NASA postponed the launch of space shuttle Atlantis by at least another day after discovering a problem with its electrical power system early Wednesday.

The fueling of the external tank had yet to start when a coolant pump that chills the system used to generate electricity on Atlantis gave an erratic reading. The electrical system is made up of three fuel cells.

The space agency planned to further examine the problem, consult with the fuel cells' manufacturer and, if possible, try to launch Atlantis at 12:03 p.m. EDT Thursday.

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Astronaut Jeff Williams, 220 miles above Earth at the space station, wasn't optimistic when Mission Control told him about the problem.

"That doesn't sound promising," he said.

Mission Control responded, "It's hard to say. We want to be ever hopeful."

The problem was discovered shortly before an overnight meeting to decide whether to start pumping the shuttle's fuel tank with supercold liquid hydrogen and liquid oxygen.

When the shuttle's fuel cells were powered up, a low voltage reading came from one of the units, and the other two spiked up to compensate.

Fuel cells have forced launch delays before and even ended missions early.

The second flight of Columbia in 1981 was delayed because of a low pressure reading on an oxygen tank that fueled a cell, and the 5-day mission was cut short when the cell failed.

In 1995, an Endeavour launch was delayed eight days so workers could remove and replace a bad fuel cell that was registering higher than allowable temperatures.

A 1997 flight of Columbia returned to Earth four days after launch when a cell failed in orbit. The cell had shown some erratic readings before the launch but the shuttle was cleared to fly. The mission was rescheduled and Columbia launched again a few months later.

The delay puts more pressure on NASA to get Atlantis off the ground this week or postpone a daylight launch until most likely late October. A Russian mission to the space station is scheduled later this month.

NASA had scheduled launch attempts for Wednesday, Thursday and Friday. The weather forecast for all three days put the chance of weather prohibiting an attempt at 30 percent.

NASA almost didn't get a chance at all this week because of Tropical Storm Ernesto. Workers started moving the shuttle back to an assembly building last week because of the approaching storm, but NASA managers reversed course and decided to leave the spacecraft at the launch pad.

The space agency missed the original launch date of Aug. 27 because a lightning strike.

If the shuttle doesn't get into space this week, NASA is considering relaxing a rule that Atlantis be launched in daylight, which would allow the space agency to make another attempt at the end of September or early October.

The daylight rule allows NASA to get clear photos during liftoff so engineers can spot any problems with foam falling from the external fuel tank, as happened in the 2003 Columbia disaster. The extra photos also would help with a redesign of the external tank.

The shuttle's three power cells are fueled by hydrogen and oxygen stored in tanks. They not only generate electrical power for the shuttle, but also drinking water for crew members.

Earlier this week, technicians noticed a slightly hazardous reading of gaseous oxygen, but Buckingham said the reading appeared to be unrelated to the fuel-cell problem.

Atlantis, set for an 11-day construction mission to the international space station, will deliver a 35,000-pound, $372 million addition to the half-built space station.

Three spacewalks are planned to resume construction on the orbiting lab, the first such work in 3½ years after Columbia disintegrated while returning to Earth.

This mission is crucial to the space station's continued expansion and must be done before the final 14 shuttle flights.

The Russians plan to launch a Soyuz capsule on Sept. 18 ferrying two new station crew members and the space station's first female tourist, Dallas-area entrepreneur Anousheh Ansari.

Officials with both space agencies wanted to avoid the shuttle and Soyuz meeting at the station, fearing a traffic jam.