Halley's Comet is the most famous comet in history, and the only bright, naked-eye comet that returns within a human lifetime. It orbits the Sun about every 76 years. Its last visit was in 1986, and its next perihelion, the closest approach to the Sun, is due on July 28, 2061. Here is the science, the history, and what to expect.
Officially named 1P/Halley, it is a short-period comet, made of ice, dust and rock, and its solid core, the nucleus, is shaped roughly like a peanut about 15 km long and 8 km wide. As the comet nears the Sun, ice turns to gas and drags dust with it, forming the glowing coma and long tail. If you are unsure how comets differ from asteroids, see Asteroid, Comet, Meteor: What's the Difference?
Comets were once thought to be one-time omens. The English astronomer Edmond Halley studied records of bright comets seen in 1531, 1607 and 1682, noticed their orbits were very similar, and concluded in 1705 that they were the same object returning. He predicted it would come back in 1758. Halley died in 1742, but the comet was recovered on Christmas Day 1758 by Johann Palitzsch, a German amateur astronomer, and it was named in Halley's honor. It was a landmark victory for Newtonian physics.
Observations of the comet go back more than two thousand years. Chinese astronomers recorded it as early as 240 BC. The comet appeared in 1066, shortly before the Norman conquest of England, and is shown in the Bayeux Tapestry. Its 1910 return was a global sensation: Earth passed through the comet's tail, and some people panicked, though the gas in a tail is far too thin to be harmful.
For the 1986 return, several space agencies sent probes. The Soviet Vega 1 and Vega 2, Japan's Suisei and Sakigake, and the European Space Agency's Giotto all studied it. Giotto flew within about 600 km of the nucleus and sent back the first close-up pictures of a comet's core, showing a dark, irregular surface with bright jets of gas and dust. The dark surface surprised scientists, as it showed that comet nuclei are not clean "snowballs".
Halley's orbit is a long ellipse that takes it out beyond Neptune and back inside the orbit of Venus. One orbit takes roughly 75 to 79 years. The period varies because the gravity of the giant planets, especially Jupiter and Saturn, tugs on the comet, and because jets of gas from the comet act like tiny thrusters. That is why scientists calculate the date with computer models. Perihelion is predicted for July 28, 2061. The 1986 apparition was unfavorable for viewing from Earth, and the 2061 apparition is expected to be better placed, though brightness predictions for comets are uncertain.
You do not have to wait until 2061 to see something of Halley. Each time the comet passes the Sun, it leaves a trail of dust. When Earth crosses that trail, grains burn up as meteors. Two annual showers come from Halley: the Eta Aquariids in early May and the Orionids in October. The Orionids peak around the third week of October each year.
No. Halley's orbit does not intersect Earth's in a way that could lead to an impact in the foreseeable future, and a comet this size would be tracked for years ahead. Comets belong to the broader group of near-Earth objects that NASA monitors, explained in What Is a Near-Earth Object?
Facts in this article are drawn from public scientific sources. For primary information, see: