CAPE CANAVERAL, Fla. - A 3 1/2-inch-long gouge in Endeavour's belly did not endanger astronauts in the shuttle's safe landing, but NASA is looking ahead to three more launches at risk for the same kind of damage.
Endeavour touched down Tuesday for an early homecoming prompted by a hurricane -- ending a nearly two-week orbital drama that centered on the gouge.
The gouge in Endeavour, carrying Christa McAuliffe's backup for the doomed Challenger flight in 1986, helped keep NASA officials acutely aware of past shuttle disasters.
Back before Columbia flew its last mission four years ago, NASA knew it had a foam problem with its fuel tanks but never imagined a piece of the airy insulation could severely wound a space shuttle.
Columbia shattered during re-entry to Earth's atmosphere, just five days before engineers were to propose possible repairs.
This time, NASA knew it had a foam problem with brackets on its fuel tanks but never imagined a stray piece would ricochet off the tank and smash into the shuttle.
Retired Navy Adm. Harold Gehman Jr., who headed the 2003 Columbia investigation, was reluctant to comment this week on the troublesome brackets, which support the fuel feed line on the tank. He said he didn't have enough information.
But he observed: "You have to assume things are going to happen and you have to mitigate the consequences, that's what our report was all about."
Endeavour's gash, although deep, was too small for scorching atmospheric gases to penetrate and cause serious damage, mission managers said during the flight. It was also on the belly, a more benign area than the nose or wings, which are subjected to much higher heat. The platesize hole that brought down Columbia pierced the left wing.
Commander Scott Kelly said he was "a little bit underwhelmed" when he saw the gouge for himself after touchdown. "We knew how big it was conceptually. We were told the dimensions. But to see it, it looked rather small," he told reporters.
Officials who checked out Endeavour on the runway said there was no apparent charring to the exposed felt fabric, the last barrier before the aluminum frame.
But now NASA finds itself playing catch-up. It's analyzing a variety of temporary bracket solutions, which may or may not be in place before the next space station construction mission in late October.
Making the brackets with titanium, which would require far less foam insulation than the aluminum version, is the permanent solution ordered after the problem first cropped up last summer.
But that won't happen until next spring. By then, NASA will be just two years from retiring its three remaining space shuttles after wrapping up a demanding schedule for finishing construction of the international space station.
Engineers are considering several short-term options: shaving some foam from the brackets or possibly applying an oil to the foam to reduce condensation and ice buildup.
Because the bracket problem has intensified for the launches since Columbia, engineers theorize it might be due to the one-hour earlier start of fueling -- a new rule intended to provide more time for ice checks. That extra hour that the super-cold fuel is in the tank could be allowing more undetected ice to form, which then can cause the neighboring foam to pop off.
NASA Administrator Michael Griffin said after Endeavour's landing that he will need to be satisfied that any change "is necessary and, in fact, beneficial" before ordering modifications. The last thing the space agency wants to do is to change something and make it worse.
NASA still doesn't know whether the debris that smacked Endeavour was foam, ice or a combination of both. Whatever it was, it broke off the bracket, fell nearly 25 feet onto a strut lower on the external fuel tank, then shot into Endeavour's belly.
Each fuel tank is covered with 4,000 pounds of foam and each bracket has only ounces of foam on it, NASA's space operations chief Bill Gerstenmaier said. "We've really taken this huge complex problem ... and we've really shrunk it down to just very, very few areas, essentially ounce-size pieces of foam that we need to go work with."
A NASA veteran who now chairs the mission management team, John Shannon, is quick to point out that once the problem was discovered, the difference in the way the Columbia and Endeavour flights were handled was "night and day."
Columbia's damage was not seriously addressed by mission managers and worried engineers did not speak up.
Endeavour's much smaller damage was analyzed for a full week using every resource available. A group of engineers went against the tide and argued it would be better to fix the gouge than return with it unmended -- because of concerns about damage to the returning shuttle. By that time, the experts had ruled out risks to the crew.
