Discovery Primed for Liftoff Today

CAPE CANAVERAL, Fla. —  Space shuttle managers will have one eye on the weather and the other on a temperamental fuel sensor as NASA makes its second attempt to launch Discovery this morning.

Liftoff is scheduled for 7:39 a.m. PDT. The long-awaited 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% chance of acceptable weather. 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 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 probably will proceed, provided they are confident they understand the problem.

“It’s an acceptable risk, and actually it’s quite a low one,” NASA Administrator Michael D. Griffin told Associated Press late Monday.

If the same fuel gauge problem occurs, “we would be good to go,” he said. “If we see some other signature than what we saw before, then we’re not going. We’re absolutely not going.”

Flight controllers planned to test the sensors shortly after fueling of the shuttle’s external fuel tank was to begin early today. Another test will be done less than an hour before launch.