NASA scrubbed Friday's launch of the space shuttle Atlantis again, this time because of a problem that has bedeviled the space agency before: a faulty fuel tank sensor.

The launch was rescheduled for 11:15 a.m. EDT on Saturday, when NASA will try a fifth time to get Atlantis off the ground and send it to the international space station to resume construction on the orbiting outpost, which has been on hold since the Columbia tragedy 3½ years ago.

Saturday is the last time NASA has to launch Atlantis before it has to go to the back of the line, behind a Russian Soyuz capsule that is slated for liftoff Sept. 18 on a flight to the space station. Atlantis and the Soyuz cannot both be at the space station at the same time.

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If Atlantis cannot lift off on Saturday, it will have to wait at least until late September — and even then, only if NASA waives a post-Columbia-disaster rule that says launches must be conducted in daylight so that the spaceship can be photographed for signs of damage.

Previous launch attempts over the past 12 days were dashed by a lightning bolt that struck the launch pad, Tropical Storm Ernesto and a fuel cell coolant pump that gave an erratic reading.

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This time, a sensor in the hydrogen fuel tank gave an abnormal reading during a test as the shuttle was being fueled early Friday.

Atlantis had been fueled with more than 500,000 gallons of supercold liquid hydrogen and oxygen, the six astronauts had donned their orange flightsuits and strapped themselves in, and the hatch to the shuttle had been closed, when NASA decided to postpone the launch with just 45 minutes to go until liftoff.

"We had a lot of discussion. ... We follow the rules," said launch director Mike Leinbach. "Ought to feel good that we did that."

Atlantis commander Brent Jett responded, "We understand. We concur 100 percent."

In order to fly, NASA would have had to waive a rule requiring that all four sensors work properly.

The fuel gauges are designed to prevent the main engines from running too long or not long enough during the climb to space.

An engine shutdown at the wrong time could prove catastrophic, forcing the astronauts to attempt a risky emergency landing overseas, or leading to a ruptured engine.

Earlier this year, the launch of Discovery was delayed by almost two months so four hydrogen fuel tank sensors could be replaced after one was found to be faulty. A similar problem briefly delayed last summer's launch of Discovery on the first shuttle flight since the Columbia disaster in 2003.

Despite four delays, Atlantis is far from the record for NASA's most snakebit shuttle.

In 1986 and 1995, NASA scrubbed the launch of Columbia six times after fueling the shuttle.

In official NASA record-keeping, Atlantis was only scrubbed once, because the other launch delays happened before the shuttle was loaded with fuel.

A launch attempt on Wednesday was called off because of some faulty wiring involving a 30-year-old motor in one of the shuttle's three electricity-generating fuel cells. But on Thursday, NASA decided to press ahead with a launch without replacing the equipment.

NASA engineers convinced themselves that the baffling electrical problem would not be a safety risk — something they did over the objections of the agency's safety managers.

Aboard Atlantis is one of the heaviest payloads ever launched into space — a 17½-ton truss section that will be added to the half-built space station. It includes two solar arrays that will produce electricity for the orbiting outpost.

One of the two girders that Atlantis is hauling up to the international space station has been waiting at Kennedy Space Center for nearly seven years.

Atlantis' crew members will make three spacewalks during the 11-day flight to install the $372 million addition.

Construction on the space station has been at a standstill ever since Columbia disintegrated in February 2003.

The current mission was slated for March 2003.

NASA spent the more than three years that followed trying to prevent the problem that doomed Columbia — pieces of insulating foam breaking off from the shuttle's big external fuel tank at liftoff.

Atlantis, which has flown 26 times, hasn't launched since October 2002.

NASA hopes this flight will jump-start a series of 15 complicated shuttle missions that will culminate in 2010 with a completed space lab.

"It's been a long road. There's been a lot of folks that have spent the better part of the last seven years of their careers here at the space center working on it," payload manager Robbie Ashley said late last month.