Two astronauts who stepped out of the international space station for an unusually risky spacewalk were quickly ordered back in Thursday when Mission Control spotted a pressure drop in one of the men's oxygen tanks.

NASA (search) stressed that the spacemen were never in any danger. They were safely in the pressurized confines of the orbiting complex within minutes, and said they were feeling fine.

The spacewalk — a critical repair effort to replace a fizzled circuit breaker — was put off until Tuesday at the earliest. Flight controllers said they need to understand what went wrong before sending astronaut Mike Fincke (search) and cosmonaut Gennady Padalka back outside.

Fincke had just popped open the hatch and floated outside when the frightening words came from Russian Mission Control (search): "You need to return. Something is not right."

Mission Control informed the spacemen that the pressure in Fincke's prime oxygen bottle was falling rapidly. They needed to get back inside, fast, and close the hatch.

They sealed the hatch 14 minutes and 22 seconds after opening it and repressurized the Russian air lock.

Fincke and Padalka were using an odd mishmash of U.S. and Russian gear, and carrying out 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 mission was fraught with risk, even before Thursday night's suit trouble. NASA had to leave the space station empty during a spacewalk for only the second time ever, forcing flight controllers on the ground to keep an eye on the outpost's systems.

NASA has been bending its own rules to keep the space station operating since the grounding of the shuttle fleet following last year's Columbia catastrophe. The grounding has all but stopped the delivery of replacement parts and reduced the size of the station crew from three to two.

The announcement that the spacewalk was officially over came soon after both crewmen were instructed to take off their spacesuits, nearly an hour after the spacewalk began.

It was a disappointing moment in space.

"Have some tea, coffee," Mission Control kindly told the crew.

Fincke later offered his own condolences to flight controllers, who teased him about almost certainly setting a record for the shortest spacewalk ever.

"We'll just live to fight another day," Fincke replied.

NASA said the suit pressure itself never faltered and that the problem was confined to the oxygen tank. Once inside, the spacemen listened for a hiss from the bottle, but heard nothing.

"We're not exactly clear on what has happened," Mission Control radioed.

The oxygen-supply problem was the latest of several complications associated with the spacewalk.

Fincke and Padalka had originally planned on leaving from the American hatch in U.S. spacesuits. But the NASA suits developed crippling cooling problems last month, forcing managers to send the two men out the Russian hatch in Russian suits.

That more than doubled the distance to the work site, on the U.S. side of the station. It also made for a trickier mission, because Russian suits are more pressurized and thus more rigid.

The last time Russian spacesuits were used, by another crew in February, the spacewalk had to be cut short because of a cooling problem that was so bad that the cosmonaut got uncomfortably warm and his helmet became wet. Padalka and Fincke were using brand new spacesuits, however.

The use of Russian equipment also created an unprecedented division of labor and raised the potential for communication blackouts, requiring the crewmen to come up with hand signals for alerting each other of danger or conveying smooth sailing.

Mission Control in Moscow was in charge of the spacewalk, with Russian the official language, but that was supposed to switch to Mission Control in Houston and English as soon as Padalka and Fincke reached the U.S. side of the complex — and vice versa on the way back.

Station operations manager Mike Suffredini said every aspect of the spacewalk — including safety — was carefully considered ahead of time.

The circuit breaker — enclosed in a rectangular box about the size of a dictionary — failed in April, just hours after Padalka and Fincke moved into the space station for a six-month stay.

Its shutdown cut power to one of three working gyroscopes, leaving the orbital complex maneuvering with just two, the bare minimum. A fourth has been broken for two years; its replacement has been grounded along with the shuttle.

NASA decided to replace the circuit breaker now with an on-board spare, rather than take the chance that another gyroscope might falter and force the spacemen to rush to make an emergency repair.

It was the first spacewalk for Fincke, 37, an Air Force lieutenant colonel, and the third for Padalka, 46, a colonel in the Russian Air Force who lived aboard Russia's Mir station (search) in the late 1990s.