Florida ? Two NASA astronauts aboard the international space station ventured outside for nearly eight 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.