Spacewalkers hindered by jammed station platform

Two astronauts used pry bars and brute force but failed to free jammed equipment outside the International Space Station yesterday during a six-hour spacewalk to prepare the outpost for its last phase of construction.

Visiting space shuttle Discovery astronauts Richard Arnold and Joseph Acaba ended up tying down the stuck cargo platform with tethers and left it for future spacewalkers to deal with.

"I know it didn't turn out the way we wanted it to, but you guys did a great job," astronaut Steven Swanson radioed to Arnold and Acaba from inside Discovery's crew cabin.

Soaring 355 kilometres over northern China, former high school science teachers Arnold and Acaba floated outside the station's airlock just before noon EDT/1600 GMT for a day's work outside the complex.

"Thanks again for going outside today," station commander Mike Fincke radioed to the spacewalkers. "This is probably the last EVA (extravehicular activity, or spacewalk) for this Discovery mission. We just want to say take your time, enjoy it and do good work."

It was the second spacewalk for the men. They also went on separate outings with lead spacewalker Steven Swanson to upgrade the station's power system, loosen battery connections and complete other tasks.

During yesterday's spacewalk, the last of three planned during Discovery's eight-day stay at the station, Mr Arnold and Mr Acaba wrestled with a cargo platform attachment that defied spacewalkers' efforts on Saturday to slip it into position.

Nasa believed the rotating mechanism needed more force - with a pry bar if necessary - to move into its proper latching position. But after several attempts to free the device, flight directors told the spacewalkers to tie it down and move on to other chores.

The astronauts were able to hammer away a locking pin that was installed upside-down, but that didn't solve the problem.

"We've ruled that out," Mr Swanson radioed to Mission Control in Houston.

Mr Arnold and Mr Acaba then braced themselves against handrails and tried to pull the platform into position.

"I didn't even feel it move," Mr Arnold said. "It's really stuck."

With that, Nasa called off attempts to fix the problem.

"We're going to need to study this thing some more," Mission Control's Rick Davis told the crew.

The astronauts previously moved one of the station's two rail carts about 19 metres from one end of the station's truss to the other, where it will be used by the next visiting shuttle crew in June to install an outdoor porch for experiments on a Japanese laboratory.