<P> Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun . Newton suspected that comets were the origin of the life - supporting component of air . </P> <P> As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition . In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion . In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon . </P> <P> In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock . This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material . </P> <P> On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt . The detection was made by using the far - infrared abilities of the Herschel Space Observatory . 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 ." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter / Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H 2CO, and dust inside the comae of comets C / 2012 F6 (Lemmon) and C / 2012 S1 (ISON). </P>

Where does the ice in comets come from