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NASA has no luck finding glitchNext week earliest possible launch date
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SPECIAL REPORTYOUR E-MAIL ALERTSKENNEDY SPACE CENTER, Florida (CNN) -- Five days of meticulous detective work by NASA engineers have failed to turn up the cause of a fuel sensor malfunction on the space shuttle Discovery, which is still sitting on its launch pad as precious days tick away toward a July 31 launch deadline. NASA officials are looking at next week as the earliest possible launch date. "It's difficult to find a glitch that won't stay glitched," said Bill Parsons, the space shuttle program manager, at a news conference Monday evening. To help find the malfunction, NASA has brought out of retirement an engineer who designed part of the sensor system back in the 1970s, said Wayne Hale, deputy shuttle program manager. NASA officials will decide by Wednesday whether they need to fill the shuttle's external tanks with super-cooled hydrogen and oxygen to help isolate the problem, which would delay a launch until at least July 26, he said. The current launch window for Discovery ends July 31, after which the launch might have to be postponed until at least September 9. NASA officials are debating whether the window could be extended to August 4, to give them more time to find and fix the sensor malfunction. The current launch window was chosen to provide ideal lighting conditions so that Discovery's ascent can be photographed by a new network of cameras installed on the vehicle, which will help the space agency evaluate design changes made after the 2003 Columbia disaster. In the first days of August, Discovery would be in deep shadow at the point where the external tanks separate from the orbiter. However, some NASA officials believe digital photographs taken out of the window of the orbiter by Discovery's crew could compensate for any dark pictures. Discovery's mission would be the first space shuttle flight since its sister ship, Columbia, disintegrated over Texas on February 1, 2003, killing all seven astronauts on board. NASA shut down the program and made numerous design changes and safety improvements to the shuttle fleet recommended by a blue-ribbon panel that investigated the disaster. Last Wednesday, less than two and a half hours from liftoff, Discovery's mission had to be scrubbed when a pre-flight test showed a sensor in the hydrogen fuel tank was not working correctly. To test the sensor, a command was sent that should have made it show the tank as empty, when it was actually full. But the reading didn't change. NASA engineers believe the problem lies somewhere along the path from the sensor in the fuel tank to computers on the orbiter which process the information, but they have so far been unable to isolate what they term an "intermittent anomaly." Tests of an electronic box in the orbiter that captures data from the sensor were successful, and checks of the wiring in the orbiter and external fuel tank, as well as the connections between the two, also turned up no problems, said Ed Mango, deputy director of NASA's orbiter project office. So far, the testing that has been done has been conducted at normal temperatures, rather than under temperature conditions at launch, when the super-cooled fuel is loaded in the tanks. The unloaded testing should be completed by Wednesday, at which point the decision will have to be made whether to perform more tests with the tanks loaded, known in NASA parlance as a "tanking test." Hale said the earliest a tanking test could be performed would be next Tuesday. Once the tanks are loaded, an immediate launch would be possible, although Hale said a more likely scenario would be to launch the following day. The sensor that malfunctioned is one of four that monitor hydrogen levels in the fuel tank. Launch protocols require that all four be working at launch, to provide redundancy in case more than one sensor fails. Hale said Monday that prior to the Challenger disaster in 1986, NASA's flight rules did allow a space shuttle to fly with just three of the four sensors working. That was changed after a post-Challenger evaluation showed that a single malfunction could knock out two sensors because of the way the system was designed, he said. If the problem cannot be isolated, NASA officials could consider allowing Discovery to fly with just three sensors working properly, although Hale said the space agency would much prefer to find and fix the problem. An in-flight malfunction of the sensors could lead to two different, but potentially catastrophic, outcomes. If the sensors fail to indicate that fuel levels are running low, fuel pumps could continue to operate after the fuel is exhausted, which could cause an explosion if engines overheat or seize up. On the other hand, if the sensors indicate fuel is running low when it isn't, on-board computers could shut down the shuttle's main engines, which could force the craft to make an emergency landing if it fails to achieve orbit. To compensate for a missing sensor, NASA might have to more rigidly time the launch so that Discovery gets to its desired orbit using a minimum of fuel. If NASA officials decide to proceed with the tanking test, engineers are considering switching the wiring between the faulty sensor and one of its working siblings, to determine whether the problem lies with the sensor or the electronics attached to it, Parsons said. Discovery's seven-person crew is led by retired Air Force Col. Eileen Collins, NASA's first female shuttle commander. Over the course of the 12-day mission, the crew will test a battery of new tools and techniques NASA engineers developed in the aftermath of Columbia to inspect the spacecraft's heat-resistant exterior tiles for any damage that might have occurred during liftoff. The shuttle will also dock with the space station to deliver much-needed equipment and supplies The panel that investigated the Columbia disaster concluded that a piece of insulating foam fell off the external fuel tank during liftoff and struck the shuttle's wing, cracking the tiles. The breach caused Columbia to disintegrate during the heat of re-entry. CNN's Kate Tobin contributed to this report. 
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