Comets

"There's something out there in the depths of space!
Something that is starting a journey towards the inner solar system.
Moving slowly at first, but increasing little by little until its fiery passage through the heavens make is visible to us here on our tiny planet."

Comets. Believe me, there are loads of them out there. They are one of the most numerous and unpredictable of all the bodies zooming through space. Hale-Bopp was one of the finest comet in recent times - it was clearly visible in the evening skies during April and May 1997 (its picture is further down the page a bit).

Of course the most famous is Halley's Comet, named after the 17th century astronomer Sir Edmund Halley. No other comet has been seen by so many people throughout history and the world - it was first recorded in 467 BC. Probably it's most remembered return was during the Battle of Hastings when the comet that was woven into the famous Bayeux tapestry commemorating William of Normandy's invasion of 1066. Bad news for King Harold whose last words were reportedly, "I think I've got something in my eye."

Comets are regular visitors to our skies, they have been chronicled through the ages by all cultures, but even the greatest thinkers had no idea what they were. Aristotle claimed they were hot dry out pourings from the ground which were carried across the sky. Simply when they got hot they caught fire, rapid burning leading to shooting stars or slow burning leading to comets. Very nice, but very wrong. Even Galileo's theory was no better - he thought they were caused by sunlight refracting through the Earth's atmosphere.

For the correct starting point on the comet trail we have to go back to Edmund Halley. His interest in comets started after seeing one in 1678. So off he went on his cometary trail of finding out all he could about their recorded appearances from writings around the world. Sir Isaac Newton's principles of gravitation, that's him with the apple, had just been published, which Halley used for his research. Before long he found that certain comets seemed to have the same orbit and the dates when they became visible were 76 years apart. Could these separate comet observations in fact be of the same comet? Halley thought so and he predicted this comet would reappear in 1758. It did, and the rest as they say is history.

From the work of Halley we learned that some comets moved in orbits like the planets, but it wasn't until the 1950's when we finally had an idea as to what a comet was made of. Astronomer of the time Fred Whipple put forward a theory that the nucleus was no more than a 'dirty snowball' about 10 kilometres in diameter. This wasn't universally accepted, but the return of that Halley's Comet in 1986 cleared everything up once and for all. For during its return a fleet of spacecraft was sent to intercept the comet at various distances. The closest approach was made by the European Space Agency's (ESA) Giotto space probe, which passed the nucleus at only six hundred kilometres on 14th march 1986. Giotto confirmed the snowball theory even though Halley turned out to look more like a large uneven potato than a round shape that had been expected.

As a cometary snowball approaches the sun the outer layers of ice from the nucleus vaporise dislodging the dust which provides material for the comet's coma - that's the halo of gas forming the bright head of the comet. Sunlight then pushes this halo into the bright dust tail that we can see. Much fainter is the blue gas or plasma tail, which is caused by the magnetic fields of the Sun's solar wind (a plasma flowing from the Sun at speeds of 400 to 720 kilometres per second). Bits of this dust blasted away from the comet can come into contact with the Earth's atmosphere where they become shooting stars.

Comets are believed to be remnants from the formation of our solar system. When the Sun switched on it blew all the light material to far beyond the orbit of the last planet Pluto into a halo known as the Oort Cloud, or so another theory goes. Here lie over 100 billion comets each waiting for a nudge by gravity which will send it falling toward the sun. After a trip reaching speeds of up to one million miles per hour, as it whips around the Sun, it may be gravitationally influenced again to become a periodic comet that returns to our skies at regular dates. On the other hand it may fly off back into the deep space to rejoin its friends in the Oort cloud.

© Anton Vamplew 2008