Once outside the station, spacewalkers Doug Wheelock and Tracy Caldwell Dyson will work towards unhooking and removing the busted module that has caused the cooling problem before they can attach a replacement.

"If all goes as planned, the spare pump will be installed during the third spacewalk," said the US space agency, adding that that operation would be possible "no earlier than Sunday."

Wheelock and Dyson on Saturday undertook the longest spacewalk outside the ISS and, at eight hours and three minutes, the sixth longest ever, but still could not wrest the faulty pump module from the outside of the station's first starboard truss.

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

If the spacewalk efforts this week fail -- a highly unlikely scenario, NASA has said -- 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.

According to NASA figures, without thermal controls the ISS's sun-facing side would roast at 250 degrees Fahrenheit (121 Celsius), while the outpost's dark side would plunge to some minus 250 degrees Fahrenheit (minus 157 Celsius).

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.

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