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What do comets and buses have in common? Well, you wait for ever, then three come along at once. True or false? Well, certainly false for buses where I live and usually for comets too. But there was a time when it happened to be true. For comets, that is. We're still waiting for the buses.....
That time was in the 1990s. Last century now, it seems a very long time ago. In fact it all happened over just three years of the 1990s. Three years and three comets. Which were they? They were Comets Shoemaker-Levy 9, Hyakutake and Hale Bopp. The title photo for this piece, by the way, is a very youthful Hale Bopp, the year before it graced our skies with unaided eye visibility.
But before looking in detail at these three comets, let's first look briefly at: What is a comet, where do we find them, what do they do and why do they do what they do?
What is a comet?
A dirty snowball. Everyone's probably familiar with that description. But snowballs don't grow tails when they're heated - well, they didn't the last time we had enough snow to make a snowball in South West England, anyway. A comet's nucleus is formed of silicate dust, ice and frozen organic - that's carbon based - compounds. The main difference between a comet and an asteroid, which we always think of as a pretty solid lump of rock, is that cometary material is volatile - that is it turns direct from a solid state to a gaseous state when heated, missing out the liquid stage in the middle - and it's this gas which forms the coma surrounding the nucleus and then the tail of the comet.
Where are comets found?
Well, it's still a bit theoretical, but it is accepted now that most comets originate in what is known as the Oort Cloud. In 1950 Jan Oort came up with the theory that a cloud of comets orbits the Sun at a radius about one thousand times larger than the solar system. He did this by using orbital mechanics to determine the origin of comets before they were perturbed by the influence of the solar system, and theorised that they came from much the same original orbit, and therefore were not just chance visitors to our solar system. There was, however, a flaw in the theory - short period comets. Oort thought that short period comets, that is, those whose orbit around the Sun is quite small, like Comet Halley which does it in 76 years, were just long period comets which had, because of gravitational influences, become trapped in an orbit very close to the Sun. However in the number of orbits it would take for a long period comet to become trapped in this way, most of them would have exhausted their volotile material and would just be inert lumps of dust. No doubt there are some which did take this route, but there are just too many short period comets around. So, in 1951, Gerald Kuiper put forward the theory of a second source - the Kuiper Belt - extending an unknown distance beyond the orbit of Neptune, and this is where the majority of short period comets originate.
It's thought that comets are some of the bits left over from the formation of the solar system, remnants that were never pulled together into the Sun and planets, or perhaps debris from collisions of early, barely formed planetissimals. With their origins in the colder outer reaches of the solar system, comets would be expected to be loosely bundled dust and ice much like the outer, gas giant planets. Orbital mechanics shows the Kuiper Belt to be a disc like the orbits of planets, which fits with the solar system model, while the Oort Cloud, much further out, is just that - a spherical cloud which surrounds the entire solar system.
How do comets get here?
This one's a bit more difficult to explain. It's thought that the inner side of the Kuiper Belt is close enough to the solar system for the outer giant planets to have a gravitational influence on it, and this would pull a few comets into the solar system, or maybe shove a few out into the Oort Cloud. The outer edge of the Oort Cloud is a bit more tricky, but again it has to be a gravitational influence. The 'passing star' theory may seem a bit far fetched, but remember that stars are massive and their gravitation is in proportion, whereas comets are tiny. Remember too that it's probably not just one event that causes a comet to blaze like Hale Bopp in our night sky. A small nudge from its state of equilibrium might take it into the field of influence of a larger comet, perhaps an asteroid, then a planet and finally the Sun. Everything out there attracts - it's how everything from a solar system to a galaxy hangs together.
So why does a comet look like it does?
We think of comets as having tails, but of course when they're out in space they don't. There, a comet is an inert nucleus with no atmsphere. It's only when a comet passes close to the Sun that it changes, because the Sun warms it. This solar warming releases the volatile substances in the nucleus which form the gaseous atmosphere or coma, and it forms mainly on the sunward side because that's where it's hottest. The comet's own gravitational field is too weak to maintain an atmosphere, so it disperses into space.
The larger particles are repelled by solar radiation and drag dust with them to form a dust tail, which points away from the Sun and is lit by the Sun shining through it. The shape, as it appears to us on Earth, depends purely on the geometry of the way we view it. If we view it face on it'll appear as a broad fan, and if we see it edge on it'll be straight and narrow. The curved shape that we sometimes see is determined by the distribution of the size of particles in the dust tail. The smaller and lighter the particle the more it will be repelled by solar radiation and the further from the nucleus it will travel. As the particles slow down, their orbit will lag behind that of the nucleus, and so the tail starts to curve, but we'll only see the curve if we are in the right position.
Sometimes a comet will appear to have more than one tail. Sometimes it will appear to have an 'anti-tail', that is a tail pointing towards the Sun. This is probably just a perspective effect. We might be seeing a very curved tail edge on, so it appears on both sides of the nucleus. Like everything else, cometary nuclii rotate, and this can cause a pulsing effect in the stream of particles, so a tail is rarely one, uniform trail of dust. It can be very uneven and have breaks and twists in it, and also solar radiation is not constant, so the tail can change. Some particles become ionised by the ultra violet light from the Sun and interact with the solar wind to form a gas, ion or plasma tail. These particles are tiny and travel very fast, so the tail they form is straight and emits its own light.
Before we look in detail at our comets a brief word about meteor showers.
Dust from a comet will follow the comet's orbit until it finally disperses into space. The regular meteor showers we see thoughout the year occur when the Earth passes through an active dust stream, the meteors themselves being dust particles entering the atmosphere. Sporadic meteors seen on many nights are particles from dispersed dust. Halley's comet is responsible for both the Eta Aquarid shower in April-May and the Orionids in October. The Perseids in August, a very reliable shower, are remnants of Comet Swift-Tuttle, and the Geminids in December are associated with the asteroid Phaeton, which may be an inert cometary nucleus.