"I really think we're going to end up with three EVAs," or extra vehicular activities, ISS manager Michael Suffredini said after US astronauts completed a first spacewalk in which they ran into trouble trying to unhook and remove the busted module that has caused the cooling problem.

A second spacewalk has already been scheduled for no earlier than Wednesday, but NASA said it was clear that a third walk was now needed.

"It would take a lot of good luck and somebody coming up with a really short tweak to the EVA for us to get to the point where we can start that (new) ammonia pump" after the second spacewalk, Suffredini told a briefing broadcast on NASA's website.

"We're going to end up being in this condition, this risk posture, a few more days than originally planned," he said.

Conditions on the ISS remained stable and the station's six-person crew -- three Americans and three Russians -- was not in danger, US space agency officials said.

But the development is a setback for NASA. Astronauts Doug Wheelock and Tracy Caldwell Dyson had just completed the longest spacewalk outside the ISS and, at eight hours and three minutes, the sixth longest ever.

Yet they could not wrest the faulty pump module from the outside of the station's first starboard truss.

"We did our best to get it disconnected, but it didn't apparently want to," spacewalk flight director Courtenay McMillan said.

At one point, Wheelock resorted to hammering on one of the stuck lines' buttons, which caused the mechanism to open successfully.

Then, an ammonia leak forced an extension of the spacewalk, as the duo needed to "bake-out" the ammonia that escaped from the lines and contaminated their suits.

NASA encountered trouble with the four valves and their quick disconnect, or QD, mechanisms that the astronauts needed to use in order to get the module completely unhooked and removed.

NASA described the QDs as "sensitive systems" designed to feed ammonia through the valves at high pressure.

Astronauts noticed ammonia had built up around one of the valves, and it started leaking ammonia when opened.

McMillan said it was safer to leave the hose connected than to risk disconnecting it in its current configuration and then being unable to stop the leak.

"If we let it leak for too long, we might actually lose more ammonia than we can afford to in terms of getting the system restarted," she said.

Experts will meet at Mission Control in Houston, Texas on Sunday to map out the next steps, NASA said.

The agency will need "probably a couple more EVAs to get the job done," McMillan said.

She added that NASA was "looking at every possible option" to ensure a rapid resolution to what has become a nagging condition for the orbiting station.

Suffredini insisted that the space agency will overcome the problem.

"The challenge is to get through this problem before the next problem hits the other cooling system," he said.

The two astronauts were hoping to move the 780-pound (355-kilogram) spare unit around 30 feet (10 meters) from the opposite side of a truss for insertion into the gap left by the defective pump.

"This is a big, unwieldy object, so maneuvering it around and handing it off to crew members... could take some time and a lot of focus," McMillan told reporters earlier in the week.

The crew had faced an unusually short lead time for such a tricky spacewalk -- less than a week, compared with the two weeks NASA usually takes to prepare for a spacewalk to fix a "Big 14 failure," when a major unit stops working.

If the second of the two ISS cooling units fails -- a highly unlikely scenario, according to NASA -- the astronauts would no longer be able to cool most of the space station components.

But the crew would not be in danger because they could move to the Russian segment of the ISS, which has its own cooling system.

The ISS, which orbits 350 kilometers (220 miles) above Earth, is a sophisticated platform for scientific experiments.

It is a 100-billion-dollar cooperation between 15 countries, and has been manned uninterrupted since October 1990.

earlier related report
US astronauts on long spacewalk fall short on repairs
Washington (AFP) Aug 7, 2010 - Two US astronauts conducted the longest-ever spacewalk from the International Space Station on Saturday but came up short in efforts to fix the orbiting lab's faulty cooling system, NASA said.

Doug Wheelock and Tracy Caldwell Dyson began the walk at 7:19 am (1119 GMT), activating battery power and life-support systems inside their suits before opening a hatch and moving into open space.

They spent eight hours and three minutes out in the void, but the National Aeronautics and Space Administration said they were unable to unbolt and remove a faulty pump that failed last weekend, one of two that circulate ammonia coolant throughout the orbiting ISS.

"It was a tough one as spacewalks go," an announcer from Mission Control in Houston, Texas conceded on a live video feed on NASA's website.

"Because of difficulties with the disconnect on the ammonia lines that feed that pump, they were unable to remove the pump" from its chamber on the starboard side of the station, the announcer said.

"As the result of an ammonia leak in the final line that needed to be disconnected from the failed pump module, the day's tasks were only partially completed," NASA said in a statement on its website.

"Teams will evaluate the go-forward plan for the second planned spacewalk" to remove the failed pump and install and activate the new pump module, scheduled for Wednesday, it added.

The spacewalk -- the fourth of Wheelock's career and the first for Caldwell Dyson -- was the longest in ISS history and the sixth longest ever, according to NASA.

The pair had slept in the Quest airlock module overnight to allow their bodies to adjust to different pressure conditions and were awakened at about 2:00 am (0600 GMT) to begin final preparations for their work.

The walk was slightly delayed because of what NASA officials described as a "communication problem" inside Caldwell Dyson's helmet, but the issue was resolved.

Throughout the spacewalk NASA was sending updates on its Twitter feed saying the astronauts were having trouble disconnecting the ammonia lines and started "troubleshooting" the problems, only to partial success.

At one point, Wheelock resorted to striking one of the stuck lines' buttons with a hammer, which caused the line to open successfully.

The ammonia leak forced an extension of the astronauts' spacewalk, as they need to "bake-out" any ammonia that escaped from the lines and may have contaminated their suits.

Experts had warned ahead of the spacewalk that the operations were challenging because the astronauts would be handling ammonia lines at full operating pressure, which makes the lines stiff during reconnection and mating.

Officials will now have to reassess the second spacewalk set for Wednesday, incorporating the updated information on the station's configuration after the first spacewalk.

The two astronauts were hoping to move the 780-pound (355 kilograms) spare unit around 30 feet (10 meters) from the opposite side of a truss for insertion into the gap left by the defective pump module.

"This is a big, unwieldy object, so maneuvering it around and handing it off to crew members... could take some time and a lot of focus," Courtenay McMillan, the spacewalk flight director for the expedition, told reporters earlier in the week.

The crew had faced an unusually short lead time for such a tricky spacewalk -- less than a week, compared with the two weeks NASA usually takes to prepare for a spacewalk to fix a "Big 14 failure," when a major unit stops working.

If the second of the two ISS cooling units fails -- a highly unlikely scenario, according to NASA -- the astronauts would no longer be able to cool most of the space station components.

But the crew would not be in danger because they could move to the Russian segment of the ISS, which has its own cooling system.

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