After two frustrating weeks of delays, space shuttle Atlantis and its six astronauts blasted off Saturday on a mission to resume construction of the international space station for the first time since the Columbia disaster 3 1/2 years ago. The shuttle rose from its seaside launch pad through a partly cloudy sky at 11:15 a.m (1515 GMT). "By our count it has been almost four years, two return to flight missions, a tremendous amount of work by thousands of individuals," said Brent Jett, Atlantis' commander shortly before liftoff. "We're confident that in the next few weeks, and the next few years for that matter, NASA is going to prove to our nation and our friends ... that it was worth the wait and we're ready to get to work." On the ground, NASA kept an eye on several cameras zoomed in on Atlantis as the spaceship streaked skyward for any signs of hard foam breaking off the big external fuel tank, the problem that doomed Columbia.