The summer has been a hot one for much of the planet, and things could get a bit hotter for the crew of the International Space Station (ISS) as well if the astronauts on board are unable to repair a broken cooling pump.

Two cooling systems circulate ammonia to vent waste heat produced by the station's many electronic components, but one of those systems, on the starboard side of the station, went down July 31, most likely due to a short circuit, said Michael Suffredini, NASA's ISS program manager, in a teleconference August 2 from Johnson Space Center in Houston. According to a fact sheet (pdf) from the European Space Agency, a partner in the ISS program, the station's typical internal temperature ranges from 65 to 80 degrees Fahrenheit.

All of the station's critical systems, and many of its noncritical systems, are now being chilled by the ISS's other cooling pump and could carry on that way indefinitely, Suffredini said. The concern is that, should the functional unit on the port side of the station fail as well, the U.S. portion of the ISS would be hobbled. If that were to happen the Russian segment of the ISS could shoulder the life-support load for the entire crew of six for several days, Suffredini said, but it would struggle to do so.

The ISS program already had a spacewalk planned for August 5, so NASA moved to use that opportunity for a repair of the faulty cooling unit. The starboard cooler, installed in 2002, was expected to fail eventually, although Suffredini said that it fell somewhat shy of its anticipated lifetime. The station carries four spares for the 350-kilogram cooling modules, one of which astronauts Douglas Wheelock and Tracy Caldwell Dyson will attempt to swap in for the failed unit in a two-part spacewalk.

Spacewalk flight director Courtenay McMillan explained that NASA personnel were working on the timeline and choreography for replacing such a bulky part. A decision on whether to begin the spacewalk August 5 or to postpone for a few days would likely be made late August 2 or on the morning of August 3, she added.*

*Update (8/3/2010): NASA has postponed the start of the repair spacewalk until August 6 at 6:55 A.M. (Eastern Daylight Time).

Photo: NASA