The Comet That Keeps Coming Back
Halley's Comet is the only known short-period comet that can be seen clearly with the naked eye from Earth, returning roughly every 74 to 79 years. It last passed through the inner Solar System in 1986 and is due back in 2061.
For most of history, people thought comets were disturbances in the air. Then, in 1705, the English astronomer Edmond Halley used Isaac Newton's new laws of gravity to study old comet sightings. He noticed that comets seen in 1531, 1607 and 1682 followed almost the same path, and decided they were one object returning about every 76 years.
He predicted it would come back in 1758. Halley died in 1742 and never saw it. But on Christmas Day 1758, a German farmer and amateur astronomer spotted it.
It was the first time anything other than a planet had been shown to orbit the Sun, and one of the earliest big tests of Newton's ideas. The comet was named in Halley's honour.
A Messenger of Doom
Halley's Comet is so bright that about one in every eight comet sightings in historical records belongs to it. The first certain record comes from China in 240 BC, describing a comet that appeared in the east and moved north.
For centuries, people took it as a warning. In 1066 it was seen in England, and later that year King Harold II died at the Battle of Hastings and William the Conqueror took the throne. The comet appears on the Bayeux Tapestry, and accounts from the time said it looked four times the size of Venus.
In 837 AD it made its closest known approach to Earth, perhaps just 4.5 million kilometres away, with a tail that may have stretched across a huge part of the sky.
The 1910 visit caused panic. Scientists found a poisonous gas called cyanogen in the tail, and Earth was set to pass through it. Despite reassurances that it posed no danger, people rushed to buy gas masks, and fake anti-comet pills were sold.
A Dark Ball of Dust and Ice
Up close, the comet is a surprise. In March 1986, a fleet of spacecraft visited it, nicknamed the Halley Armada. The European probe Giotto made the closest pass, becoming the first spacecraft to study a comet in detail.
The solid centre, the nucleus, is small: about 15 kilometres long and 8 kilometres wide, shaped a bit like a peanut or a potato. It is made of ice and dust loosely held together. As it nears the Sun, ice turns straight into gas, a process called sublimation, and that gas and dust form a glowing cloud around it called the coma. The solar wind then pushes out a tail that can stretch more than 100 million kilometres.
Scientists expected a bright, icy surface. Instead, the nucleus reflects only about 4 percent of the sunlight that hits it, about the same as coal. Halley's Comet is actually pitch black.
It also travels backwards, going around the Sun in the opposite direction to the planets.
In With the Comet, Out With the Comet
The comet gave one writer a famous farewell. Mark Twain was born on 30 November 1835, two weeks after the comet came closest to the Sun. In his autobiography, published in 1909, he wrote that he came in with Halley's Comet, expected to go out with it, and that missing it would be the greatest disappointment of his life.
He got his wish. Twain died on 21 April 1910, the day after the comet's closest pass to the Sun.
Others saw it twice. In Japan, Emperor Hirohito watched it in 1910 at the age of eight, and again in 1986 at 84.
The 1986 visit was the worst for viewing in 2,000 years, because the comet and Earth were on opposite sides of the Sun, and city lights made it even harder to see.
Its next closest pass to the Sun is predicted for 28 July 2061. This time it will be on the same side of the Sun as Earth and far brighter in the sky. But the comet is slowly wasting away, and it may break apart or fade out within the next few tens of thousands of years.
