HOUSTON - NASA called off tests of the International Space Station's urine recycler on Sunday after problems developed and revamped plans for Monday's spacewalk to fix 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 cargo ship into its docking port.

But the spacewalkers ran into problems installing an attachment needed to mount a cargo platform. Lead spacewalk officer Glenda Laws-Brown told reporters at a briefing at the Johnson Space Center that a locking pin apparently was installed upside down. The astronauts used tethers to secure the attachment and moved on to other tasks.

Monday's spacewalk schedule was already tight due to the cancellation of a planned fourth outing. Nasa shaved a day off the mission and cancelled 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 that a failure of one of the station's gyroscopes does not take two devices out of operation. The shuttle is due back at the Kennedy Space Center on Saturday. -- REUTERS