HOUSTON Like pent-up teenagers who could wait no more, Atlantis astronauts Jim Voss and Jeff Williams got a jump Sunday night on an expected 6 hour-spacewalk to make critical repairs to the International Space Station (ISS).

Running more than a half-hour ahead of schedule, the pair began NASAs first spacewalk of the century at 9:48 p.m. EDT Sunday inside Atlantis airlock. There, they switched to battery power on their bulky white spacesuits, officially kicking off the start of their spacewalk.

Jim Voss, left, and Jeff Williams assemble the Russian "Strela" crane before moving it into position on the International Space Station.

The spacewalk came some 21 hours after Atlantis pulled up and docked with the ISS following a two-day chase.

The Spacewalk: What They'll Do

1. Secure a wobbly crane that juts from station 2. Assemble a Russian-made construction crane on side of station 3. Replace a malfunctioning communications antenna 4. Install a cable for a remote camera outside station 5. Attach handrails along station's sides for future spacewalks 6. Take pictures of station

Assisting the men in the round of repairs was astronaut Mary Ellen Weber, a 37-year-old chemist who operated the shuttles 50-foot-long robotic arm from inside Atlantis. The arm helps move the spacewalkers around the cargo bay much like a "cherry picker" does for a telephone repairman.

Webers job was particularly tough because the seven-story station standing on its end in the cargo bay blocked her vision.

"This space station stack is just a couple of feet away from our windows so we have extremely limited views," she said.

As a result, Weber had to rely on camera views and radioed instructions from Voss and Williams to put the arm precisely where the spacewalkers needed it.

Choreographing it all from inside Atlantis was pilot Scott "Doc" Horowitz, a 43-year-old Air Force colonel and test pilot.

Voss, a 51-year-old retired Army colonel and the oldest member of the crew, was making his second spacewalk. He was attired in a plain white spacesuit.

Williams, a 42-year-old Army lieutenant colonel, wore a spacesuit with red stripes on the arms and legs to avoid confusion by mission controllers when the two men were close together.

First high-priority task: Secure a wobbly U.S.-made construction crane that juts from one side of the station.

The crane was attached to the station on a spacewalk done almost a year ago on STS-96 in May 1999 but either was not properly latched at the time or somehow worked itself loose in the interim.

NASA managers need to have the crane locked in place so that the next piece of the station the crucial Russian-built Service Module or Zvezda ("Star) can be safely docked by remote control once the module is launched to the ISS in July.

"We dont want anything happening that might change the center of gravity (of the ISS) or create any leanings we cant control," said Paul Hill, lead flight director for the ISS.

The next task: Finish assembling a Russian-made construction crane called Strela ("Arrow").

Half of the crane was installed on last years spacewalk on the ISS. Voss and Williams were to attach the remaining pieces carried into orbit on Atlantis and then move the entire device to a different side of the ISS.

Jim Voss attaches the Russian "Strela" crane together before moving it to the station.

The third task: Replace a malfunctioning communications antenna one of two on the outside of the station that were installed on the first ISS assembly flight in late 1998. The old antenna, measuring two feet long and 1 foot wide, will be returned to Earth aboard Atlantis.

If enough time remained, Voss and Williams were to install an electrical cable for a remote camera outside the station and attach eight handrails along the sides of the ISS for use on future spacewalks.

One of the last scheduled chores was for the spacewalkers to play orbital tourists, snapping photographs of the ISS so engineers on Earth can study its condition after 17 months in space.

Flight engineers estimated it should take Voss and Williams about 45 minutes to latch the U.S. crane, 1 hour and 45 minutes to assemble and move the Russian crane, 1 hour to replace the communications antenna and 30 minutes to install the camera cable and handrails. The rest of the 6 hours included time to set up and stow their tools and equipment.

"The bottom line is, we want to leave the station in a position where its ready to receive the Service Module," Horowitz said.

The spacewalk was the 85^th in NASA history, the 49^th by a space shuttle crew and the fifth done for the assembly of the ISS. The station is expected to need about 40 more spacewalks before it is completed in late 2005.

Next up for the Atlantis crew will be opening the hatch and entering the station Monday night. Theyll be the first humans inside the ISS in nearly a year.

The astronauts will spend most of the week making emergency electrical repairs and hauling aboard supplies needed for the first resident crew that is scheduled to arrive in November.

That crew, NASA hopes, will kick off what is expected to a permanent human presence in orbit around Earth.

"We are going to have people hundreds of years from now wondering what it was like when we first began exploring space," Weber said. "This mission is one small part of it but its absolutely critical."

Atlantis is scheduled to end its 10-day mission with a landing early on Memorial Day at Floridas Kennedy Space Center.