A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail.

Halley's Comet or Comet Halley, officially designated 1P/Halley, is a periodic or short-period comet and returns to Earth's vicinity about every 75-76 years, making it possible for a human to see it twice in his or her lifetime. The last time it was here was in 1986, and it is projected to return in 2061. It is arguably the world's best known comet.

The comet is named after English astronomer Edmond Halley (1656-1742), who examined reports of a comet approaching Earth in 1531, 1607, and 1682. He concluded that these comets were actually the same comet returning over and over again, and predicted the comet would come again in 1758. He didn't live to see the comet's return, but his discovery led to the comet being named after him.

When Halley's Comet sweeps by Earth in 2061, the comet will be on the same side of the Sun as Earth and will be much brighter than in 1986. At least one study has pointed out that it is difficult to predict Halley's orbit on a scale of more than 100 years, and that the comet could collide with another object (or be ejected from the solar system) in as little as 10,000 years, although not all scientists agree with the hypothesis.

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