The reason is that this is the consensus mainstream theory – but why do they insist on keeping strictly on song when all the evidence appears to contradict it. Or does it?
Tim Cullen at http://malagabay.wordpress.com/2014/03/19/the-moby-dick-of-astronomy/ … which is a reference to the upcoming Rosetta mission which aims to send a couple of harpoons on to a comet in order to secure a robotic landing craft. Hence, he begins with a great white whale pursued by Captain Ishmael Ahab in a whaling boat and leads seamlessly into Fred Whipple writing a hundred years afterwards (in 1949) and providing mainstream with its consensus theory. Cullen claims it was pulled out of a hat – without any evidence to support it. Whipple claimed comets were made up of a conglomerate of ices (water and gases). See also http://malagabay.wordpress.com/2012/10/10/inventions-and-deceptions-dirt…
Mainstream belief in the comet as a dirty snowball has dominated astronomy for 60 + years, and although it is currently somewhat diluted an actual diry snowball has yet to be encountered. We live in an age of space exploration. In 1986 probes failed to locate surface water on Halley's Comet. In 1994 no volatile gases were observed when Shoemaker-Levy 9 broke apart. In 2000 Comet Linear brok apart and again, no evidence of water, and the same goes for Comet Borelly in 2001. Same again with Comet Wild2 in 2004 – and lastly, Deep Impact only showed a very weak eruption of water vapour (but see also http://malagabay.wordpress.com/2013/02/26/running-hot-and-cold-extraterr…
Tim Cullen says the faithful have their fingers crossed in anticipation of the Rosetta Mission which will rendevous with Comet Churyumov-Gerasimenko in 2015. He ends the piece with Captain Ahab harpooning Moby Dick and getting caught up in the harpoon line, ejected from the boat and carried down into the depths by the great white whale. What will happen when the harpooning of the comet occurs?
Along the way he directs the reader to www.thunderbolts.info/tpod/2006/arch06/060216deepimpact2.htm