With the completion of the fourth and final spacewalk of Atlantis' mission, NASA managers today declared the new $790 million S0 truss mounted atop the international space station fully operational. While attention today was focused on spacewalkers Jerry Ross and Lee Morin, engineers at the Johnson Space Center successfully activated a final set of critical internal systems that eventually will route power to the station from huge solar arrays mounted on the ends of the completed truss.

"We fully activated and checked out all of our electrical distribution equipment," said Ben Sellari, NASA's S0 launch package manager. "We also verified a very important new feature that S0 adds to the power distribution system, which is what we call a hot crosstie. We are able to take other primary and secondary channels and cross over between those in the event we have certain power losses. So it gives us some additional flexibility and redundancy in the future to do power management and respond to failures."

Summarizing the testing to date, Sellari said "all of the S0 systems have been working essentially flawlessly. I mean, we haven't seen - and I'm almost afraid to say this because we might jinx ourselves - we haven't seen a single heater problem, we haven't seen a single unexpected or anomalous annunciation. The hardware and the software has been working very, very well."

Lead spacewalk planner Dina Barclay was equally pleased, calling today's spacewalk the final chapter in "an extraordinary series of spacewalks that brought our newest element of the international space station to life."

"Jerry and Lee, our grandfather duo, came inside having rounded out an incredible set of spacewalks," she said. "Today the crew far exceeded our expectations and we were even able to complete an objective that had been previously deleted and another one that was learned of post launch."

Repressurization of the space station's Quest airlock module began at 5:06 p.m., officially ending a six-hour 37-minute spacewalk.

For the statistically minded, this was the 38th spacewalk devoted to space station assembly since December 1998. Thirty NASA astronauts, one Canadian and five Russian cosmonauts have now logged 236 hours and 27 minutes of spacewalk time building the international outpost. Of that total, 13 spacewalks totaling 69 hours and 12 minutes were logged by astronauts using space station airlocks. The rest were staged from the airlocks of visiting shuttles. The four spacewalks carried out during Atlantis' mission added up to 28 hours and 22 minutes.

Jerry Ross, veteran of a world-record seven space flights, now holds the U.S. record for spacewalks with nine excursions totaling 58 hours and 18 minutes. Astronauts Steven Smith, also aboard Atlantis, is in second place with 49 hours and 48 minutes of spacewalk time. First place belongs to cosmonaut Anatoly Solovyov, who has logged 77 hours and 41 minutes of EVA time in 16 spacewalks.

The only problems today involved a gas analyzer that appeared to stop working shortly after its installation and problems removing a stuck bolt from a cable cutter. But these were minor issues in a day that saw the astronauts tighten or loosen another 59 bolts and nine cable connections.

"With these four spacewalks behind us, for our team it's mission accomplished," Barclay said. "We're really proud of what our crew and team have accomplished."

Going into mission STS-110, the astronauts and flight controllers faced three tiers of mission requirements: 10 category 1 "mandatory" requirements, eight category 2 requirements for continuing station assembly and 31 tasks in category 3 that were required to assist future assembly crews. Sellari said that as of today, only three items have yet to be accomplished: The transfer or more fresh water, nitrogen and oxygen to the station. By the time Atlantis undocks Wednesday, however, the crew should be 100 percent complete.

"This has been an extraordinary mission in a lot of ways," Sellari said. "We seem to make the missions (look) very easy. But I can assure you, nobody who has followed this flight very closely believes this has been an easy mission. The proficiency by which the element teams, our contractor teams, our operations teams and our flight crew teams perform their operations are both a blessing and a curse. It's a blessing in that we try to accomplish a high percentage of our objectives, it's a curse because getting that high efficiency and having a lot of success makes it look very easy. And I can assure you this was not an easy flight.

"We've had an extraordinarily successful mission," he summarized, "we are very, very proud of the hardware on board station. S0 is where it belongs, it's in space, it's an integral part of international space station, it is poised with its new capabilities to allow the space station now to expand its capabilities, its core systems, to meet its full potential as a world-class on-orbit laboratory."

Aboard the space station, meanwhile, the astronauts took a break early this evening for a final joint dinner before Atlantis undocks Wednesday. Undocking is targeted for 2:31 p.m.