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NASA shoots for Tuesday shuttle launchCountdown starts Saturday; engineers continue looking at glitch
 The new countdown for the shuttle launch will begin Saturday afternoon. RELATED
SPECIAL REPORTYOUR E-MAIL ALERTSKENNEDY SPACE CENTER, Florida (CNN) -- NASA officials decided Wednesday to go ahead with the launch of the space shuttle Discovery Tuesday morning, even though they have still not definitively found what caused a fuel sensor malfunction that forced the mission to be scrubbed last week. Shuttle program manager Bill Parsons said space agency officials were "comfortable" with moving toward launch while the grounding is repaired and additional tests are conducted. "We believe the best way to go through this is to do a countdown. If the sensors work like we think they will, then we'll launch on that day. If anything goes not per the plan we have in front of us, then we'll have a scrub," he said at a news conference. The countdown will begin Saturday afternoon, with liftoff scheduled for 10:39 a.m. Tuesday. NASA engineers suspect electronic grounding problems or electromagnetic interference to be the most likely causes of the problem. John Muratore, NASA's shuttle systems manager, said that as a safety measure, engineers plan to develop a detailed program of monitoring and testing for the the run-up to the launch to detect any additional problems with the fuel tank sensor. Last Wednesday, less than two and a half hours from liftoff, Discovery's launch was 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. The malfunctioning sensor is one of four sensors on the vehicle. The mission had to be postponed because NASA's flight protocols required all four to be working. The redundancy is designed to prevent a misreading of fuel levels in the event multiple sensors fail, which could cause the spacecraft's engines to overheat and explode or leave it short of its necessary orbit. Parsons said NASA officials, as a contingency, are preparing for the possibility of waiving the requirement and sending up Discovery with just three working sensors. However, he said no decision has been made to change the protocol, and he said it wouldn't be done unless engineers are confident they understand the problem, even if they can't isolate it. Since the mission was postponed, Discovery has been sitting on its pad at the Kennedy Space Center while NASA engineers around the country worked feverishly to determine the cause of what they call an "intermittent anomaly." Muratore said tests at the Johnson Space Center in Houston indicated that electromagnetic interference might be a cause of the problem, and more tests will be conducted on the Discovery vehicle to test that hypothesis. However, because some of the equipment on the shuttle can only be safely used during the launch process, patterns of electromagnetic interference can only be totally replicated with Discovery in launch mode, he said. Also at launch, the shuttle vehicle has a different temperature profile because the fuel tanks are loaded with super-cooled hydrogen and oxygen. One test that NASA engineers plan to run before launch is to switch the electronic junction box attached to the malfunctioning sensor to one of the other sensors, in an effort to determine whether the problem lies with the electronics or the sensor itself. While a problem with the electronics might be rectified before launch, or at least give NASA managers confidence to go forward with three sensors, a problem in the sensor -- which is located in the bottom of the external fuel tank attached to Discovery's orbiter -- would likely result in a launch delay, Parsons said. 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. The current launch window for Discovery ends July 31, after which the launch might have to be postponed until at least September 9. This 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 seven-person crew is led by retired Air Force Col. Eileen Collins, NASA's first female shuttle commander. Over the course of the planned 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 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. If the launch takes place next Tuesday, the shuttle will also dock with the space station to deliver much-needed equipment and supplies 
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