Between the Moon and the Sun: The Art of Knowing What is Possible

Not long ago, I asked this guy who knew a great deal about power plants whether "clean coal" was possible. He reminded me that once, a trip to the Moon was science fiction and now it's history.
Perhaps clean coal technology, today's science fiction, will be tomorrow's boon. Perhaps not.

Okay, so a manned voyage to the Moon was a long shot, but it was obviously doable, but that doesn't mean everything is possible. How about sending people on a trip to the Sun? I'm going to bet that a trip to the Sun is humanly impossible. Physics--let alone economics--is just against it. When you dream big, how do you know if your shooting for the Moon or flying into the Sun?