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{ Practical astronomy | Astronomy | Comets }


Comets are small bodies in the solar system. Unlike planets, dwarf planets or asteroids, their orbits are far from circular and often may not even be closed curves. We can observe them around the time when they are closest to the Sun, but any given comet spends most of its time far away and invisibly faint.

"Short-period" comets return regularly at intervals between two or three years at the short end and one or two hundred years at the long end. The most famous comet, 1P/Halley, takes 79 years for one orbit. As many or more comets have very long periods – thousands to millions of years – or may in fact not return at all.

Unlike asteroids, comets consist mostly of ice, albeit with a lot of dust mixed in. When these objects approach the Sun, they warm up, the ice starts to evaporate, dust grains are released, and molecules are released, dissociated and ionised. Comets then develop in general two tails. Sunlight acts upon the dust grains and drives them away from the comet's head in a direction pointing away from the Sun; this forms the curved dust tail. The solar wind of ionised particles acts upon the gas to form the more straight ion tail. Although sunlight moves much faster than the solar wind, its impact on the dust grains is much less. Combined with the comet head's movement around the Sun, this leads to a curvature in the dust tail. The ions from comet and Sun are of similar mass, so that the ion tail is quite fast and hence appears straight.

Comets are notoriously unpredictable in terms of how bright they will be, but also whether they might break up into pieces close to the Sun. This is because so much depends on the unknown detailed structure of the comet and how it will respond to the increased influence from the Sun. So it is possible that a predicted "comet of the century" becomes the flop of the century, while some average comet might develop a sudden burst of activity and brightness.

Although the tails are the hallmark of the comets, the tails exist only while the comet is amongst the inner planets, relatively close to the Sun, and even then it is fainter and harder to photograph than the head.

There are two ways to name a comet; both involve the names of their discoverer(s). An exception is 1P/Halley, whose discoverer is an anonymous cave person, while Halley was the astronomer who predicted it to be periodic. Periodic comets have a number assigned; add the letter P for "periodic" and you get something like 17P/Holmes. One-off comets are not numbered, but they have the time of discovery in the name; instead of a P they have a C. So C/2007 N3 Lulin was discovered by someone named Lulin in 2007, and it was the third comet to be discovered in the 14th (N) fortnight of that year.

C/1995 O1 Hale-Bopp

Comet C/1995 O1 Hale-Bopp 1997-03-28.

Comet Hale-Bopp was an exceptionally bright comet; this did not come as a surprise but had been predicted about two years in advance. In the image the comet has passed through the constellation Cassiopeia (top of the image). Both the blue straight ion tail and the curved white dust tail are visible. The image is affected by sodium-illuminated cloud at the bottom right and by a general background gradient.

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153P/Ikeya-Zhang 2002-03 and 2002-04.

This is a montage of seven images of this bright comet of spring 2002. All images are adjusted to the same brightness representation, all are adjusted to the same scale (~106 pix/° in the full version), and all are rotated to be oriented in equatorial coordinates. Observe how the direction of the tail changes as the comet moves across the sky from the evening West horizon to high declination low above the North horizon, the tail always pointing away from the Sun. Also observe how the balance between the broad, curved dust tail and the thin, straight ion tail changes over time.

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The images were taken (left to right) on the evenings of 2002-03-13, 2002-03-25, 2002-03-29, 2002-04-01, 2002-04-06, 2002-04-07, 2002-04-16. Black corresponds to a surface brightness of −1 MJy/sr, pink to +5 MJy/sr, and white to 50 MJy/sr.

C/2004 Q2 Machholz

Comet C/2004 Q2 Machholz 2005-01-29.

At the end of January 2005, comet Machholz passes the double cluster of χ and h Persei, which is caught just in the bottom right corner. The blue ion tail can be seen emerging to the left from the head (the coma). A second tail points to the bottom left, apparently caused by dust that is lagging behind the comet in its orbit.

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Comet 17P/Holmes 2007-11-11.

In late October 2007, comet Holmes brightens about 500,000-fold from being beyond the reach of amateur astronomers' telescopes to competing with the brighter stars of the night sky – easily visible to the naked eye. From day to day its coma grows, while its total brightness remains more or less constant. Here it is almost the apparent size of the Sun.

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C/2007 N3 Lulin

Comet C/2007 N3 Lulin 2009-03-01.

In February 2009, comet Lulin passes both opposition and closest to the Earth. It passes through the constellation of Leo past Saturn and later Regulus.

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103 P/Hartley 2 2010-10-07.

This periodic comet comes closest to the Sun just outside the Earth's orbit. The 2010 perihelion occurs just as the Earth itself is on that side of the Sun, both helping the apparent brightness of the comet and allowing it to be seen all night. In this image the comet is approaching the open star clusters χ and h Persei. The picture on the left is made from 8 frames taken over a period of somewhat over one hour with the frames aligned on the stars. The comet makes a trail due to its movement during that period. On the right the same frames are aligned on the comet, turning all stars into trails.

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