CAPE CANAVERAL, Fla. (AP) _ For the fourth time in two weeks, NASA nixed the launch of the space shuttle Atlantis, this time for a faulty fuel tank sensor _ the same glitch that has thwarted two other missions.
A fifth liftoff attempt will be made at 11:15 a.m. EDT Saturday to get the spacecraft headed on a mission to resume construction of the international space station.
On Friday the six astronauts were already strapped in with the hatch closed when the space agency called off the launch with just 45 minutes to go. Although the fuel sensor had malfunctioned hours earlier, NASA wanted to keep discussing the problem before scrubbing the flight.
But they knew the odds for a liftoff weren't good. The agency has a new rule requiring a stand-down of 24 hours when one of the hydrogen tank's four engine cutoff sensors doesn't work properly; such a delay would allow engineers to gather more data on the problem.
``We had a lot of discussion. ... We follow the rules,'' launch director Mike Leinbach radioed Atlantis' crew, notifying them about the scrub. ``Ought to feel good that we did that.''
``We understand. We concur 100 percent,'' responded Atlantis' commander, Brent Jett. ``It was given a lot of thought by a lot of smart people ... It's the right thing to do.''
A large number of managers favored flying, but opposition to launching was led by NASA's flight crew operations director.
``... When you get into the launch countdown, there are emotions running a little bit higher,'' said shuttle program manager Wayne Hale. ``But at the end of the day, we decided staying with the plan ... was the prudent thing to do.''
The new plan for the 24-hour delay was implemented after two previous encounters with faulty fuel tank sensors. Earlier this year, Discovery's launch was delayed by almost two months so four tank sensors could be replaced after one was found to be faulty, and a similar problem briefly delayed last summer's launch of Discovery on the first shuttle flight since the Columbia disaster in 2003.
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. NASA managers are confident the shuttle can launch successfully with only three of the four sensors working properly.
``If everything is performing as expected and we just have one sensor that continues to be a bad actor, we'll launch tomorrow,'' Hale said Friday.
Previous Atlantis launch attempts over the past dozen 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.
Atlantis is far from the record for NASA's most delayed liftoff. In 1986 and 1995, NASA scrubbed the launch of Columbia six times after fueling the shuttle.
This mission is in a scheduling squeeze, however. Atlantis must lift off on Saturday or wait until the end of September at the earliest so as not to interfere with a Russian Soyuz trip to the space station. The Soyuz launch ferrying two new station crew members and the first female space tourist is slated for liftoff Sept. 18.
NASA needed approval from the Russians to launch the shuttle as late as Saturday and had to promise to undock the spacecraft no later than the day the Soyuz launches.
If NASA attempts a mission later this month, it will have to waive a post-Columbia rule that says launches must be done in daylight so that the spaceship can be photographed for signs of damage.
Aboard Atlantis is one of the heaviest payloads ever carried into space _ a 17 1/2-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.
Atlantis' crew members will make three spacewalks during the 11-day mission to install the $372 million addition.
Construction on the space station has been at a standstill ever since Columbia broke apart on its return home in 2003, killing its seven astronauts.