Halley’s Comet (Pronounced to rhyme with “Sally’s Vomit") is probably the most famous short-period comet in the Solar System. It was named after the second Royal Astronomer, Sir Edmund Halley, in 1758 when his prediction that it would return every seventy-six years came true. The comet was last here in early 1986, but unlike its previous appearance in 1910, was not very bright. It is on course to return once more in 2062.
History
It is theorised that the comet was formed at the same time as the Solar System several million or even billion years ago, when a collision of two massive objects sent Halley out into the far reaches of the solar system on an unconventional orbit that lasts nearly eighty years.
The first recording of the comet is in 239 BC (although a drawing of a comet has been found that dates to 1059 BC and may well be Halley) when Chinese astronomers mentioned its appearance in the skies. To the ancients, the appearance of a new, fast moving, star seemed as if the gods were travelling closer to the Earth than usual and so, at this time, comets were taken to be omens of change.
The comet continued to return regularly for the next two hundred years and speculation has lead some to believe that it was this comet that was the star that lead the Magi to the birthplace of Jesus. Unfortunately for propagators of this theory, a closer study of records at the time reveals that the comet never appeared during Jesus’ lifetime; returning in 11 BC and 66 AD.
The next recorded significant appearance of the comet was in 1066 when it appeared in the skies at the time of the Norman invasion of England. King Harold II of England is pictured in the Bayeux Tapestry, sitting on his throne, being warned by his astrologers that the comet signifies a great change. Harold went on to lose the battle of Hastings, and all of England fell to the rule of the invading William of Normandy.
The comet’s 1145 appearance was documented by a Monk in Eadwine Psalter (book of Psalms) when he drew a small picture of it in the margin.
In 1301, the artist, Giotto di Bondoni painted his Nativity Fresco, and, inspired by the Comet’s presence in the skies that year, incorporated it into the picture as the Star of Bethlehem.
In 1705, Edmund Halley made the critical observation; he had noticed that roughly every seventy six years a bright comet had been observed crossing the heavens and had sought to prove that they were one and the same. Using his friend Isaac Newton’s new Theory of Gravity, Halley made the prediction that the comet was orbiting the sun like the Moon orbits the Earth and would return in the year 1758. Halley died in 1742, but on Christmas night, 1758, a bright comet was observed streaking across the skies. The comet was named in his honour.
The comet continued to return regularly; being observed in 1835 and photographed for the first time in 1910 when, due to it passing especially close to the Earth it was unusually bright. In 1982, the comet was observed passing Saturn by astronomer David Jewitt and in December 1985, the comet became visible from Earth with the naked eye. Unfortunately, the comet was unusually far away for this, the most recent passing, and, as a consequence, was not as impressive as it had been before.
Between 1984 and 1985 several spacecrafts were launched by the USSR, Japan and Europe to study Comet Halley. In 1986, Giotto and Vega passed close enough to comet to photograph it, and to analyse a sample of its tail. The comet proved to be much darker than expected, and may be the darkest object in the solar-system. During the fly-past, a small lump of Halley hit Giotto and its course was changed. Unfortunately its camera was disabled and so we did not get any photographs of the comet from exciting new angles.
As it moves away from the sun for the twenty-ninth time since it was first observed, a decrease in surface activity, but in 1991 the comet was rocked by a huge explosion, causing a dust cloud that expanded for several months it is unknown whether this was surface activity or whether the comet was hit by some unknown body. The comet was once again observed in 1994, but no activity was registered. The most recent sighting of the comet was in January 2004 when the Very Large Telescope based in Chile spotted it moving towards Neptune. The comet is currently an inactive lump of ice and it is not expected that it will be active again until it returns in nearly sixty years.
The Comet
The Nucleus
The nucleus of the comet is nine miles (fifteen kilometres) wide, five miles wide (eight kilometres) long and five miles high. It is shaped rather like a potato and it is pitted with around twenty visible craters, the biggest of which are several miles across. Its surface is made up of frozen gasses such as Hydrogen and Carbon Dioxide and a few other metallic minerals. The nucleus was found to be much darker than expected, reflecting about four percent of the sun’s light, this makes it even darker than coal, and it is perhaps the darkest object in the solar system. There is one group of people who believe the comet to be made of anti-matter, but I have been unable to verify this.
The nucleus is surrounded by a dense cloud of sublimed water, Carbon Dioxide and other gasses known as the coma. This can stretch up to one hundred thousand kilometres (seventy thousand miles) above the surface of the comet.
The Tail
Three jets that spew molecules from the comet’s surface when it becomes heated up by the sun form the most visible part of the tail. These jets are approximately ten million kilometres (seven million miles) long, and are the part of the tail visible from Earth. The tail leaves behind a stream of particles and every October when the Earth passes through the orbit of Halley, it gets a meteor shower known as the Orionids.
Trailing behind Halley for several hundred million miles is the ion trail. This is composed of streams of plasma, formed by interactions with the Sun’s solar winds and can be visible from earth, especially with telescopes.
Surrounding the comet, millions of miles in diameter is the hydrogen cloud. This is very sparse and cannot be easily seen.
It is expected that in another three hundred orbits or so, Halley will lose its tail and surface completely and become a “dead comet” or “near Earth asteroid” which will eventually either collide with a planet or the sun, or get slung out of the solar-system by a larger planet’s gravity.
Halley’s orbit
Halley’s orbit is around 18 degrees below the plane of the elliptic and takes it out almost as far as Pluto. Currently (early 2004) Halley is just passing Neptune and it is expected to reach the furthermost point in December 2024. Halley’s orbit is retrograde, which means that in comparison to the planets, it is travelling the other way around the sun, the comet is also rotating in the opposite direction to the planets.
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