Cape Canaveral, Fla. Space shuttle managers will have one eye on the weather and the other on a temperamental fuel-level sensor as NASA makes its second attempt to launch Discovery this morning.
The long-awaited 114th shuttle launch would be the first since the Columbia accident 2 1/2 years ago.
"All of our hardware is ready," NASA Test Director Pete Nickolenko said. "The launch and flight teams are ready and our flight crew is ready for a successful mission."
Forecasters are predicting a 60 percent chance of acceptable weather. Rain showers and storm clouds are the main concerns.
Florida isn't the only place where the weather is an issue. At least one of the shuttle's three emergency runways in Europe must have good conditions at launch time. Two of them, in Zaragoza, Spain, and Istres, France, are forecast to have unfavorable weather. The third, in Moron, Spain, is predicted to have winds just below the limit.
The launch team also is closely watching four fuel sensors in the shuttle's liquid hydrogen tank that are part of a backup safety system. If a serious problem arises, the sensors cut off the shuttle's three main engines before the ship's fuel supply runs dry.
Running the engines without fuel could lead to disaster.
One of the four sensors failed during the countdown for a previous launch attempt on July 13. A week and a half of troubleshooting has yet to pinpoint the problem. However, engineers have come up with a strategy to press ahead.
Technicians switched the circuit leading to the failed sensor with another one. If either of those sensors malfunctions during today's countdown in the same way as before, mission managers likely would proceed. That's provided they are confident they understand the problem.
Any other type of sensor failure would stop the launch, deputy shuttle program manager Wayne Hale said Sunday.
Flight controllers planned to start testing the sensors shortly after fueling of the shuttle's external fuel tank began around 12:30 a.m. today. About 7:15 a.m., launch computers will do an automated check of the sensors, commanding them to switch back and forth from indicating "wet" to "dry." Another test will be done during the final hold in the countdown less than an hour before launch.
"We're going to continuously check the validity of the signal from the sensor all the way to the orbiter's computers during the tanking, so if at any time any one of these circuits fails, we will know about it," Hale said Sunday.
Discovery's seven astronauts are set to embark on a 12-day mission that will test improvements made to the shuttle since the Columbia accident. The flight also will deliver 29,000 pounds of hardware and supplies to the international space station.
The White House announced Monday that first lady Laura Bush was expected to be among the VIPs attending today's launch. Fewer members of Congress will be here than on July 13, however, because lawmakers are wrapping up their final week of work before the August recess. Many plan to watch instead from screening rooms on Capitol Hill.
Local law enforcement agencies are expecting tens of thousands of spectators to line roads and waterways around Kennedy Space Center to see the liftoff. But the media contingent covering the launch has thinned considerably, perhaps by a third or more, since the previous attempt almost two weeks ago.