Cape Canaveral - Sticky bolts proved too much for spacewalking astronauts, forcing them to leave a new power-switching box dangling from the International Space Station instead of firmly bolted down.
Nasa scrambled to reduce the power demands of the orbiting lab and balance the electrical load, while mapping out a plan that could have the astronauts going back out as early as next week to tackle the problem.
It was a major disappointment for Nasa's Sunita Williams and Japan's Akihiko Hoshide, who spent hours struggling with the bolts. They used all sorts of tools and tactics as the spacewalk went into overtime, but nothing worked.
With time running out, Mission Control finally told them to tie down the box and head inside.
"We'll figure this out another day," Mission Control radioed.
Thursday's spacewalk was supposed to last 6½ hours but stretched past eight hours. It ended up in Nasa's top 10 list for longest spacewalks - at the No 3 spot.
VulnerableThe power router is one of four, and Nasa stressed that the other three are working fine. Nonetheless, electrical usage will need to be closely monitored at the 420km-high lab given the failed effort.
"The team may have to manage power loads a little bit, but this is familiar territory," said Nasa's space station programme manager, Mike Suffredini. "We'll be able to deal with that while we decide what our next plan is."
While the space station remains in stable condition, Nasa would like to take another crack at securing the box as soon as possible - perhaps next week - because of the mid-September departure of half the six-member crew, including the second US astronaut, who ran the robot arm from inside the station.
And the longer this situation goes on, the more vulnerable the space station is to additional failures, Suffredini noted.
Until the problem is resolved, the space station is able to draw power from just three-quarters of its solar wings - six instead of all eight.
The old switch box started acting up last year, and Nasa decided to replace it before it failed. This was the first spacewalk by Americans since the final shuttle flight a year ago.
Williams and Hoshide had trouble getting the old unit out because of two sticky bolts, and they found metal shavings in the sockets. They squirted in compressed nitrogen gas to clear the holes, and some debris came out. But still, the main bolt would not go in properly; the companion bolt was left undone.
FrustrationThe frustration mounted as the minutes and hours ticked by. At one point, Mission Control radioed: "We've tried almost every backup we have on this stupid bolt."
At a news conference later in the day,
Nasa officials said possible solutions might involve lubricating the thick, sturdy bolts or applying more torque.
Putting in a new switching box was the No 1 priority of the spacewalk. In separate work, the astronauts managed to hook up one power cable and get another cable halfway connected. They never got around to replacing a bad camera on the space station's big robotic arm.
Mission Control did its best to cheer up the spacewalkers as they re-entered the space station. "You guys are rock stars, just so you know," Mission Control said.
It was the second spacewalk in less than two weeks. On 20 August, two Russians worked outside the orbiting complex, installing shields to protect against micrometeorite strikes.
It's no longer common for astronauts to step into the vacuum of space. That's because after almost 14 years, the space station is virtually complete.
Williams is the lone woman at the space station. She and Hoshide arrived a month ago, launching from Kazakhstan aboard a Russian rocket.
