Discovery's Goal: A Quiet Trip

After exhaustive wind-tunnel tests and computer analyses, engineers determined that "we are safe to fly structurally" without the PAL ramps, Hale said, and they were removed. Getting rid of the ramps, he added, is "the largest aerodynamic change that has ever been made in the history of the shuttle."

What engineers did not do, however, was find a satisfactory way to redesign foam insulation for 34 metal brackets -- known as ice frost ramps -- that hold the fuel pressure lines and electrical cables in place. Without PAL ramps to shield them, they will be exposed to different aerodynamic forces than they were in previous flights.

"We know that pieces [of foam] can come off and can be harmful," Hale said. "It's a hazard we know we need to fix," he added, but planners decided to go ahead with the current launch and redesign the brackets later.

This was not a decision taken lightly or quickly. Mission planners first came to an impasse at a meeting in late April, with dissenters arguing that "as-is" brackets were too dangerous and that the shuttle should fly only after engineers had redesigned the brackets and tested them.

Griffin carried the day at that meeting by pointing out that only one major change -- removing the PAL ramps -- should be flight-tested before undertaking a second.

But the impasse reappeared during subsequent meetings, culminating in the dissent by O'Connor and Scolese at Discovery's flight readiness review on June 17. Again Griffin cast the deciding vote, this time arguing that crew members would not be in danger because they could make on-orbit repairs -- or, if that proved impossible, take refuge on the space station.

O'Connor appeared to accept this judgment, suggesting that he had maintained his opposition because he wanted Griffin to be put on record. "If the administrator could accept it," O'Connor said at a news conference last week, "I felt I was not going to throw my badge down."

The lingering dilemma, described by External Tank Project Manager John Chapman, is that the ramps must be aerodynamically sound without the PAL ramps, while insulating the metal brackets to keep ice from forming on them as they are cooled by the liquid hydrogen and liquid oxygen inside the tank. Having ice fall from the tank during launch could be even more hazardous to the orbiter than if foam broke off the tank.

"You always want to guard against unintended consequences," Chapman told reporters at Johnson Space Center. "You want to make sure you understand fully before you make any change at all." When redesigned bracket insulation performed worse in wind-tunnel tests than the existing version, engineers stayed with the original.

Discovery's upcoming mission is the second of what NASA has deemed "test flights" to evaluate post-Columbia safety upgrades. But it also resumes the shuttle's main task -- assembling and operating the space station.

Since Columbia, the station has been in something of a caretaker state, with the two crew members -- one American and one Russian -- spending much of their time keeping vital operating systems in running order with minimal resupply from Russia's Progress cargo spacecraft.

This will change when Discovery delivers European Space Agency astronaut Thomas Reiter to join Commander Pavel Vinogradov and flight engineer Jeffrey Williams, bringing the station crew back up to its normal pre-Columbia complement. Reiter, a German national and veteran spacewalker aboard Russia's former Mir space station, will stay aloft for about six months, returning to Earth aboard a later shuttle flight.

Besides Reiter and Lindsey, who flew the shuttle twice before as pilot and once as mission commander, Discovery's crew includes pilot Mark E. Kelly and spacewalker Piers J. Sellers, a British-born astronaut. Each will be flying on his second mission.

Three rookies round out the crew: Lisa M. Nowak and Stephanie D. Wilson, who will handle cargo and operate "robotic arm" cranes during damage surveys and spacewalks; and the second spacewalker, Michael E. Fossum.

The mission includes two scheduled 6 1/2 -hour spacewalks for Sellers and Fossum, focusing on equipment testing and repair, and will add a third if there is enough fuel and other consumables for an extra day in orbit.

If a third spacewalk takes place, Sellers and Fossum will test the effectiveness of a black adhesive called "Non-Oxide Adhesive Experimental," or "NOAX," in repairing cracks to the reinforced carbon-carbon heat shielding.