BRIAN BARNWELL has a nose for historical events. In 1955, he attended the splashy opening of Disneyland in Anaheim, Calif. In 1969, he visited Florida to witness the launching of Apollo 11, which sent men to land on the Moon. And, last week, Mr. Barnwell, a 52-year-old real estate appraiser from Santa Barbara, Calif., took his sons, 14-year-old Brendan and 10-year-old Sean, to Boulder, Colo., for another special moment in the chronicles of American culture — the founding convention of the Mars Society, a group of people who fervently believe it possible to send humans to Mars with existing technology at an affordable cost if only the American people will get behind the idea.