NASA delayed tests of the International Space Station's urine recycler on Sunday after problems developed with centrifuge-like device and revamped plans for Monday's spacewalk to deal with an improperly installed cargo platform attachment.
Flight directors also repositioned the station and the visiting space shuttle Discovery to avoid a piece of space junk, which was expected to come too close during Monday's spacewalk, the last of three during Discovery's mission.
The primary goal of Discovery's flight was to deliver and install the station's last set of solar panel wings, which was accomplished on Thursday. The shuttle blasted off from the Kennedy Space Center on March 15 for a 13-day mission.
With the shuttle fleet due to be retired next year, NASA is counting on the Discovery astronauts to also complete several maintenance tasks that will ease the burden on future crews.
The station, a project of 16 nations, has been under construction for more than 10 years.
During the mission's second spacewalk, astronauts Steven Swanson and Joseph Acaba loosened connections on batteries that will be replaced during the next shuttle mission in June and installed a GPS navigation antenna needed to guide Japan's new unmanned cargo ship into its docking port.
But the spacewalkers ran into problems installing a mount for a cargo platform. A locking pin was installed upside-down and may be blocking the mechanism from rotating into its proper position.
By applying more force -- with pry bars and other tools if necessary -- astronauts should be able to free the mechanism, said lead space station flight director Kwatsi Alibaruho.
"We thought initially that the incorrect installation of the pin was the reason we were not able to rotate the (mechanism) down to its proper configuration. We now believe that (it is) just much, much stiffer than expected," he said.
A BUSY WORKLOAD
Monday's spacewalk schedule was already tight due to the cancellation of a planned fourth outing. NASA shaved a day off the mission and canceled the last spacewalk when Discovery launched five days late due to a hydrogen fuel leak.
The shuttle needs to depart the station by Wednesday to avoid a schedule conflict with a Russian Soyuz capsule carrying the station's next crew.
In addition to fixing the cargo carrier attachment, astronauts Acaba and Richard Arnold are expected to reposition one of the station's rail carts and rewire a circuit breaker so a failure of one of the station's gyroscopes does not take two devices out of operation. They also are scheduled to lubricate part of the station's robot arm.
Sunday afternoon, NASA repositioned the shuttle-station complex so it would encounter more friction from atmospheric particles and slow down by about 0.1 feet per second -- enough to change its orbit so a piece of debris from a Chinese satellite launch in 1999 would pass harmlessly by during Monday's spacewalk.
Orbital debris is a growing concern in space, with last month's crash of a U.S. commercial communications spacecraft with a defunct Russian satellite, as well as the 2007 destruction of a Chinese satellite by the Chinese military as part of a weapons test.
At speeds exceeding 17,500 mph, shards as small as a fleck of paint have enough energy to damage or destroy spacecraft.
Also Sunday, engineers were trying to figure out why a water purification system that recycles urine and condensate into drinking water was not working properly.
The Discovery crew delivered a new distiller for the device that was successfully tested without liquids Saturday. Problems developed during the first test run Sunday using urine.
NASA wants to have the urine recycler working before expanding the station's live-aboard crew from three to six members in May.
The shuttle is due back at the Kennedy Space Center on Saturday.