CAPE CANAVERAL, Florida - NASA's countdown clocks are ticking again for the next-to-last space shuttle launch.

The countdown began Friday morning for the second time for Endeavour's final mission. A launch attempt late last month was foiled by an electrical snag aboard Endeavour. NASA says repairs have fixed the problem.

Commander Mark Kelly and his five crewmates returned to Kennedy Space Center on Thursday. Their families will follow this weekend, including Kelly's wife, Arizona Rep. Gabrielle Giffords, who was shot in the head in January.

Endeavour will fly to the International Space Station and deliver a $2 billion science experiment. Four spacewalks are planned during the 16-day mission.

Only one other shuttle flight remains, by Atlantis in July.