CAPE CANAVERAL, Fla. (AP) _ Space shuttle astronauts Piers Sellers and Mike Fossum swapped out balky equipment to fix the international space station's rail transporter Saturday and started tests to see whether a station boom can be used to make repairs to a space shuttle.

While initial tests went well for Sellers on their spacewalk, Fossum's turn at the end of the boom was delayed because of a glitch. A tether that connected him to the boom just wouldn't retract. The duo decided to try to ``coil it up'' like a garden hose and tied it up with a metal twistie-tie.

Finally, Fossum got his turn on the 50-foot boom attached to the station's 50-foot robotic arm.

``It's beautiful,'' Fossum said. ``The thin glow of the moonlit Earth below.''

The planned 6 1/2-hour jaunt, which began as the space station passed over Asia 220 miles below, was the first spacewalk for Fossum and the fourth for Sellers, who both arrived at the station on the space shuttle Discovery.

``Enjoy the view, gentlemen!'' NASA communicator Megan McArthur said from Houston.

Both men did. When Mission Control pointed out to Sellers that he could see Britain over his left shoulder, the British-born astronaut said: ``Wow! Oh, my goodness. It's a beautiful day in Ireland.''

As he looked down at the Caspian Sea several minutes later, Fossum said: ``Ha, ha, ha. This is a good view. I'm in a dream; nobody wake me up.''

They had quickly dispatched their first task, immobilizing a cable cutter on the station's mobile transporter, or railroad car, and rerouting a cable. A duplicate cable cutter accidentally cut a cable leading to the transporter late last year, and NASA wanted to make sure it doesn't happen again because the cable is a conduit for power, data and video images.

The transporter moves along the space station and is used for constructing the complex. The severed cable will be replaced during a second spacewalk set for Monday.

In the most daring part of the spacewalk, the astronauts simulated repair-related movements to test the 100-foot long boom-arm combination as a long-planned possible work platform in case emergency repairs are ever needed on a shuttle's heat shield.

While there was swaying and oscillating during the work, Sellers was able to accomplish all his simulated tasks in the first series of maneuvers. Fossum's turn was delayed by the tether problem, which pushed the astronauts further behind on their already behind-schedule spacewalk.

As he was being moved into position on the long extension, Sellers called the ride ``a very slow gentle sway in and out of the bay.''

And when he got his turn, Fossum described the feeling as ``smooth, very smooth.''

Lead spacewalk officer Tomas Gonzalez-Torres described the expected effect as similar to trying to paint a house from the top of a rickety ladder.

While waiting for Fossum to finish, Sellers did a couple of flips, and the two let out frequent ``woo-hoos.''

The technique was developed to make sure there is never a repeat of the Columbia disaster, which killed seven astronauts in 2003. Foam from the shuttle's external tank struck Columbia's wing during launch, creating a breach that allowed fiery gases to penetrate the shuttle during the return flight to Earth.

Fossum and Sellers may get a chance to use the boom for a real repair on their third spacewalk, now scheduled for next Wednesday. NASA managers are evaluating whether a piece of fabric filler protruding from the thermal tiles on Discovery's belly needs to be removed by the spacewalkers.

Two pieces of gap filler had to be removed from Discovery's belly during a spacewalk last year because of concerns they would cause problems during re-entry.

In their morning message, flight controllers told the Discovery crew that they wanted to take additional pictures of slightly damaged thermal blankets using a camera on the space station.

NASA managers do not think two of the blankets pose any problems but want to make sure the other two small blankets don't tear off during re-entry into Earth's atmosphere. The thermal blankets are used to protect the shuttle against searing heat during ascent and descent.

The flight controllers also had a request for the shuttle crew: Stop pouring unused drinks down the shuttle's toilet. ``An example of how closely Big Brother watches,'' they wrote.

The space shuttle crew awoke Saturday to ``God of Wonders,'' a popular Christian music recording chosen by Fossum's family.

``I do think it's particularly appropriate as I prepare to step outside for about 4 1/2-trips around this chunk of creation we call Earth,'' Fossum radioed Houston.

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