NASA hopes to figure out what caused the latest problem keeping Atlantis earthbound: an electrical short in a 30-year-old motor.

If the agency determines by Thursday night that the cause of the short is not serious, NASA can try to launch the shuttle Friday morning.

On Wednesday, NASA scrubbed Wednesday and Thursday launch attempts because of the short in one of Atlantis' electricity-generating fuel cells. The delays follow postponements last month due to a lightning strike on the launch pad and the approach of Tropical Storm Ernesto.

• Click here to visit FOXNews.com's Space Center.

If a Friday launch is viable, there's a 70 percent chance of good weather for an 11:40 a.m. EDT attempt.

Atlantis and its six astronauts plan to haul 17½ tons of girders and solar panels into orbit and resume construction of the international space station, which has been on hold since the Columbia disaster 3½ years ago. Astronauts will make three spacewalks to put the pieces together.

NASA is caught in a schedule squeeze. The space agency made an agreement with the Russians not to attempt a launch after Friday because Russia is sending a three-person Soyuz capsule to the space station on Sept. 18.

If it doesn't launch Friday, the space agency may have to wait until late October — or relax daylight launching rules instituted after the 2003 Columbia accident and try again at the end of September.

Once the Russian Soyuz comes back, NASA may attempt a launch as early as Sept. 28 or 29 even though the launch would be in darkness, spokesman Allard Beutel said.

NASA rules say shuttles have to be launched in daylight so that the big external fuel tank can be photographed for evidence of any broken-off pieces of foam of the sort that doomed Columbia.

There is a slight chance of a Saturday launch, but NASA would have to shorten its construction mission on the international space station, something Wayne Hale, space shuttle program manager, has said he would not like to do.

In a four-hour, 15-minute mission management team meeting Wednesday afternoon, engineers split evenly on what to do next.

"If you want high drama, this is about as good as it gets," Hale said in an evening news conference. "If you were an engineer you would really love this."

What happened in the fuel cell is something NASA has never seen in 25 years of flying shuttles, said deputy orbiter project manager Ed Mango.

The three fuel cells generate electricity for the shuttle during flight, and all three must work.

Hours before adding fuel to the shuttle's massive external tank, engineers discovered that a coolant pump that chills fuel cell No. 1 was giving strange readings.

One of three current paths in the fuel cell isn't working. If a second one doesn't work, the fuel cell can only run for nine minutes before being shut down. The tiny motor involved was built in 1976 and validated for flight in 1981, Hale said. The problem could be the motor or wiring.

Complicating everything is the fact that NASA does not really know the innards of the system.

"The vendor sold us the thing and didn't exactly tell us how it works, amazing as that might be," Hale said.

NASA has to squeeze 15 shuttle launches into the next four years to finish the construction of the half-built space station by 2010, when NASA is scheduled to stop flying shuttles.