ધૂમકેતુ
બાહ્ય લાક્ષણીકતાઓનુ અધ્યયન
આઈઝ॓ક ન્યૂટન described ધૂમકેતુન્॓ as compact, ધન, fixed, અન્॓ durable bodies: in one word, a kind of planets, which move in very oblique ભ્રમણકક્ષા, every way, સંપૂર્ણ આઝાદીથી, persevering in their motions even against the course and direction of the planets; and their પૂંછ as a very thin, slender vapour, emitted by the head, or nucleus of the ધૂમકેતુ, ignited or heated by the સૂર્ય. ધૂમકેતુઓ also seemed to Newton absolutely requisite for the conservation of the પાણી તથા ભેજ of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the ધૂમકેતુs. Another use which he conjectured ધૂમકેતુs might be designed to serve, is that of recruiting the સૂર્ય with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary — As early as the 18th century, some scientists had made correct hypotheses as to ધૂમકેતુs' physical composition. In 1755, Immanuel Kant hypothesized that ધૂમકેતુs 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 in the 1835 apparition of ધૂમકેતુ Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a ધૂમકેતુ's ભ્રમણકક્ષા and argued that the non-gravitational movements of ધૂમકેતુ Encke resulted from this mechanism. However, another ધૂમકેતુ-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the ભ્રમણકક્ષા of the Perseid meteors, and based on ભ્રમણકક્ષાal similarities, correctly hypothesized that the Perseids were fragments of ધૂમકેતુ Swift-Tuttle. The link between ધૂમકેતુs and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the ભ્રમણકક્ષા of ધૂમકેતુ Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.A "gravel bank" model of ધૂમકેતુ structure arose, according to which ધૂમકેતુs consist of loose piles of small rocky objects, coated with an icy layer. By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, ધૂમકેતુs were icy objects containing some ધૂળ and rock.This "dirty બરફball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's ધૂમકેતુ in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of ધૂમકેતુ Borrelly on September 21 2001 and confirmed that the લાક્ષણીકતાઓ of ધૂમકેતુ Halley are common on other ધૂમકેતુs as well. Forthcoming space missions will add greater deપૂંછ to our understanding of what ધૂમકેતુs are made of. The Starધૂળ spacecraft, launched in February 1999, has already collected particles from the coma of ધૂમકેતુ Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. In 2005, the Deep Impact probe will blast a crater on ધૂમકેતુ Tempel 1 to study its interior. And in 2014, the European Rosetta space probe will ભ્રમણકક્ષા ધૂમકેતુ ધૂમકેતુ Churyumov-Gerasimenko and place a small lander on its surface.