Two spacewalking astronauts supplied the international space station with a fresh tank of nitrogen gas Wednesday, one of them a German who was too sick to venture outside a few days earlier.

It was a chance for Hans Schlegel to prove himself.

Never before in 27 years of space shuttle history was an astronaut replaced on a spacewalk and then given a second chance.

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Looking and sounding fit, Schlegel and Rex Walheim completed their primary job halfway through the planned 6½-hour spacewalk: removing a depleted nitrogen tank from the space station and installing a full one weighing 550 pounds.

The high-pressure nitrogen gas is needed to flush ammonia through the station's cooling lines.

As soon as the new tank of nitrogen was powered up, Mission Control radioed up the news along with congratulations for the spacewalkers' "great work."

"Hot dog!" said Walheim.

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The spacewalk began, coincidentally, as the linked shuttle Atlantis and the station soared more than 200 miles above Cologne, Germany.

"Hello to all the people of Germany," Walheim said. "What a pleasure it is to be up here spacewalking with one of your native sons, Hans Schlegel."

Noted Schlegel: "It's great to be a part of an international team ... doing research in space."

It was the first spacewalk ever for the 56-year-old Schlegel.

He was supposed to go out on the first spacewalk of the mission to help hook up Europe's space station lab, Columbus, which was ferried up by Atlantis.

But he became ill after reaching orbit last week. Installation was delayed by one day to Monday, and he was replaced by a U.S. colleague.

Neither Schlegel nor anyone else at the European Space Agency or NASA will say what was wrong with him. Schlegel has said it's a private medical matter.

His crewmates offered encouragement as the spacewalk passed the five-hour mark and Schlegel struggled to put covers on the Columbus lab's protruding pins.

"Hey, Hans, you're doing great work. Take your time and pace yourself," shuttle pilot Alan Poindexter radioed from inside.

Once Schlegel completed the chore, one of the crew called out to him: "Outstanding work on Columbus today, thanks a lot ... and it's great to see you outside my friend."

Replied Schlegel: "It's a pleasure to work on it."

The crew began preparing for the spacewalk after waking up to Jimmy Buffett's "Oysters and Pearls," which opens with lyrics about Charles Lindbergh's historic solo non-stop flight across the Atlantic.

It was played for Poindexter, who coordinated Wednesday's spacewalk from inside the station.

Schlegel and the nine other space travelers spent Tuesday opening up the $2 billion Columbus lab and getting its equipment running. They were continuing that work Wednesday.

The astronauts — wearing goggles to guard against floating dust, metal chips or other debris — opened up floor panels to get to the equipment underneath, and turned on computers, heaters and fans.

A few of the systems had startup trouble, but that's normal for a brand new piece of hardware, NASA managers said.

The European space station program manager, Alan Thirkettle, said it was thrilling to see astronauts inside Columbus, which was immaculate and brilliantly white.

"They're doing the first thing that the crew does, which is to make a complete mess of what was a beautiful piece of clean hardware inside," Thirkettle joked.

One more spacewalk is planned for Atlantis' space station visit.

On Friday, Walheim and Stanley Love — Schlegel's replacement on Monday — will hang scientific experiments on the outside of Columbus.

Atlantis is supposed to undock from the space station on Sunday, but may stay a day longer so the seven shuttle astronauts can help with Columbus' setup.