All systems go for historic shuttle launch

Alan Boyle Science editor • Profile • E-mail

CAPE CANAVERAL, Fla. - The shuttle Discovery's seven astronauts received the final "all systems go" Tuesday for NASA's first shuttle launch in more than two years.

"I think our long wait may be over," launch director Mike Leinbach told shuttle commander Eileen Collins. "Good luck, Godspeed, and have a little fun up there."

Discovery is due to lift off at 10:39 a.m. ET, beginning a 12-day mission to resupply the international space station and test safety procedures that were developed in the wake of the shuttle Columbia's catastrophic breakup in February 2003.

Prospects for a trouble-free liftoff brightened in the morning, with picture-perfect weather and no sign of the fuel-gauge glitch that stopped Discovery's first countdown 13 days earlier.

Among the VIPs attending the launch are first lady Laura Bush and her brother-in-law, Florida Gov. Jeb Bush, as well as families of the fallen Columbia and Challenger astronauts. Thousands lined viewing areas on Florida's Space Coast.

Weather forecasters said there was a 90 percent chance of acceptable conditions for launch, an improvement over previous forecasts.

There was also positive news about Discovery's external fuel tank: After filling the tank with 526,000 gallons (2 million liters) of cryogenically cooled liquid hydrogen and oxygen, controllers reported that all of the tank's fuel-level sensors were working just as expected.

During Discovery's first launch attempt on July 13, one of the tank's low-level sensors appeared to give bad readings, forcing mission managers to stop the countdown. The problem went away once the tank was emptied, and for days, hundreds of engineers struggled to figure out what caused the intermittent glitch.

The low-level sensors work roughly like a fuel gauge in an automobile, alerting the shuttle's computers when propellants are close to running out. If the system reads "empty," the computers would start shutting down the main engines.

Failure of the system could shut off the engines too early or keep them running on empty, risking serious damage. The sensor system is designed to work even if two of the four low-level sensors go bad, but since the Challenger explosion in 1986, NASA has required all four to be in working order before launch.

Engineers narrowed down the causes of the glitch to electromagnetic interference or grounding problems that have since been fixed.

But the only way to make sure the glitch had disappeared was to check the sensor system under cryogenic conditions, mission managers said. So on Tuesday, the launch team set up the same situation that resulted in the bad readings during the earlier countdown. They also switched the connections between the questionable sensor and a good sensor.

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