Halley's Comet
TO know about Halley's Comet, firstly you should know what is a comet?
Halley's Comet, officially designated as 1P/Halley, is a short-period comet returns to Earth vicinity about every 75–76 years, making it possible for a human to see it twice in his or her lifetime.
Halley is the only known short-period comet that is regularly visible to the naked eye from Earth. The last time it was here was in 1986, and it is projected to return in 2061. It is the first comet whose return was predicted and, almost three centuries later, the first to be imaged up close by interplanetary spacecraft.
Predicted next perihelion: 28 July 2061
Orbital period: 75 years
Radius: 5.5 km
Discoverer: Edmond Halley
Orbits: Sun
Meteor shower spawned: Eta Aquariids, Orionids
In 1705 English astronomer Edmond Halley published the first catalogue of the orbits of 24 comets. His calculations showed that comets observed in 1531, 1607, and 1682 had very similar orbits. Halley suggested that they were really one comet that returned approximately every 76 years, and he predicted that comet’s return in 1758. As a result of this discovery, the comet is now named after Halley. 1P/Halley is often called the most famous comet because it marked the first time astronomers understood comets could be repeat visitors to our night skies. When the comet was visible in the late of1758, passed perihelion (closest distance to the Sun) in March 1759. Its periodic returns demonstrated that it was in orbit around the Sun and, thus, that at least some comets were members of the solar system. Earlier passages of Halley’s Comet were later calculated and checked against historical records of comet sightings. Some have speculated that a comet observed in Greece between 467 and 466 BCE may have been Halley. However, the generally accepted date for its earliest recorded appearance, which was witnessed by Chinese astronomers, was in 240 BCE. Its passages have taken place every 76 years on average, but the gravitational influence of the planets on the comet’s orbit has caused the orbital period to vary from 74.5 to slightly more than 79 years over time. During the comet’s return in 1910, Earth passed through Halley’s dust tail, which was millions of kilometres in length, with no apparent effect.
LAST SEEN COMET:-
The most recent appearance of Halley’s Comet in 1986 was greatly anticipated. Astronomers first imaged the comet with the 200-inch Hale Telescope at Palomar Observatory in California on October 16, 1982, when it was still beyond the orbit of Saturn at 11.0 AU (1.65 billion km) from the Sun. It reached perihelion at 0.587 AU (88 million km) from the Sun on February 9, 1986, and came closest to Earth on April 10 at a distance of 0.417 AU (62 million km ).
Halley's Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation Five interplanetary spacecraft flew past the comet in March 1986: two Japanese spacecraft (Sakigake and Suisei), two Soviet spacecraft (Vega 1 and Vega 2), and a European Space Agency spacecraft (Giotto) that passed only 596 km from the comet’s nucleus. Images of the nucleus obtained by Giotto showed a dark potato-shaped object with dimensions of about 15 × 8 km. As expected, the nucleus proved to be a mixture of water and other volatile ices and rocky (silicate) and carbon-rich (organic) dust. About 70 per cent of the nucleus surface was covered by a dark insulating “crust” that prevented water ice below it from sublimating, but the other 30 per cent was active and produced huge bright jets of gas and dust. Thus, these observations supported several longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile ices. As the comet rotated on its axis, the rate of dust and gas emission varied as different active areas on the surface came into the sunlight.
Halley’s closest approach to Earth took place on April 10, 837, at a distance of only 0.04 astronomical units (AU; 6 million km [3.7 million miles]). It was the large bright comet.
This discovery put to rest an alternate explanation known as the sandbank model, promoted by English astronomer R.A. Lyttleton from the 1930s to the 1980s, that the nucleus was not a solid body but rather a cloud of dust with adsorbed gases.
