Leaving their orbiting outpost unmanned, the international space station's two astronauts floated outside on a spacewalk Friday and plugged in new antennas and replaced a worn-out piece of cooling equipment.

Gennady Padalka and Mike Fincke popped open the hatch on the Russian side of the spacecraft and quickly stepped through the fourth and final spacewalk of their six-month mission.

Their job: install three antennas for a new type of cargo carrier to be launched by the European Space Agency (search) to the space station late next year and replace a 2-foot-square Russian pump panel, part of a critical system for cooling station equipment.

"Be careful," Mission Control repeatedly warned the spacewalkers, telling them something like snow might float out when the 6-year-old pump panel comes out. "Go slowly."

The new pump assembly went in neatly. Then the spacewalkers installed a few hooks on exterior handrails, for use during spacewalks. The antenna job followed; the men tossed antenna covers overboard as they connected the three devices, one by one.

Because no one was left inside the 225-mile-high complex, flight controllers in both Moscow and Houston kept close watch over the two men and all systems. The spacewalk lasted 51/2 hours, and everything was accomplished. "OK, guys, thank you very much," Mission Control radioed.

NASA (search) prefers having a crew member inside during spacewalks but has had to settle for one less person on board for more than a year because of the grounding of the shuttle fleet. This was the fifth spacewalk with an empty outpost.

The space station has been relying on Russia's much smaller spacecraft for deliveries ever since Columbia broke apart over Texas during re-entry on Feb. 1, 2003.

Padalka and Fincke have just 11/2 months remaining aboard the space station before they return to Earth. They will be replaced by another Russian and American.

During Friday's outing, flight controllers hoped to better understand the mysterious force that tilted the space station 80 degrees off-center during the men's last spacewalk, one month ago.

Engineers suspect the extra force could have come from air gushing out when the hatch was opened, or from the spacesuits themselves.

To test those theories, Mission Control (search) had the spacewalkers remain as still as possible for 15 minutes in an "immobility experiment."

As Padalka and Fincke hunkered down and tried to be motionless, they asked flight controllers to talk to them so they wouldn't get bored.

"You want news?" Mission Control asked. "It is raining in Moscow." The chitchat went on, and on.

About 3 1/2 hours into Friday's spacewalk, well after the astronauts' motionless test, one end of the space station pitched upward and the entire outpost eventually tilted 70 degrees off-center, Mission Control reported. As they did during last month's tilt, the U.S. gyroscopes reached their stabilizing limit and Russian thrusters had to take over control of the complex, officials said. None of this disrupted the spacewalk.