When Ben finds out the Losties have a sailboat Juliette gave a very reveling response. She said something like, "Who cares! Let them sail in circles."
This would have brought to mind that Desmond could not "sail away" from the island.
Michael was given "very" specific coordinates to get off the island.

I do not think or suggest that the island is in outer space. I think that there is a sort of Black Hole or Worm Hole. (Think Jody Foster in Contact) That has to be entered and exited from perfectly.
This could be why desmond's girlfriend is searching with such high doller equipment insted of "Google Earth!"

Remember only half of my ideas turn out to be total bunk!

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I will tell you the truth at all times, unless I am pulling your leg.