CAPE CANAVERAL, Fla. Sticky bolts proved too much for spacewalking astronauts Thursday, forcing them to leave a new power-switching box dangling from the International Space Station instead of bolted down.

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 back inside. The work will be left for a future spacewalk, presumably sometime soon.

"We'll figure this out another day," Mission Control radioed.

Thursday's spacewalk was supposed to last six and a half hours but instead went for eight hours and 17 minutes. It ended up in NASA's top 10 list for longest spacewalks at the No. 3 spot.

The power router is one of four, and NASA stressed that the other three all of them redundant are working fine. Nonetheless, the electrical system will need to be reconfigured at the 260-mile-high (418-kilometer-high) lab, given Thursday's failed effort.

The old box started acting up last fall, and NASA decided to replace it before it failed altogether. 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 area. They squirted in compressed nitrogen gas to clear the holes, and some debris came out. But still, the bolts wouldn't go back in to secure the new box. None of the tools seemed to do the trick.

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Spacewalkers do repairs at space station