Franklin Chang-Diaz and Philippe Perrin chat with CBS' Peter King and William Harwood during an interview Friday. Photo: NASA TV/Spaceflight Now

"We still wake up with smiles on our faces even though we're tired," he said, floating in the station's Quest airlock module with fellow spacewalker Philippe Perrin. "It was a beautiful experience, it was for me, overcoming, especially the first EVA we did, when I was riding on the arm and got to see just a huge panorama of the world. It was to me an overcoming experience, I could hardly contain the emotions."

During their third and final spacewalk Thursday, Chang-Diaz and Perrin replaced the wrist joint in the station's $600 million robot arm, a critical piece of surgery that had to go well to permit continued assembly of the orbital outpost.

"It was a little bit challenging," Perrin told CBS Radio today. "We had to deal with things we did not expect. It was a long EVA, it took more than seven hours, which is a long, long time to spend in the suit, very tiring. So it was quite challenging.

"We could have expected better, but we also could have expected much worse, because we were doing major surgery on the arm. After you take the arm apart, you better (be able to) put it together, otherwise the station could not continue the assembly sequence."

At one point during one of his three spacewalks, Chang-Diaz told mission controllers it was difficult to find his way about in orbital darkness.

"I found that to be one of the biggest learning components that is not really reproduced well in the training," he said. "That is, the station is such a large spacecraft with so many appendages and directions that you have to think in a three-dimensional sense without reference to gravity at all.

"When we train in the water, we always have (an awareness of) the direction of gravity. It is impossible to eliminate that from your senses," he said. "And here, that does not exist anymore, so you have to rely entirely on visual cues. And of course when it's dark, you have very limited visibility. ... So it is easy to become disoriented and we take great pains to understand the road and have a clear roadmap to go from point A to point B."

During Endeavour's return to Earth Monday, Perrin will be strapped in on the shuttle's lower deck, along with the station's returning crew members. Expedition 5 commander Yuri Onufrienko, Daniel Bursch and Carl Walz were launched to the station Dec. 5. When they land Monday, they will set a new U.S. endurance record of 194 days.

All three will make the return to Earth resting on their backs in cushioned recumbent seats. Perrin will be sitting with them on the lower deck to provide assistance if needed.

"My duty there is to make them as comfortable as we can before we enter the atmosphere and then as soon as we land and the wheels stop, I need to be able to help them if they don't feel well, or give them some water, very basic things," Perrin said. "My role would be very important in case of an emergency. If we had to, for some reason, bail out of the orbiter or do anything fancy, I would have to take them out of the orbiter because they would not be able to walk out."

NASA managers currently are re-assessing the long-range goals of the space station project. Plans to increase crew size to six or seven, which would include full-time fliers from other partner nations, are on hold pending completion of budget and technical reviews.

For his part, Chang-Diaz said it is "essential" to get astronauts representing the station's partner nations on board at some point.

"The space program has become a world wide activity and the merging of all these nations together, I think, in a very clear way contributes to peace and the understanding of nations.

"We have brought people from many different backgrounds into space and I think the environment they are beginning to see, as they view the whole planet from the vantage of orbital altitude, creates this sense of wholeness and (a) planetary sense rather than a very nationalistic or very narrow view of how society ought to be. So I think the space program brings along with it not only the technical advances but also the social changes that are going to improve the quality of life on the planet."