Halley's Comet, perhaps the most famous of all comets, which reappears approximately every 76
years. It is known to have been seen on 30 separate occasions, from 240 bc to 1986. Its next appearance is scheduled for 2061. (These dates refer to the time at which the comet passed perihelion, the point in its orbit closest to the Sun.) Although the most recent apparitions, or appearances, have been disappointing, owing to unfavourable positions of the comet with respect to the Sun and the Earth, it remains one of the largest and most active comets known.

Perhaps the best display was that of 837, when the comet emerged suddenly from the direction of the Sun and approached the Earth to a minimum distance of less than 6 million km (3.7 million mi). A famous appearance was that of 1066, which, although presaging defeat in the Battle of Hastings for the English king, Harold, augured well for the Norman invader, William. The comet's arrival that year was recorded for posterity in the Bayeux Tapestry.

The comet is named after the English astronomer Edmond Halley. The theory of gravitation developed by Isaac Newton was a key ingredient in Halley's discovery that the bright comet that appeared in 1682 had an orbit that brought it back to the inner solar system at regular intervals. Previously there had been three main suggestions concerning the paths of comets: that they were straight lines, sharply curved parabolas, or greatly elongated ellipses. Although the question was finally settled for most comets in favour of parabolic orbits, Halley was foremost in recognizing that an alternative elliptical solution fitted the observations of the 1682 comet.

This solution was consistent with the observed positions of the comet appearances of 1456, 1531, and 1607, and Halley was the first to propose that these apparently different comets were actually the same. Moreover, by applying Newton's gravitational theory and allowing for the gravitational effects of Jupiter and Saturn, Halley was able to predict, successfully, the comet's approximate time of return. It was seen again in late 1758.

Nucleus of Halley's Comet

In March 1986 the European Space Agency’s spacecraft Giotto made a close fly-by of Halley’s Comet during its perihelion passage (the part of its orbit nearest the Sun). Although Giotto was knocked off course and temporarily disabled by a speck of dust from the comet travelling at several miles per second, it had managed to capture the most detailed images of a comet’s nucleus ever taken. The nucleus is peanut-shaped, and about 16 km (10 mi) along its longest axis. The top left of the nucleus shows its true colour, which is very dark; the white arrow in the black box shows the direction of the Sun, and gases are erupting off the surface from spots facing in that direction.

Whether Halley's comet will die with a bang or a whimper is still not known. Many comets moving in similar orbits end their lives by colliding with the Sun, while others are ejected from the solar system; a few collide with the planets, sometimes the Earth. Collisions of cosmic bodies with planets have profound implications not only for the origin and evolution of life on Earth but also for our understanding of the formation and long-term evolution of all the planets in the solar system.

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