Space shuttle Atlantis thundered into orbit Monday with a new part for the international space station, a 44-foot section of latticework that is equipped with a railcar and track.

Atlantis and its crew soared into a clear sky at 4:44 p.m., four days late because of a leaky fuel line on the launch pad that had to be repaired. A last-minute computer problem in the firing room almost forced another delay, but controllers quickly reloaded the system software and launched Atlantis with just 11 seconds to spare.

"You spent a few extra days in Florida, but it's time for you guys to take a ride," launch director Mike Leinbach told the seven astronauts. "So we wish you luck."

"We'll see you back here in about 11 days," replied commander Michael Bloomfield.

Security at the launch site was tight, as it has been since Sept. 11, with fighter jets and attack helicopters on the lookout for intruders in no-fly and no-sail zones. Three small planes violated the air space and were escorted down by F-15s; a ship was also chased away.

A new NASA security policy kept the astronauts' whereabouts under wraps until they arrived at the pad. Only then was a video of the crew's breakfast, suit-up and departure for the pad — normally seen live — broadcast by the space agency.

"We are living in a different world post-September 11th and that means we ought to always be diligent about what we're doing to make this less of an attractive or considered target," NASA Administrator Sean O'Keefe told reporters after liftoff.

The space station and its three-man crew were flying over the Atlantic, due east of Newfoundland, when Atlantis took off. The shuttle should arrive Wednesday, and a day later the astronauts will begin installing the girder that will serve as a backbone for the space station.

O'Keefe, accompanied by Interior Secretary Gale Norton and a few other high-level Washington guests, congratulated controllers for beating the end of the five-minute launch window.

"Shoot, you had 11 seconds to spare. Another day at your office, right? No big deal," O'Keefe joked.

Astronaut Jerry Ross made history at liftoff, becoming the first person to rocket away from Earth seven times. He also happens to be NASA's most experienced spacewalker ever, having performed seven in his 22 years as an astronaut.

When the 54-year-old retired Air Force colonel ventures outside this weekend with Lee Morin, a 49-year-old Navy doctor, the two will become the first pair of grandfathers to take a spacewalk. Their nickname: the Silver Team.

In all, the shuttle astronauts will perform four spacewalks to install the girder being delivered by Atlantis and perform other exterior work.

This is no ordinary aluminum beam. The $600 million truss has four computers and more than 10 miles of wiring and 664 feet of fluid lines — 475,000 parts in all. It weighs nearly 27,000 pounds.

A $190 million railcar and track are attached to the girder. The space station's mobile robot arm eventually will hop on the railcar and slowly move from one end of the complex to the other, hauling cargo and attaching new parts.

Over the next few years, NASA will add additional girders. The entire framework eventually will exceed 350 feet and support a suite of solar panels and radiators needed for additional space station laboratories.

Atlantis' astronauts will be the first visitors for the space station crew, four months into a six-month stay. The shuttle flight will last a scant 11 days.

Monday's launch marked the debut of a new type of main engine that features stronger and safer high-pressure fuel turbopumps, thanks to the elimination of welds. This was the first time that a full cluster of these new engines was used to propel a shuttle.