A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger, and if sufficiently bright may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many different cultures. Comets have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space along hyperbolic trajectories. Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Suns light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[1] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[2][3] The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets. As of July 2013 there were 4,894 known comets,[4] and this number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System may number one trillion.[5] Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular.[6] Particularly bright examples are called Great Comets. On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet, Ceres, largest object in the asteroid belt.[7] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[8] The finding is unexpected because comets, not asteroids, are typically considered to sprout jets and plumes. According to one of the scientists, The lines are becoming more and more blurred between comets and asteroids. The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης (wearing long hair), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant long-haired star, comet in Greek. Κομήτης was derived from κομᾶν (to wear the hair long), which was itself derived from κόμη (the hair of the head) and was used to mean the tail of a comet. The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[12] As such, they are popularly described as dirty snowballs after Fred Whipples model.[13] However, some comets may have a higher dust content, leading them to be called icy dirtballs.[14] The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[15][16] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASAs Stardust mission.[17] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[18][19] The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halleys Comet reflects about four percent of the light that falls on it,[20] and Deep Space 1 discovered that Comet Borrellys surface reflects less than 3.0% of the light that falls on it;[20] by comparison, asphalt reflects seven percent of the light that falls on it. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes. The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the coma, and the force exerted on the coma by the Suns radiation pressure and solar wind cause an enormous tail to form pointing away from the Sun.[35] The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun.[36] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[36] Larger dust particles are left along the comets orbital path whereas smaller particles are pushed away from the Sun into the comets tail by light pressure.[37] Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[38] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[39] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[40] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun.[40] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.[40] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[39] Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation.[41] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[42] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[39] In 1996, comets were found to emit X-rays.[43] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, stealing one or more electrons from the atom in a process called charge exchange. This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.
Posted on: Sat, 12 Jul 2014 16:12:53 +0000