Spacewalkers set out for 4th station service call

By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - Two shuttle Atlantis astronauts floated outside the International Space Station on Sunday for a fourth and final spacewalk before the shuttle departs on Tuesday.

Patrick Forrester and Steven Swanson left the station's airlock about 12:40 p.m. EDT/1640 GMT to finish work on a rotary joint so a pair of electricity-producing wing panels can track the sun for power.

The wings were installed last week during the first spacewalk of Atlantis' 13-day flight.

During the installation, the space station's primary computer network crashed, raising concerns that the still-under-construction, $100 billion outpost would have to be temporarily abandoned.

The German-built computers were provided to Russia in exchange for equipment to dock Europe's cargo ship, called the Automated Transfer Vehicle, at the station early next year.

Russian flight controllers and cosmonauts aboard the station figured out a way to bypass suspect protection circuits and successfully revived the network. The computers control steering rockets that are needed to periodically readjust the station's position in space.

NASA wants to make sure the thrusters will kick in to dampen any motion caused when the 100-ton shuttle detaches and plans a test of the system on Monday.

The shuttle has enough supplies to stay at the outpost until Wednesday if any problems arise.

"Things are looking very good," station flight director Holly Ridings said.

Engineers have not been able to pinpoint the exact cause of the computer crash, which set off an around-the-clock, multinational surge of tech support.

The best guess is that the electrically charged environment around the station changed as a result of the addition of another 17 tons of mass to the complex, triggering interference of some sort in the computer's power system.

While the station's computers appear to have recovered, problems may just be beginning.

Both the Columbus module, Europe's main contribution to the space station program, and its cargo ship have the same computer systems. They were developed by Astrium-Space Transportation in Bremen, Germany.

Investigations are under way to determine if any changes need to be made before the lab is launched in December and the scheduled January debut flight of the ATV, the European Space Agency said in a statement.

The addition of Columbus, a Japanese-built module named Kibo and an Italian connecting node will pave the way for the station's crew to double in size from three to six members beginning in 2009 or 2010.