CAPE CANAVERAL, Fla. – Two NASA astronauts aboard the international space station ventured outside for more than seven hours Wednesday to turn on the orbital lab’s permanent cooling system.

Spacewalkers Michael Lopez-Alegria and Sunita Williams disconnected an interim setup that supported the station during its first phase of construction. Then they reattached fluid lines and electrical cables to one of two coolant systems that will service the outpost for its remaining life. The other will be switched over during a spacewalk scheduled for Sunday.

Flight controllers had no problems powering up the new system shortly after the astronauts finished the job.

“You guys have been doing really good work,” a NASA manager radioed from the ground. “I know it’s been hard,” he said, referring to the tight places the astronauts had to work in.

“You’re right about that access,” Lopez-Alegria replied. “It’s pretty abysmal.”

The spacewalk lasted seven hours and 55 minutes. Although the excursion ran up to an hour behind schedule, Williams and Lopez-Alegria were able to complete almost all of the major tasks. The pair also secured a retracted radiator that was used by the old cooling system.

Time constraints didn’t allow the astronauts to finish the final chore of the day, which involved removing fluid lines from an ammonia container that was part of the old system.

That container will be jettisoned from the station later this year.

During the day’s final task, Williams spotted several frozen flakes of toxic ammonia leaving one of the fluid lines. Although the ammonia didn’t come in contact with either astronaut, additional safety procedures lengthened their stay in the station’s airlock after Wednesday’s work.

“We don’t think we have contamination,” Lopez-Alegria told flight controllers, “but that (added precaution) is probably for the lawyers.”

The spacewalk was the first of three scheduled for Williams and Lopez-Alegria during a nine-day period. The next excursion, planned for Sunday, consists of tasks similar to Wednesday’s. A third spacewalk is scheduled for Feb. 8 to remove thermal shrouds from a solar array structural truss.

Lopez-Alegria and Russian cosmonaut Mikhail Tyurin are in the midst of a seven-month stay aboard the station that began Sept. 20. Williams arrived on shuttle Discovery in December to begin a planned six-month stay. Lopez-Alegria will team up with Tyurin to perform yet another spacewalk on Feb. 22 to secure or remove an antenna on a Russian cargo ship that failed to properly retract.