The comet Hale-Bopp with two tails. Photograph taken in April 1997.
This composite image of Tempel 1 was built up from scaling all images to 5 meters/pixel, and aligning images to fixed points.
In 1986, Giotto's encounter with Comet Halley provided the first ever opportunity to take images of a comet nucleus. The images were obtained with the Halley Multicolour Camera on Giotto.
At home in the distant reaches of the Solar System, comets are fragile, irregularly shaped chunks of dust and ice harbouring organic molecules with diameters ranging from several hundred metres to a few tens of kilometres. Located far from the Sun, moving in a slow orbit, comets remain bitterly cold and are hence able to preserve volatile constituents in the form of ice over extremely long periods of time, meaning they possess information on the entropic states of the early Solar System, which adds to their interest within the scientific community.
Missions to space and ground based observations have shown that there is no such thing as 'the' comet; instead, we find porous, low-density bodies displaying a wide range of physical and chemical properties without a homogeneous composition.
Even despite the massive advances in scientific understanding in recent years, important questions remain to be clarified or fully answered, for instance the importance of comets in the emergence of planetary systems and life itself. After all, the number of comets deflected toward the Sun shortly after the formation of the planetary system were significantly greater, leading to more frequent collisions with the terrestrial planets and the Moon. It seems probable that comets impacting Earth brought with them essential building blocks for life, among them water and organic molecules, thus influencing the emergence of a biosphere.
Where do comets come from?
The largest reservoir of comets is found in the Oort Cloud, spherically enclosing the Solar System at a distance of up to 100,000 Astronomical Units (15 trillion kilometres) and most likely containing over a trillion objects.
But just beyond the orbit of Neptune there is a second, significantly smaller comet reservoir, stretching to around 50 Astronomical Units from the Sun. The so-called Transneptunian objects (TNOs) found there form a scattered plane of far-flung, diminutive planetary objects, although they do include larger bodies akin to planets in the Kuiper-Edgeworth Belt: among them is Pluto with its moon Charon and celestial bodies located farther afield such as the dwarf planet Eris, discovered in 2003. Sized at over 1000 kilometres in diameter, they bear scant resemblance to the 'traditional' image of a comet and are therefore known as 'plutonides'. There are probably several billion TNOs in total, of which only around 1200 have been identified. And whereas the asteroid belt strung out between Mars and Jupiter principally contains chunks of rock and metal, TNOs mainly consist of volatile substances such as water ice, carbon dioxide, methane and ammonia.
Where does the comet's tail come from?
Even slight disturbances to the trajectory, caused by the gravitational pull by planets, may deflect comets from their path. As they approach the Sun, the frozen constituents gradually melt and escape into space, releasing dust from the surface as they flow. This so-called cometary activity produces a hazily diffuse, extremely thin comet atmosphere with a diameter ranging between 10 to 100,000 kilometres around the small nucleus – the coma. Interacting with solar radiation and winds, it creates the impressive sweep of dust and plasma that at times may stretch for millions of kilometres. Occasionally they are visible to the naked eye as a magnificent celestial display.
Last modified:20/01/2014 10:52:58