei and Sakigake (Japan). Giotto came to within 370 mi (596 km) of Halley’s nucleus, capturing for the first time fascinating images of a potato-shaped, 9x5x5 mi (15 x 8 x 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 that create the gigantic, glowing coma and tail. The comet was also observed by the International Cometary Explorer as it orbited the sun from space.

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 the 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 to 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.