Caught in a scheduling squeeze, NASA decided to try to launch space shuttle Atlantis on Friday without replacing a troublesome electrical component.
Friday had been the last launch day available before the US space agency ran into a scheduling conflict with the Russian space agency. But NASA managers now believe they can try Saturday, if needed, and they were finalizing negotiations with the Russians.
There was a 30 percent chance bad weather would interfere at the 11:40 a.m. EDT (1540 GMT) Friday launch time.
On Thursday, NASA decided not to change out an electricity-generating fuel cell whose coolant pump had given erratic readings, causing a scrub a day earlier. Replacing the 250-pound (112.5-kilogram) pump could have delayed any launch attempt by several weeks and might have been riskier than leaving it in place, said Steve Poulos, shuttle orbiter projects manager.