Atlantis pulls away from the International Space Station as seen from camera mounted on Zarya module. Photo: NASA TV/Spaceflight Now

NASA managers praised the Atlantis astronauts Friday for pulling off a complex overhaul to restore the international space station to good health. Even so, James van Laak, manager of space station integration and operations, took a moment to point out that not every station assembly mission will be as trouble free as this one, saying "it would be perhaps inaccurate or dishonest, even, to not make it clear that we are undertaking one of the greatest engineering challenges humankind has ever undertaken.

"And I guarantee you over the next few years we will stumble and scrape our knees a few times and I wouldn't be human if I didn't admit to some concern about that," he said. "But we have great confidence we can do this safely, we're not going to hurt people and we don't believe we're going to do damage to the hardware. We're just going to occasionally stub our toes and have to take a step backward and address those problems."

Atlantis undocked from the space station at 7:03 p.m. EDT (2303 GMT) on Friday evening. After looping around below the lab complex, pilot Scott Horowitz fired the shuttle's maneuvering jets to leave the station behine, setting the stage for a major milestone in July: Arrival of a new Russian command center called the service module.

"This mission was so successful because the two vehicles performed so well and the teams the planned, designed and trained and then executed the mission performed so well," said Jeff Bantle, a mission operations representative at the Johnson Space Center.

A view of the International Space Station as Atlantis completes partial flyaround. Photo: NASA TV/Spaceflight Now

"We really gave the space station a major overhaul. We replaced more than half of the battery storage capability. All six battery sets are working very well and I think the ISS is ready for what I consider a very key event, this rendezvous with the next module. That's going to be probably the most important thing the (Russian Zarya module) needs to do. So I think we put it in great shape for that."

The service module, which will take over propulsion chores from Zarya while providing crew quarters and additional electrical power, is scheduled for launch atop a Proton rocket around July 12. Once in space, the service module will serve as a target for the Zarya module, which will carry out a remotely controlled rendezvous and docking using a Russian-built guidance system. Over the next few days, Russian flight controllers plan to put the docking system through a series of tests to verify its readiness for the main event. The system has generated suspect data on one channel in recent weeks and engineers want to make sure they fully understand its performance before the service module's launch.

"We're wrapping up the docked portion of an absolutely outstanding mission," van Laak said. "All of our mission objectives have been accommplished, we've left the space station in fantastic mechanical condition, ready to proceed to the docking of the service module in July. All the maintenance items have been done, the air quality problems we might have had on the last mission have been absolutely corrected and there are no concerns whatever about this crew or any other crew visiting the station. Basically we're ready to go and very, very pleased."