Richard Arnold and Joseph Acaba, two former science teachers turned astronauts, carried out a six-and-a-half hour walk in space, which included attempts to unblock the external cargo carrier.

The carrier became stuck after Acaba and another astronaut inadvertently inserted a restraining pin upside down on Saturday. Although Arnold and Acaba managed to dislodge the pin, the cargo system remained stuck.

The two emerged from the decompression chamber at 1537 GMT, six minutes earlier than planned, after mission control told them to stop trying.

"Because the problem is not yet understood, Mission Control canceled Acaba and Arnold's installation of a similar payload attachment system on the starboard side" of the ISS, NASA said.

"We're going to need to study this thing some more," said Rick Davis from mission control in Houston.

The astronauts secured the system with "long-duration tethers," which are to protect the hardware until another attempt can be made in the future.

Another controller later offered a more upbeat message: "You've left the space station in much better shape and we heartily thank you for your work."

The International Space Station and Discovery crew will likely receive similar praise from US President Barack Obama, who -- along with a group of middle school students -- is to link up with the voyagers by telephone on Tuesday.

The failure to unblock the cargo platform was the latest hiccup to afflict Discovery and long-running ISS.

The two vessels maneuvered into lower orbit Sunday to avoid a piece of floating debris that could have passed close by during Monday's space walk.

The object, which measured a little over 10 centimeters (four inches), came from a Chinese rocket launched in 1999 that broke up in March 2000.

The astronauts did achieve their main tasks of repositioning an equipment cart from one side to another of the Mobile Transporter, the ISS's rail line.

Elsewhere on the ISS, experts were working on a water-recycling unit that processes astronauts' urine and sweat into drinking water. A replacement part was brought by the Discovery and testing began Sunday. The samples will be taken back to Earth for analysis.

The machine, which was delivered to the ISS in November -- and has not yet functioned properly -- is key to sustaining a bigger crew on the orbiting station and for long-term space expeditions, such as moon landings or missions to Mars.

Carrying large amounts of water aboard the shuttle or other space vessels is expensive and takes up room needed for other equipment.

The two astronauts also greased the hand joints of the ISS's robot arm.

One of the Discovery mission's biggest tasks was to deliver the last set of solar arrays, which were successfully unfurled Friday.

The ISS now has four solar panels, two per wing, containing 32,800 cells that convert sunlight into electricity.

They will boost the outpost's full power generation from 90 to 120 kilowatts, providing the power the space station needs to carry out scientific experiments aboard Kibo and the European Columbus laboratory.

The additions also make it possible to double the space station's crew from three to six, beginning in May.

Discovery's latest mission, which blasted off last Sunday from Florida with a crew of seven astronauts, is one of the last major efforts in a decade-long push by 16 countries to build the 100-billion-dollar outpost in space.

NASA has scheduled nine shuttle flights through 2010 to finish building the space station. Upcoming shuttle flights also include the last mission to service the orbiting Hubble telescope in May.

Discovery is due to land back on Earth on March 28 at 1742 GMT, two days after a Russian Soyuz mission takes off for the ISS carrying a crew of three, including US billionaire businessman Charles Simonyi, who has shelled out 35 million dollars for his second trip as a space tourist.

On Discovery, crew member Koichi Wakata became the first Japanese astronaut to join the ISS for a long stay. He is scheduled to remain on the orbiting station until June.

The Discovery mission, delayed five times, is the first by a US space shuttle in 2009.

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Washington (AFP) March 23, 2009
Discovery and the International Space Station maneuvered to avoid debris during Monday's spacewalk, dropping into a lower orbit, NASA officials said.