Article Posted on : - Aug 02, 2010

The International Space Station. A NASA photo

HOUSTON (PTI): One of the two cooling systems of the International Space Station suddenly shut down early Saturday night, forcing a wave of equipment power-down, even as six Russian and NASA astronauts aboard the facility were safe.

Two of the four gyroscopes were initially shut down, part of the space station's pointing and navigating system.

But the crew installed a jumper cable to bring up a third gyroscope, leaving the station in a much more stable position. On board are three Americans and three Russians who are out of danger.

The cooling system which broke down was serving the US, European and Japanese laboratories.

NASA officials gave preliminary approval for a pair of space walks, the first of which is likely to take place later this week.

However, the station is in a stable configuration with most systems receiving cooling and many systems operating with redundancy following the installation of jumper cables from the Destiny Lab's power system overnight.

The crew is monitoring the systems and relaxing on an otherwise off duty day. Officials said the orbiting complex is in a stable situation.

Two of the Americans on board were already scheduled to conduct a spacewalk Thursday for routine maintenance, though the repairs would supersede the original chores.

Much of the station is operating on a single string, however, with no safeguard in case of further cooling system failures.

The trouble arose Saturday night, when one of the two ammonia-fed cooling loops shut down.

Alarms sounded throughout the sprawling outpost as the circuit breaker for the pump in that line tripped, causing the pump to stop working.

The cooling system is critical for on-board operations. The two ammonia lines ensure that all the station's electronic equipment does not overheat.

Astronaut Tracy Caldwell Dyson hustled through the equipment shutdown procedures and, with crewmate Douglas Wheelock, installed a jumper cable to keep all the rooms cool.

Flight controllers tried to restart the disabled ammonia pump early Sunday, but the circuit breaker tripped again.

No further repair attempts were planned, at least for now. In fact, the astronauts were allowed to sleep in because of all the late-night disruptions.

Any repairs later this week will involve replacing that ammonia pump, a difficult job that would require two spacewalks.

Two spare pumps are stored on the outside of the station. The first repair spacewalk likely would occur Thursday at the earliest, with the second excursion two or three days later.

A final decision on whether to proceed with the task will be made following additional engineering review.

Among the equipment powered off for now: the Global Positioning System circuit, several power converters and a set of devices that route commands to various equipment.

No space shuttle visits are planned before November.

Only two US shuttle missions remain before the fleet is retired; a third and final flight for next summer is under consideration for which Indian American astronaut Sunita Williams is shortlisted.