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 clear blue sky at 11:15 a.m.

“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 as it streaked skyward for any signs of hard foam breaking off the big external fuel tank, the problem that doomed Columbia.

Atlantis carried one of the heaviest payloads ever launched into space — a 17 1/2 ton truss section that will be added to the half-built space station. It includes two solar arrays that will produce electricity for the orbiting outpost.

The astronauts will make three spacewalks during the 11-day flight to install the $372 million addition.