CAPE CANAVERAL, Fla. (AP) — Two astronauts pulled off a riskier and trickier-than-usual spacewalk Wednesday, replacing a failed electric motor and giving the international space station a much-needed power boost.

The station's power system still has problems; a joint for rotating one set of solar wings is mysteriously clogged with metal shavings and can't be fixed until later this year. Wednesday's successful operation, however, added to the power margin at the orbiting outpost and cleared the way for the deliveries of two science labs.

Atlantis is supposed to lift off with the European Space Agency's Columbus lab next week after a two-month delay, but a bent radiator hose could force yet another postponement.