All systems go for historic shuttle launch

No sign of glitch
During Tuesday morning's tests, the system was set artificially to give an "empty" or "dry" reading. If one of the two sensors involved in the switch didn't show the right reading, that would help engineers narrow down the root of the problem — and mission managers would be willing to go ahead with the countdown as long as they feel the glitch is well-understood. But if any other kind of problem arose, the countdown would be stopped.

All the sensors were "performing as expected" as the clock ticked down, NASA reported. An automated test at T minus 3 hours verified that the sensor system could operate correctly in different modes, as did a final check during a built-in hold at T minus 9 minutes.

After the fueling, an inspection crew went to the launch pad to check the tank for any signs of excessive ice buildup — but found nothing that would prevent launch.

Meanwhile, Discovery's crew seemed to be in a jovial mood, wearing Hawaiian-style shirts as they sat down for a photo opportunity. NASA reported that commander Eileen Collins had a light midnight breakfast of toast and mixed fruit, while other crew members selected from fruit, English muffins and sandwiches.

NASA TV A worker helps strap in a Discovery crew member Tuesday.

Later in the morning, the astronauts traded the gaudy shirts for their bulky orange spacesuits. They walked out of their quarters onto a waiting van, cheered by onlookers and guarded by a gun-toting SWAT team as well as a hovering helicopter.

Within minutes, the convoy pulled up at the base of Launch Pad 39B, and one by one, the astronauts were helped into their seats to await liftoff. Launch-pad workers sealed the hatch at about 9 a.m. ET.

Safety first
The main focus of this shuttle mission is to test new safety procedures in the wake of the Columbia tragedy in 2003, which investigators traced to damage done to the shuttle by a piece of foam insulation flying off the external fuel tank shortly after launch. The breakup of the shuttle during its re-entry killed all seven astronauts aboard and led to the grounding of the rest of the shuttle fleet.

Since then, NASA has made dozens of upgrades in the shuttle and the fuel tank, and has also taken measures to reform its "safety culture." More than 100 cameras will record Discovery's launch, and engineers have developed a new extension boom for inspecting the shuttle in orbit. During the first of three spacewalks, astronauts will test techniques that could eventually be used to repair damage to the shuttle during this mission.

However, a task force monitoring the return to flight determined that NASA has not yet met three of the Columbia Accident Investigation Board's recommendations for heading off potential damage from ice or foam flying from the fuel tank, or for repairing such damage.

The idea that NASA would consider launching the shuttle with an unresolved glitch raised additional questions about the agency's commitment to safety. When asked about the controversy over the fuel-gauge glitch, NASA Administrator Mike Griffin said that he felt comfortable with the process mission managers went through, and that he was more concerned about "doing what's right, not what necessarily is obvious or popular."

Looking ahead
If the countdown is halted Tuesday, due to a last-minute glitch or a change in the weather, mission managers could try again Wednesday. In all, four launch attempts could be made between now and the end of the month — with a possible extension of the launch window several days into August. After that point, NASA would have to wait until September, due to the lighting conditions and the position of the international space station.

Discovery's crew is due to take up tons of supplies to the station — and bring back tons of trash from orbit. In between, spacewalkers are to install a replacement guidance gyroscope on the station, as well as a storage platform for future construction jobs.

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