CAPE CANAVERAL, Fla. - For the second time in two weeks, the space shuttle Discovery is ready for launch to the international space station, on the first shuttle mission since the Columbia disaster two and a half years ago.
It may seem like deja vu all over again, but there's one big difference for Tuesday's launch attempt: On a flight designed to signal NASA's heightened commitment to safety, mission managers are geared up to grant a launch-pad exception to its rules, for the first time in recent memory.
The exception would clear Discovery for launch if a fuel-gauge glitch that arose during the previous countdown turns up again. The last time, on July 13, bad readings from one of the fuel sensors forced a postponement of liftoff, setting off a full-bore effort by hundreds of engineers to track down the glitch's cause.
They narrowed down the cause to grounding problems in the circuitry and electromagnetic interference — but in the end, they could only assume that they had resolved the problem.
"We've done an extensive degree of troubleshooting and analysis ... to really as best we can understand what we've got," test director Pete Nickolenko told reporters Monday at NASA's Kennedy Space Center. "I believe that we're ready, and I feel very confident going into the terminal countdown that the entire team's ready to execute this mission. And we fully expect that it should work as designed."
Nickolenko said there were "no significant issues or problems" being worked, and preparations are on track for launch at 10:39 a.m. ET Tuesday.
But first, Discovery has to pass a crucial test beginning in the wee hours of Tuesday morning, soon after the launch team starts loading the external fuel tank with liquid hydrogen and oxygen. Mission managers admit there's a chance that the fuel-gauge glitch might arise only when the tank contains the superchilled propellants — and so they'll start duplicating the conditions that set off the glitch almost as soon as the liquid hydrogen comes up to the sensors' level.
Last time, one of the four low-level sensors appeared to give bad readings during a simulated "empty" condition. Under NASA's current rules, all four sensors have to work in order for launch to proceed — even though the sensor system is designed to work with just two good sensors.
This time, the connection to the questionable sensor, known as No. 2, has been swapped to a good sensor, No. 4. And this time, the launch team will simulate the "empty" or "dry" condition for most of the time that the tank is filled.
If either the No. 2 or the No. 4 sensor registers bad readings in the simulation — that is, "wet" when it should read "dry" — mission managers will be able to narrow down the problem further. In fact, deputy shuttle program manager Wayne Hale said "some of us are kind of hoping" that the glitch reappears, so that engineers can figure out definitively whether it lies in the shuttle's circuitry or the tank's wiring.
With a single glitch, the managers are prepared to sign a waiver and allow the countdown to go forward with three good sensors, Nickolenko said. But they'd hold up the launch if other, unexpected problems crop up — for example, if both No. 2 and No. 4 show bad readings, or if the other sensors exhibit problems.
Nickolenko said he couldn't recall a time when a waiver had been granted under such conditions on launch day, and NASA representatives said they were reviewing their records to find out whether there was any precedent.
The safety issues are particularly sensitive this time around because of all the changes in safety procedures made in the wake of the Columbia tragedy in 2003. Just after the launch of that mission, a piece of foam insulation flew off the fuel tank and hit the shuttle's left wing. At the time, engineers thought no damage was done. But after the catastrophic breakup of the shuttle — in which Columbia's seven astronauts lost their lives — investigators determined that the debris cracked a hole in the wing, allowing hot atmospheric gases to enter and destroy the craft from within.
NASA has made dozens of upgrades in the shuttle and the fuel tank since then, 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 12-day mission's 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 prospect of launching with a known defect could add to the perception that NASA is potentially cutting corners on safety.
When asked about such perceptions, NASA Administrator Mike Griffin said that he felt "comfortable" with the process mission managers have gone through, and that he was more concerned about "doing what's right, not what necessarily is obvious or popular."
An unexpected glitch isn't the only thing that could delay launch: Anvil clouds or storms in the vicinity of Kennedy Space Center also could force a scrub. Launch weather officer Kathy Winters put the chances of acceptable weather for liftoff at 60 percent.
If the countdown is halted Tuesday, 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.
In addition to testing safety, inspection and repair procedures, Discovery's crew is due to bring up tons of supplies to the station. Spacewalkers are to install a replacement guidance gyroscope on the station, as well as a storage platform for future construction jobs.