Nasa says it is "at a loss" on how to fix a stuck bolt on the International Space Station after a six-hour space walk failed to fix the problem.
Its astronauts said the jammed bolt is making it impossible to install a vital new power system.
The unit - one of four on the station which distributes power from its solar panels - needs to be in place ahead of the arrival of a new Russian module in 2013.
But a a six-hour spacewalk to install the unit on Thursday ran into trouble almost immediately, when metal shavings were found around the old, 220-pound power switcher.
An attempt to clean the housing with nitrogen gas was made, but the new unit proved impossible to install.
Astronauts said a bolt needed to connect up the the unit to the station's power systems had jammed.
After several attempts to free the bolt, Nasa's Mission Control said they were out of ideas.
"We're kind of at a loss of what else we can try," astronaut Jack Fischer at Nasa;s Mission Control centre in Houston said to the crew. "If you guys have any thoughts or ideas or brilliant schemes on what we can do, let us know."
After rejecting an attempt to try and force the new unit into place, the engineers gave up and returned to the station's airlock.
The failure of the mission cuts the electricity available on the station by 25%.
That does not put astronauts at risk but means Nasa will have to perform a "balancing act" to keep on schedule, flight director Ed Van Cise said.