Experts are researching oxygen leak out of U.S. flight engineer Michael Fincke's suit while he was on a spacewalk outside the station.
The Russian space station commander Gennady Padalka and US astronaut Mike Fincke floated into space but were ordered back just minutes into the planned six-hour excursion. Flight controllers in Moscow noticed a problem soon.
The spacemen sealed the Russian hatch 14 minutes and 22 seconds after opening it and re-pressurised the air lock.
NASA said the two were never in any danger, but stressed that the problem needed to be understood before they could go back out.
Fincke and Padalka were using US and Russian gear, and carrying a spare circuit breaker.
The new breaker is needed to restore power to one of the gyroscopes that help keep the station stable and pointed in the right direction.
The station's four 4-foot-wide (1.2-meter), 800-pound (362- kilogram) gyroscopes were installed at the end of 2000 and keep the outpost's solar panels aimed at the sun. One failed in June 2002, and rocket thrusters have enough fuel to control the position for six months to a year if a third fails.
The next space shuttle mission is scheduled to bring a replacement for the failed gyroscope. Boeing Co. and Lockheed Martin Corp. are the major contractors for the station, which the U.S. is building with the help of 16 partners.
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