Cape Canavarel, Fla. ? Two NASA astronauts-turned-electricians ventured outside the international space station Thursday to successfully rewire half of the outpost’s power grid.

Shuttle Discovery spacewalkers Bob Curbeam and Christer Fuglesang deftly unhooked and reattached more than a dozen electrical cables during a five-hour excursion that ended about an hour ahead of schedule.

The work switched two of the station’s four main power circuits from the temporary setup they have been in since the outpost’s infancy to a permanent system.

Another, almost identical spacewalk, scheduled for Saturday, the third of Discovery’s 12-day mission, will reroute power on the station’s two remaining circuits. The task is a crucial one that paves the way for the addition of European and Japanese science laboratories during future shuttle flights.

“It’s not good news, it’s great news,” astronaut Steve Robinson radioed the shuttle and station crews from Mission Control after Thursday’s rewiring was complete. The two electrical circuits “have powered up successfully.”

Mission specialist Christer Fuglesang, of Sweden, works on the international space station Thursday during a spacewalk to rewire the station in this image from NASA television.

Minutes later, the astronauts received more welcome news: The station systems that had been taken offline were being reactivated and a coolant unit for the new power grid was working. There had been concern that a pump needed to begin the flow of ammonia through the coolant system might not start up properly after being dormant for years.