NASA delayed the launch of the space shuttle Atlantis until Sunday at the earliest, to give managers more time to decide whether to change longtime launch rules to address fuel gauge problems.
Shuttle managers met into the night on Friday, trying to decide how best to proceed. After six hours of discussion, they determined it was too late to try for a Saturday liftoff. The launch time for Sunday would be 3:20 p.m. ET — that is, if managers decide Atlantis is ready to go.
"I find this extremely frustrating," shuttle program manager Wayne Hale told reporters late Friday.
Atlantis’ countdown was halted Thursday after a pair of gauges at the bottom of the external fuel tank mysteriously failed a routine test at the launch pad.
The day-by-day delay was especially disappointing for the many visitors from the European Space Agency. Atlantis is supposed to carry a huge European-built science lab, Columbus, to the international space station.
Before a launch is allowed, NASA managers must be convinced that flight controllers and the astronauts can safely work around the fuel gauge problem while the shuttle is zooming to orbit.
Changing the standards
Since last year, the rule has been that three of the four fuel sensors must be working — before then, the requirement was four-out-of-four good sensors. On Friday, managers considered changes in their standards, in part because of some new instrumentation added to the shuttle fleet.
At first, the team talked about relaxing the requirements further, to launch even if two sensors were bad. But Hale said engineers found that a third sensor in Atlantis' tank was doing "unusual things" when the fuel was removed. That led managers to debate going back to a four-out-of-four standard, with further data factored in from instruments recently added to the shuttle fleet.
After thrashing out whether to revise the flight rules, the managers were told to "go home and think about it," then review the plan again on Saturday, Hale said.
"We don't want to get launch fever," he said.
Air Force Capt. Chris Lovett set the chances of acceptable weather for launch at 70 percent for Sunday and Monday.
NASA has until next Thursday or Friday to launch Atlantis. If it isn’t flying by then, the mission will be delayed until January because of computer concerns and unfavorable sun angles for the shuttle when it’s docked to the space station.
Engineers suspect the sensors are fine and that the problem is with an open circuit somewhere in the extensive wiring. Any repair would take days.
How the sensors work
Each shuttle fuel tank is equipped with four engine-cutoff sensors, also known as "ECO sensors," that keep track of whether the tank is empty or full of liquid hydrogen. The sensors are part of a backup system that would kick in if the tank was leaking during the climb to orbit, for example, and safely shut down the engines. The engines could ignite or explode if they kept running without fuel.
NASA has been bedeviled by these fuel-tank sensors ever since 2005, when shuttle flights resumed following the Columbia disaster. Last year, the space agency eased its sensor rules for launch, and also conducted an investigation into why the sensors failed. NASA thought they had isolated a bad batch of sensors, but Hale said he was disappointed to see the problem crop up again.
Over the past couple of years, engineers have added more instrumentation that can tell flight controllers whether the sensors are working right. If enough sensors failed during liftoff and, possibly, a sizable leak was detected, Mission Control could instruct the astronauts to manually shut down the engines early. But the procedure has never been given a test run during an actual launch.
Even though the flight rules may go back to a four-for-four requirement for the sensors, Hale said that would not rule out a launch on Sunday. In previous cases, the problems with the sensors seemed to clear up once fuel was removed from the shuttle's tank and then replaced.
"Our experience has been that they magically work," Hale said.
This report includes information from The Associated Press and msnbc.com.