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KENNEDY SPACE CENTER, Florida (CNN) -- NASA scrubbed the much-delayed liftoff of space shuttle Atlantis on Friday amid concerns about a faulty sensor on the external fuel tank.
Another attempt at a launch will be made Saturday after the fuel tank is emptied and refilled, NASA spokesman George Diller said. The "preferred" launch time is around 11:15 a.m. ET Saturday, Diller said.
A long discussion between NASA engineers, managers and safety officers Friday morning ended with a conclusion that the three functioning sensors were not enough to ensure the main-engine cutoff system would work safely during launch.
A similar sensor has plagued previous missions, and a malfunctioning fuel cell held up the launch of Atlantis earlier this week.
Shuttle managers on Thursday evening defended their decision to proceed with the launch.
Extensive review of engineering data convinced a NASA troubleshooting team that the problem involving the fuel cell's coolant pump is isolated and would not lead to a failure during launch or the shuttle's 11-day mission.
Managers said they were confident that the risk of a fuel cell's failure is low.
The launch of the Atlantis, commanded by Navy Capt. Brent Jett, will be the first shuttle mission since the 2003 Columbia disaster to deliver a major new portion of the international space station.
Under NASA regulations, three operational fuel cells are required to proceed with a launch. They generate electricity that powers the onboard systems during flight and generates water for the crew.
The Atlantis crew will conduct three spacewalks to install a massive set of solar arrays to the station's main truss.
NASA has been negotiating with Russian space officials to get more time to complete the mission.
The shuttle needs to leave the space station before a Soyuz space capsule arrives to avoid a traffic jam.
Several fuel cell problems
Shortly after midnight Wednesday, workers powered fuel cells up before fueling the shuttle's external tank for launch and observed a voltage fluctuation in a fuel cell.
The problem happened again when technicians turned the unit off and on again, and program managers ultimately scrubbed the Thursday launch to allow engineers more time to study the problem and consider possible fixes.
The fuel cells, located under the floor of the shuttle cargo bay's upper portion, are difficult to access on the launch pad. .
"Until we take this fuel cell apart, it's a detective story. ... We have clues and evidence," NASA program manager Wayne Hale. "Because we don't know for sure, there is still the element we can be wrong. If clues lead you to the wrong place, there is risk."
CNN's Kate Tobin contributed to this report.
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