April 24th: Halley's Comet

On 24 April 1066 Halley's comet was seen in the skies above England. To the people of that time, the unusual star had to be an omen of some kind. A bad omen for King Harold, but a good one for William. Here are 10 things you may not know about Halley's comet:

1. It is approximately 15 kilometres long, 8 kilometres wide and 8 kilometres thick, and is shaped like a peanut. It is actually a pile of rubble, loosely held together.
2. The comet has an elliptical orbit around the sun, which takes around 75 years to complete. At its farthest point, or aphelion, it is 35 times further away from the sun than the earth is, At its perihelion it is nearer to the sun than Venus.
3. We can only see it from Earth because of the substances which evaporate from it when it approaches the Sun, causing a coma, or atmosphere, of vapour and dust which reflect sunlight. The tail is caused by solar winds pulling the coma out into space. The tail can be over 100 kilometres long.
4. The coma and tail consist of 80% water vapour, 17% carbon monoxide and 3–4% carbon dioxide with traces of hydrocarbons and traces of methane and ammonia. The dust particles are a mixture of carbon-hydrogen-oxygen-nitrogen (CHON) compounds and silicates.
5. Halley did not discover the comet. It had been observed by astronomers for many centuries before Halley was born. The first records known to mention it are from China in 239 BC. Halley was the first modern scientist to work out that it was orbiting the sun and would return on a regular basis. Most people believed then that comets were just passing through the Solar System in a straight line, never to be seen again. He correctly predicted when it would be back, and although he didn't live to see it, the comet was named after him. I say "modern scientist" because there is a passage in the Talmud which mentions "a star which appears once in seventy years that makes the captains of the ships err," which suggests astronomers in the first century were aware of it, too.
6. There is speculation that the Star of Bethlehem could have been Halley's comet, since it appeared in 12 BC.
7. American author Mark Twain was born two weeks after the comet's perihelion in 1835. He predicted: "I came in with Halley's comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don't go out with Halley's comet. The Almighty has said, no doubt: 'Now here are these two unaccountable freaks; they came in together, they must go out together.'" He was right. He died on April 21 1910, the day after the next perihelion.
8. By the time Halley's comet appeared in 1910, mankind had invented photography and so it had its photograph taken for the first time, and scientists were able to study it using spectroscopy. Superstition about its appearance was still widespread, though, and hoaxers had a field day. One hoax said that a religious group in Oklahoma were about to sacrifice a virgin to ward off the evil omen, but were stopped by police. When scientists discovered that the tail contained cyanogen, a toxic gas, an astronomer called Camille Flammarion declared that when Earth passed through the tail, the gas would extinguish all life on Earth. Panic buying of gas masks and "anti-comet umbrellas" ensued until other astronomers pointed out that there were only traces of the gas and that it would not cause any problem.
9. By the next appearance, in 1986, space travel had been invented, and so a veritable armada of space probes were sent up to meet it. A Space Shuttle mission was scheduled to observe the comet from space, but sadly, it was the Challenger mission which exploded just after launch in January 1896.
10. Halley's comet will be back in the middle of 2061.