CAPE CANAVERAL, Fla. - NASA put off Friday's launch scrub decision for the space shuttle Atlantis as long as possible to determine whether a fuel sensor issue warranted keeping the orbiter Earthbound for one more day, top shuttle officials said today.

According to NASA's own flight rules, questions over a problematic engine cutoff sensor in Atlantis' external tank made today's launch scrub inevitable, though shuttle officials held out on the off chance that more data could make the space shot possible, NASA shuttle program manager Wayne Hale told reporters here at the Kennedy Space Center (KSC).

"At the end of the day, we decided that staying with the plan we established...was the prudent thing to do," Hale said.

Atlantis and its six-astronaut crew - which had already boarded the spaceplane before today's scrub - are now slated to launch towards the International Space Station (ISS) Saturday at 11:15 a.m. EDT (1515 GMT). Weather forecasts predict an 80 percent chance of favorable flight conditions tomorrow for the shuttle's STS-115 mission to resume construction of the orbital laboratory.

Early Friday morning, engineers detected an errant reading in one of four engine cutoff sensors (ECO) near the base of the liquid hydrogen portion of Atlantis' 15-story external tank. The sensor, part of a backup system design to shut down Atlantis' three main engines if the orbiter's fuel tank runs dry, failed a standard preflight test to give a simulated 'dry' - or empty - reading when directed by launch controllers.

NASA has seen similar situations in the past - an ECO sensor glitch delayed the launch of Discovery's STS-115 return to flight mission for 13 days in 2005 - and designed a plan that requires a 24-hour scrub to retest the afflicted part as engineers drain, then refill, a shuttle's external tank.

"If everything is performing as we expect and we just have one sensor continue to be a bad actor, we'll launch tomorrow," said Hale, who defended the decision to wait on the scrub decision since the astronaut and launch teams were in place and ready to discuss the issue. "We feel really good that we really need only three of those engine sensors to work."

But some mission managers and shuttle engineers felt there was a technical case for launching Friday despite the fuel sensor anomaly and its related regulations, though they were eventually overruled. Ken Bowersox, head of NASA's Flight Crew Operations Directorate, and STS-115 Mission Management Team (MMT) chairman LeRoy Cain - who carries the final say - opted to follow the flight rules, a move also supported by several other non-voting members.

"We feel good about the fact that we executed the plan that we put in place," Cain said.

A tight turnaround

Tucked inside Atlantis' payload bay is a 17.5-ton pair of new trusses and two packed solar arrays awaiting delivery to the ISS. The $371.8 million payload is slated to be the first major addition to the ISS since late 2002. Space station construction was stalled in 2003 as NASA worked to recover from the Columbia accident.

The lighted flight window for Atlantis' STS-115 mission opened on Aug. 27 and has slowly ebbed away due to a launch pad lightning strike, a tropical depression, a fuel cell cooling system glitch and - most recently - the ECO sensor anomaly.

Saturday marks the last available day to launch Atlantis, under the current lighted conditions required by flight rules, before NASA must stand down to avoid conflicts with the planned Sept. 18 liftoff of an ISS-bound Russian Soyuz spacecraft carrying a new crew and one tourist to the orbital laboratory.

But shuttle pad workers have their work cut out for them to once more ready Atlantis for flight and avoid anticipated afternoon thunderstorms that could stall launch pad activities later today.

Along with NASA's normal operations associated with a launch scrub - draining Atlantis' fuel tank, testing its ECO sensors, then ramping back up for more tests and the refueling process Saturday - shuttle officials also hope to replace a lost thruster cover near the spacecraft's nose to prevent rain water from collecting in its nozzle. But that process would have strain NASA's already strained scrub turnaround timeline.

"If we don't get unlucky in terms of the timing of the weather, it sounded like we maybe could pull it off," Cain said. "It's very tight."

Meanwhile, Atlantis' STS-115 crew - commanded by veteran spaceflyer Brent Jett - are scheduled to wake up at 11:50 p.m. EDT (0350 Sept. 9) tonight to prepare for their next launch attempt.

Mission managers plan to meet at 12:30 a.m. EDT (0430 GMT) tomorrow to decide whether to refuel Atlantis external tank and press ahead with a Sept. 9 space shot. If approved, external tank fueling would begin at about 1:20 a.m. EDT (0520 GMT), according to a NASA schedule.

NASA will provide live coverage of Atlantis' launch preparations beginning at 5:00 a.m. EDT (0900 GMT). You are invited to follow the briefing using SPACE.com's NASA TV, which is available by clicking here or using the button at the left.

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