Halley's Comet

Halley's Comet is a periodic comet whose official designation is 1P/Halley. It takes about 76 Earth years on average to complete an orbit, although the interval varies because the gravitational pulls of planets perturb its path. Its orbit is retrograde: the comet travels around the Sun in the direction opposite to Earth's orbital motion, on a plane inclined about 18 degrees to Earth's orbit. At its farthest point from the Sun, it passes beyond Neptune's orbit.

The comet's nucleus is an irregular, very dark body roughly 15 by 8 kilometres in size. As it approaches the Sun, warming causes frozen material to escape from the surface as gas and dust. This activity forms a surrounding coma and tails, making the comet visible under favourable conditions. Material released along its orbit also contributes to the annual Eta Aquariid and Orionid meteor showers.

People recorded appearances of the comet long before modern astronomy; astronomers have linked observations of it across more than two millennia. In 1705, English astronomer Edmond Halley compared the similar calculated orbits of comets reported in 1531, 1607 and 1682. He proposed that the reports described the same object on successive returns and predicted that it would appear again in 1758. The comet was sighted late that year and passed perihelion—the point in its orbit nearest the Sun—in March 1759. Halley had died in 1742, before the prediction was confirmed. Its periodic return became an important demonstration that at least some comets orbit the Sun, and the comet was later named in his honour.

The comet's 1986 return was studied by an international group of spacecraft, including Japan's Suisei and Sakigake, the Soviet Vega 1 and Vega 2, and the European Space Agency's Giotto. Giotto passed approximately 600 kilometres from the nucleus and returned the first close-up images of a comet nucleus. Its observations showed an irregular, dark surface and jets of gas and dust; the encounter helped transform comets from distant points of light into objects that could be studied directly. NASA's historical account also records contributions from other spacecraft observing the comet from different vantage points.

Halley was last seen in Earth's skies during its 1986 apparition. Because the comet's orbital period is not perfectly constant, predictions account for gravitational perturbations by the planets. NASA and the NASA Planetary Data System give 2061 as its next expected return to the inner Solar System. The date refers to the comet's return, not a guarantee of how bright or easy it will appear from every location on Earth.