Endeavour lifts off from the Kennedy Space Center, bound for the International Space Station. Photo: NASA/KSC.

The shuttle Endeavour rocketed into orbit this afternoon, hot on the trail of the international space station for a critical crew exchange and three spacewalks next week to upgrade the lab's robot arm and to fix what amounts to a broken wrist.

Under heavy security, Endeavour vaulted away from pad 39A at 5:22:49 p.m., climbing through a hazy sky atop a brilliant jet of 5,000-degree flame from its twin solid-fuel boosters.

The launching was timed to coincide with the moment Earth's rotation carried the launch pad into the plane of the space station's orbit. After wheeling about to line up on the proper trajectory, Endeavour streaked away on a course paralleling the East Coast.

Launch came six days late because of stormy weather and work to replace a nitrogen regulator in the shuttle's left-side rocket pod. While today's launching came under an overcast sky, meteorologists said the clouds were thin and electrically benign, posing no danger to the crew.

At the controls were commander Kenneth Cockrell, pilot Paul Lockhart and flight engineer Franklin Chang-Diaz, only the second person in history to make seven space flights.

Their crewmates are French astronaut Philippe Perrin and the next crew of the international space station, Expedition 5 commander Valeri Korzun, a veteran of the Mir space station, NASA biochemist Peggy Whitson and rookie cosmonaut Sergei Treschev.

Korzun and company will replace the space station's current occupants, Expedition 4 commander Yuri Onufrienko, Daniel Bursch and Carl Walz, who were launched to the lab complex Dec. 5.

Had Endeavour taken off on its first attempt last Thursday, Onufrienko and his crewmates would have landed June 11 to close out a voyage lasting 187 days and 19 hours. That would have been just a few hours shy of Shannon Lucid's U.S. space endurance record of 188.4 days, set in 1996 aboard the Russian Mir space station.

Thanks to Endeavour's launch delays, Bursch and Walz are now scheduled to land June 17. In so doing, they will set a new U.S. record of 193 days 17 hours and 30 minutes in space.

Korzun's crew plans to spend about 135 days aboard the space station before returning to Earth in October, replaced by yet another three-person crew. The station has been continuously staffed since Nov. 2, 580 straight days as of today.

"I think we've trained well enough and long enough together that I don't anticipate we'll have any problems," Whitson said. "We have a good mix of personalities. Valeri's pretty outgoing and quite the joke teller and story teller, so he entertains us quite a bit. Sergei and I, we can add our own entertainment at times, so we're looking forward to working together."

If all goes well, Cockrell will guide Endeavour to a docking with the space station just after noon on Friday.

The primary goal fo the 110th shuttle mission is to replace the Expedition 4 crew and to deliver nearly 5,600 pounds of supplies and equipment, including some 1,500 pounds of scientific equipment.

The astronauts also plan to stage three spacewalks to upgrade and repair the station's Canadian-built robot arm.

The upgrade refers to installation of a $254 million mounting platform that will enable the Canadarm2 space crane to ride a motorized cart back and forth along a huge truss that eventually will extend the length of a football field.

The cart is mounted on rails attached to the forward face of the only truss element currently in orbit, a $600 million component that is bolted to the top of the Destiny laboratory module.

The seven-joint Canadarm2 currently is attached to the other side of the lab module. It is not long enough to attach any of the outboard truss segments and for station assembly to continue, it must be attached to the truss rail car.

The mobile base system, or MBS, mounting platform is designed to be bolted to the top of the mobile transporter cart. It features four robot arm attachment points, each one of them capable of providing power and relaying data, including video from the arm's cameras, back to operators inside the station.

Walz and Whitson will use Canadarm2 to lock onto the MBS Sunday, near the end of the first spacewalk by Perrin and Chang-Diaz, and pull it from Endeavour's cargo bay. The next day, they will use the arm to attach the MBS to the mobile transporter cart, activating a large mechanical claw to temporarily hold it in place.

Then on Tuesday, Chang-Diaz and Perrin will complete the attachment task during a second spacewalk, bolting the MBS down, hooking up two power cables and six video and data lines.

That will set the stage for the mission's third and final spacewalk next Thursday when Chang-Diaz and Perrin replace the arm's wrist-roll joint.

Shortly after the space station remote manipulator system - SSRMS - robotic crane was installed last year, engineers discovered a subtle electronic glitch in the redundant circuitry used to control the movement of the arm's wrist-roll joint.

For redundancy, independent electronic control systems are built into each of the arm's seven joints. In one of the two systems that can be used to control the wrist-roll joint, a malfunction can occur that prevents the release of brakes locking the joint in one position.

That, in turn, can trigger an automatic shutdown, preventing controllers from using the arm even with the healthy avionics "string."

After intensive troubleshooting, Canadian software engineers developed a computer programming patch that forces the arm's main computer to ignore the problem in the wrist-roll joint and not to order a shutdown before switching to the healthy avionics string.

Before the most recent shuttle-station assembly mission, another patch was put in place that essentially tells the arm's computer to ignore the joint entirely when working through the faulty avionics string.

The wrist-roll joint works normally with the other control system, but NASA managers want full redundancy to protect against problems down the road. As it now stands, a failure in the healthy control system could shut the arm down, interrupting assembly operations.

"Certainly for normal operations, for any of the build outboard of where we are now, we need a fully functional arm," said shuttle flight director Paul Hill. "The arm we have now is fully functional on (one) string but if we fail this string, that takes us down to either one degraded string or, depending on the failure, two degraded strings."

And so, on March 20, station managers decided to add a third spacewalk to Endeavour's mission so Perrin and Chang-Diaz could replace the faulty wrist-roll joint.

Launch was delayed a month to give engineers time to develop a payload bay mounting fixture for the replacement joint and to give the spacewalkers additional time to train.

"We've got one of those missions that has almost got too much in it to get done," said shuttle commander Kenneth Cockrell. "We're looking forward to a very jam-packed timeline when we get on orbit."