Cape Canaveral, Fla., 13 July 2005--Today's planned launch of the space shuttle Discovery was scrubbed with just two and a half hours to go. The crew had already boarded and was being strapped into their chairs by NASA's close out crew when Kennedy Space Center's firing room, which controls launches, detected a failure in one of four sensors in the massive orange external tank that supplies propellants to the space shuttles main engines.
These sensors warn when the fuel level in the tank is running low during flight and allow the shuttle's engines to be shut down before the fuel is completely exhausted. If the engine were to continue running on empty there is a strong risk of what NASA calls an "uncontained failure," which in practice means anything from an individual engine meltdown to an explosion.
Diagnosing the problem requires emptying the external tank, meaning that there was no way they could have the shuttle ready again to fly before the 3:51 deadline. The shuttle would have to have launched within five minutes of this time if it was to have any hope of rendezvousing with its destination, the International Space Station.
The sensor problem came as surprise on a day when the weather was perceived as the main risk to a launch. The United States Air Force's meteorological service provides weather forecasts for NASA's launches. Its blue shirted officers were mobbed at the press site when it was announced around 11 am that the chances of conditions being suitable for launch had fallen from 60 per cent to 40 per cent. Things looked especially grim around noon, when a thunderstorm passing over the area prompted NASA officials to order journalists to take cover indoors for fear they would be struck by lightening. However, the downpour missed the Discovery sitting on its launch pad and the crew began boarding on schedule around 12:30 pm, only to have to disembark an hour later when the sensor failed.
It will not be until later this evening, when engineers get a chance to examine the faulty sensor, that we will know if the shuttle can make another attempt tomorrow afternoon. If extensive repairs are needed it could be next week at the earliest before another attempt is made. The Discovery can continue making attempts up until August 1st, after which date, new mission rules that mandate that launches and landings must occur during daylight, mean the next available chance to launch will not occur until September.