CAPE CANAVERAL, Fla. -- 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 31/2 years ago.

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SLIDESHOW: Launch Photos

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The shuttle rose from its seaside launch pad through a partly cloudy 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 streaked skyward for any signs of hard foam breaking off the big external fuel tank, the problem that doomed Columbia.

There was no immediate word on any problems with shedding foam before the shuttle separated from its external fuel tank, safely in space. The fuel cells that had forced launch delays earlier this week were working as expected, NASA spokesman Kyle Herring said.

Atlantis carried one of the heaviest payloads ever launched into space -- a 171/2 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.

"In terms of spacewalk tasks, clearly these are the most complicated spacewalk and assembly tasks that ever have been done before," said Wayne Hale, NASA shuttle program manager.

Construction of the space station has been on hold since Columbia disintegrated over Texas, killing seven astronauts on their return to Earth in February 2003. Since then, NASA has struggled to find ways to prevent the breakaway of hard foam chunks like those that had fatally damaged Columbia's wing during liftoff.

The space agency plans 14 more flights besides the Atlantis mission to complete the station before the shuttle fleet is retired in 2010 and NASA turns its attention to flying to the moon and Mars.

Atlantis' mission was supposed to take place in 2003. After training for the flight for a record 41/2 years, the six astronauts had to wait a little longer, when the launch was scrubbed four times over the past two weeks: first because of a lightning bolt that hit the launch pad, then Tropical Storm Ernesto, then a problem with the shuttle's electrical system, and then a faulty fuel gauge.

On Friday, the astronauts were already strapped in and the hatch sealed when NASA scrubbed an hour before the launch time because a sensor in the hydrogen fuel tank gave an abnormal reading as the shuttle was being fueled. That delay cost NASA $616,000.

If the shuttle didn't get off the ground this week, NASA would have had to wait until late September or even late October to try again.

Russia plan to send a Soyuz capsule to the space station on Sept. 18, and it would be too crowded to have both spacecraft there at the same time. The Soyuz will bring two new crew members to the space station and Anousheh Ansari, a Dallas-area entrepreneur who is to become the first female space tourist.

Atlantis' six crew members are commander Brent Jett, pilot Chris Ferguson and mission specialists Joe Tanner, Dan Burbank, Heidemarie Stefanyshyn-Piper and Steve MacLean of the Canadian Space Agency.