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Sources: NASA exploring shuttle optionsStill no firm launch date for Discovery
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YOUR E-MAIL ALERTS(CNN) -- Despite ongoing troubleshooting throughout the weekend and into Monday, NASA engineers still have not reached any firm conclusions as to what's wrong with a faulty hydrogen fuel sensor that prompted the space agency to scrub the launch of the shuttle Discovery last Wednesday. But, as the first space shuttle mission since the Columbia disaster nearly 2 1/2 years ago comes down to the wire, NASA officials reiterated their determination not to let "launch fever" compromise safety. Discovery's crew was onboard and strapped in on July 13 when one of four redundant engine cutoff or "ECO" sensors malfunctioned. The faulty sensor indicated the shuttle's external fuel tank, which had been filled just hours before, was running dangerously low on liquid hydrogen propellant. Though the three other sensors were functioning normally, and only two are needed to achieve orbit, NASA scrubbed the launch because procedures require all of the ECO sensors to be operational for liftoff. NASA sources familiar with the ongoing engineering discussions told CNN that shuttle managers are considering several options on how to proceed: -- Engineers could decide to do another "tanking test," in which super-cold liquid oxygen and hydrogen fuel would be pumped into the external tank to see how the tank and its systems responded to launch-day conditions. NASA conducted two tanking tests earlier this spring, with the hydrogen fuel sensor problem cropping up in the first one but not the second. For reasons mostly unrelated to the sensor malfunction, NASA decided to roll Discovery back to the Vehicle Assembly Building and swap out the external tank for another one. Engineers concluded at the time -- mistakenly, it turned out -- that the intermittent problem with the sensor would almost certainly be fixed by the swap-out. If a third tanking test were done, engineers would switch the wires that connect the malfunctioning sensor with one of its siblings. If the errant sensor continued to malfunction, the problem would be in the sensor, and engineers would know they would have to go into the external tank to fix it. If the malfunction "moved, " the problem would probably be in the electronic circuitry and junction boxes of the orbiter. These electronics are considered to be in excellent condition, but as they date from the 1980s, aging is an issue. -- To buy themselves a little more time, NASA officials are debating whether to extend the end of the current launch window from July 31 to August 4. NASA committed to daytime launches for the first two return-to-flight missions to ensure ideal lighting conditions during liftoff for the new and improved network of cameras that are to photograph the shuttle's ascent. The external tank separation that occurs when the shuttle is nearly in orbit would be in deep shadow during an August 1 to 4 launch, but some at NASA think digital photographs taken out the window of the orbiter by Discovery crew members could compensate for any dark pictures. -- Shuttle program managers could decide to launch with only three operative ECO sensors. Launch controllers had planned for a five-minute window in which to launch Discovery on a rendezvous trajectory with the international space station. Under this option, the shuttle would need to launch in an instantaneous window that would permit it to achieve the desired orbit using a minimum amount of fuel. An in-flight malfunction of three ECO sensors during ascent could lead to two different but potentially catastrophic outcomes. If the sensors "failed wet," they would fail to determine when fuel levels were running dry, and the fuel pumps would continue to operate, even after all the fuel is exhausted. In such a situation, the engines could overheat or seize up, resulting in an explosion. Alternatively, the sensors could "fail dry," meaning they would erroneously determine that fuel reserves were dangerously low. In that event, computers would automatically shut down the main engines, potentially leaving the shuttle short of achieving orbit and in need of an emergency landing. Officials had hoped that troubleshooting efforts would lead to a quick resolution of the problem, but so far the source of the malfunction remains elusive. Discovery continues to sit on launch pad 39B, with workers doing all they can to work the problem there. A rollback to the Vehicle Assembly Building (VAB) would eat up valuable time and could push the launch beyond the current launch window, which closes July 31. The shuttle fleet has been grounded since February 1, 2003, when Columbia broke apart over Texas while on landing approach to Florida's Kennedy Space Center, killing all seven astronauts on board. Seven months later, the Columbia Accident Investigation Board (CAIB) concluded that foam insulation broke off during launch from the shuttle's external fuel tank, striking and cracking a panel on the orbiter's wing. When the shuttle re-entered the Earth's atmosphere 16 days later, searing hot gases seeped into the wing and incinerated the spacecraft. Discovery's seven-person crew is led by retired Air Force Col. Eileen Collins, NASA's first and only female shuttle commander. Over the course of the 12-day mission, shuttle astronauts will test a battery of new tools and techniques that NASA engineers developed in the aftermath of Columbia to inspect the shuttle's critical thermal protection system (TPS) for any damage that might have occurred during liftoff. Discovery will also dock with the international space station to deliver much-needed equipment and supplies If Discovery does not launch during the current launch window, the next opportunity will be September 9 to 25. 
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