NASA postponed the launch of the space shuttle Atlantis to January after a sensor in an emergency engine cutoff system failed again on Sunday, once more delaying Europe's major contribution to the International Space Station.
Two attempts to launch Atlantis were stymied by erratic fuel sensors, which are part of an emergency engine cutoff system.
Atlantis and its seven astronauts -- five Americans, one German and one Frenchman -- are due to deliver Europe's Columbus science laboratory module to the space station, ending a quarter of a century in which European space pioneers had to run their experiments on orbital outposts owned by others.
"We're disappointed that we're not flying. We'd love to fly, but we want to fly safe and it's not believed to be safe at the moment," said Alan Thirkettle, the European Space Agency's station program manager.
Two of the four sensors in the ship's hydrogen tank failed during NASA's first launch attempt on Thursday. One of them failed again when the tank was being filled for a second launch try on Sunday.
The sensors, which operate like dipsticks to determine fuel levels, are part of a backup system to cut off the shuttle's three hydrogen-burning main engines if the tank runs dry because of a leak or other problem.
Running the engines without propellant could cause their pumps to break and possibly trigger a catastrophic explosion.
"Our hope is that we can go do some trouble-shooting and nail down what part of the system the failure might be in," said LeRoy Cain, head of NASA's Mission Management Team.
"We're determined to get to the bottom of this," Cain said.
Out of time
NASA had until Thursday or Friday for launch attempts this year, but any analysis and repair of the problem would take longer than that to complete, officials said.
Disasters in 2003 and 1986, which killed the crews aboard the shuttles Columbia and Challenger, have made NASA especially cautious about launch safety.
After Thursday's scrubbed launch, managers met for two days and had decided to attempt the Atlantis launch again on Sunday despite the erratic sensors, which have a history of glitches.
Engineers had believed they had traced the problem to a subtle manufacturing issue and thought they had solved it by replacing all suspect devices.
NASA will start another round of investigations and engine upgrades to try to determine the cause of the problem. Atlantis will remain at the launch pad, positioning NASA for another launch try as early as on January 2.
NASA has 10 shuttle flights remaining to complete construction of the $100 billion space station. It also wants to make two re-supply flights and a servicing call to the Hubble Space Telescope before the shuttle fleet is retired in 2010.
"It's not that big of an impact to us overall," Bill Gerstenmaier, NASA's associate administrator for space flight, said after Sunday's launch was canceled. "There's enough margin that can accommodate this move into January."
The launch delay will keep station flight engineer Dan Tani in orbit for another month. He was to be replaced by France's Leopold Eyharts, one of Atlantis' crew members, who will oversee the setup of the Columbus laboratory.
Europe had planned to debut its science laboratory for the space station in 2002, but the launch was put on hold by delays largely caused by the grounding of the U.S. space shuttles after the 2003 Columbia disaster. With the delays, the laboratory's costs have climbed to more than $1.5 billion.
(Edited by Jane Sutton and Vicki Allen)