Comets have always been special. In ancient times, the sky was a stately ordered place, with the Sun, Moon, Stars and Planets all moving in their stately and predictable paths. But every now and then, something strange and new would appear. It would start as a fuzzy star with a misty trail behind it, growing from week to week until it formed a spectacular arc across the sky before fading away and vanishing. This was not a part of the normal pattern, and was seen as a terrifying and awesome omen. To many cultures, the tail of a comet looked like a head of long hair streaming in the wind, and so English speakers refer to this sight as a Comet (from the greek aster kometes – “long-haired star”). Many people think that the star of Bethlehem may have been a comet, although the evil connotations held by comets make it unlikely to have been interpreted as signifying a birth!
These great, bright, obvious comets are quite rare, though – perhaps a few per century. If they were more common, they might not have held such significance for the ancient astrologers. But fainter, less obvious comets are far more common. Since the invention of the telescope, and as optical technology improves, the number of comets discovered each year keeps going up. There is usually at least one comet in the sky visible to amateur equipment at all times and the SOHO orbital space telescope has discovered 125 new comets per year since it was launched 16 years ago!
So what are they? For decades, comets were affectionately described as ‘dirty snowballs’, although that’s something of an oversimplification. The nucleus of the comet is composed mostly of various ices and dirt, but most of that dirt is made up of carbon compounds: everything from soot to cyanide to tar. They’re more oily than dirty, and almost pitch black in colour. They’re also quite small, ranging from about a hundred kilometers across down to only a few tens of meters, meaning that they should be invisible from earth. So how do they manage to put on such impressive shows?
The secret lies in their icy nature. Much of a comets mass comes from frozen water and carbon dioxide. These two different ices are frozen quite solid for most of a comet’s orbit, but as it approaches the Sun it starts to warm up. First the Carbon Dioxide ice (commonly known as Dry Ice) starts melting, then the water ice. In the vacuum of space, water boils at freezing point, so both types of ice bypass the liquid phase and sublimate straight into their gaseous forms. This gas blows off, and forms a very thin atmosphere around the comet, which we call the coma. As the comet gets nearer to the Sun, and the temperature rises, more and more ice melts causing the coma to get larger and larger. In some comets, the coma can be over a million kilometers across — that’s bigger than all the planets put together, and almost as large as the Sun. Although the coma is extremely thin and sparse (it’s not even visible in photographs of the nucleus taken by robotic spacecraft which have visited comets), their sheer size lets them reflect enough sunlight to become visible from Earth. Incidentally, it is this gaseous nature, reliant on outgassing from melting ice, which makes it so hard to predict a comet’s brightness in advance. Astronomers learned this the hard way when Halley’s Comet returned in 1986 and failed to match up to the spectacular expectations which had built up from its impressive performance in 1910.
As the comet comes in still closer, the Sun starts having a second effect on the comet. The solar wind, a stream of charged particles constantly blasting out from the solar surface, begins to push and distort the comet. The closer the comet gets to the Sun, the stronger this distortion grows until eventually the coma streams out away from the Sun into a spectacular tail, giving the familiar swooshing comet shape. Sometimes the coma is filled with fine dust, blown up by ice outgassing from beneath the comet’s surface. The dust and gas, having different masses, are affected differently by the solar wind, so that a comet’s tail sometimes splits into two: one of gas, one of dust.
At some point, the comet will reach its closest point to the Sun, the point in its orbit called perihelion. It then heads back out into deep space, cooling down and fading away. Some large comets have managed to repeat this cycle for centuries (Halley’s Comet being the most famous example), while others swing so close to the Sun that they boil away entirely on a single pass.
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