By WILLIAM HARWOOD
c.2010 New York Times News Service
One of two critical cooling systems aboard the International Space Station malfunctioned late on Saturday, setting off alarms and triggering an extensive reduction in power. Mission managers gave tentative approval on Sunday to plans for a complex two-spacewalk repair job late this week.
The cooling system, which dissipates heat generated by the lab’s electronics, initially shut down around 8 p.m. Eastern time on Saturday when a circuit breaker tripped, setting off warning alarms that awoke the crew. With reduced cooling, flight controllers were forced to shut down two of the space station’s four stabilizing gyroscopes, used to help control the lab’s orientation; one communications channel; and several computer control units.
The station’s flight engineers, Col. Douglas H. Wheelock of the Army and Tracy E. Caldwell Dyson, also had to install jumpers between the Russian Zarya propulsion module and the U.S. segment of the station to prevent additional cooling problems.
NASA officials said the station’s six-member crew — including a third American, Shannon Walker, and three Russians, the commander Aleksandr Skvortsov, Mikhail Kornienko and Fyodor Yurchikhin — was not in any immediate danger. But the loss of a cooling system will cripple station operations until the problem can be resolved.
“We’re going to be working hard overnight to figure out what’s going on,” the astronaut James M. Kelly radioed from Mission Control shortly before the astronauts returned to bed. “By tomorrow, a little bit later on, hopefully we’ll be able to send you up a little bit better idea of where we stand on everything.”
A few hours later, the alarms blared again when flight controllers tried to restart the stalled coolant pump. Once again, a circuit breaker blew, indicating a problem with the pump module.
Later in the morning, one of the two powered-down gyroscopes was reactivated. Although the station’s main switching units, which direct power to various subsystems, were running a bit hotter than normal, engineers said the laboratory was in a stable configuration.
The International Space Station is equipped with two independent cooling systems, or coolant loops, mounted on the lab’s main solar power truss that use ammonia circulating through huge radiator panels to dissipate the heat from the station’s electronics.
While the loops are independent, the station cannot operate at full power with just one. Spare components are on board, including two coolant system pumps, but installation is considered difficult and two spacewalks will be required.
Caldwell Dyson and Wheelock were already planning to conduct a spacewalk on Thursday to mount a robot arm attachment fixture to the Zarya module and to rig the central Unity module for attachment of a storage compartment during a shuttle flight in November.
That excursion will now be deferred. NASA engineers and managers met Sunday at the Johnson Space Center to discuss spacewalk repair options and gave preliminary approval for “an initial spacewalk no earlier than Thursday by Wheelock and Caldwell Dyson to replace the pump module,” NASA said in a statement.
The two have not trained for a pump module replacement, but they participated in extensive spacewalk training together before launching.
The spare 780-pound coolant pump, built by Boeing, is mounted on an external platform just in front of the station’s airlock module, relatively close to the faulty pump module. The pump module measures five and a half feet long, a bit more than four feet across and three feet high.
The repair plan calls for the astronauts to first remove the failed unit, temporarily stow it and then move the new pump into position. The second spacewalk would be devoted to making a variety of electrical connections and hooking up ammonia coolant lines.
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