ધૂમકેતુ: આવૃત્તિઓ વચ્ચેનો તફાવત

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.{{hnote|Whipple (1950)}} This "dirty બરફball" model soon became accepted. It was confirmed when an armada of [[spacecraft]] (including the [[European Space Agency]]'s [[Giotto mission|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)|Starધૂળ spacecraft]], launched in February [[1999]], has already collected particles from the coma of [[81P/Wild|ધૂમકેતુ Wild 2]] in January [[2004]], and will return the samples to Earth in a capsule in [[2006]]. In [[2005]], the [[Deep Impact (space mission)|Deep Impact]] probe will blast a crater on [[9P/Tempel|ધૂમકેતુ Tempel 1]] to study its interior. And in [[2014]], the European [[Rosetta space probe]] will ભ્રમણકક્ષા ધૂમકેતુ [[67P/Churyumov-Gerasimenko|ધૂમકેતુ Churyumov-Gerasimenko]] and place a small lander on its surface.

== પ્રખ્યાત ધૂમકેતુઓ ==

While hundreds of tiny ધૂમકેતુs pass through the inner સૂર્યમંડળ every year, only a very few ધૂમકેતુs make any impact on the general public. About every decade or so, a ધૂમકેતુ will become bright enough to be noticed by a casual observer — such ધૂમકેતુs are often designated [[Great ધૂમકેતુ]]s. In times past, bright ધૂમકેતુs often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's ધૂમકેતુ in [[1910]], the Earth passed through the ધૂમકેતુ's પૂંછ, and erroneous newspaper reports inspired a fear that [[cyanogen]] in the પૂંછ might poison millions, while the appearance of [[ધૂમકેતુ Hale-Bopp]] in [[1997]] triggered the mass suicide of the [[Heaven's Gate (cult)|Heaven's Gate]] cult. To most people, however, a great ધૂમકેતુ is simply a beautiful spectacle.