CAPE CANAVERAL, Fla. (Reuters) - It's been 3.5 years and hundreds of millions of dollars since the space shuttle Columbia disintegrated. Yet NASA faces the same vexing problem that doomed the orbiter when it tries to launch shuttle Discovery on Saturday.
Insulation foam -- it seems a trivial part of launching a complex spacecraft. But the problem of falling foam has perplexed the U.S. space agency capable of doing what no other country does, landing a space freighter like an airplane.
Outfitted with a second round of upgrades to its fuel tank following the 2003 Columbia disaster, Discovery is on the launch pad and scheduled to fly at 3:49 p.m. EDT (1949 GMT) on July 1.
If successful, NASA plans up to 17 more shuttle missions, including a possible servicing call to the Hubble Space Telescope, before the shuttle fleet is retired in 2010.
If not, and if the U.S. space agency were to lose a third shuttle to disaster, NASA Administrator Michael Griffin has said he would likely pull the plug on all future missions.
NASA lost Challenger on Jan. 28, 1986, due to the failure of an O-ring seal on a solid rocket booster.
Then Columbia disintegrated during re-entry on Feb. 1, 2003, when its wing was damaged by foam debris that fell from its external fuel tank during launch.
The 1.6-pound (0.7 kg) chunk of insulating foam smashed into the ship's left wing. It pierced a hole in the orbiter's protective heat shield that caused its breakup and the loss of all seven crewmembers aboard as Columbia slammed into the atmosphere on its fatal re-entry toward Earth.
In July 2005, NASA launched shuttle Discovery to test-fly a new fuel tank design -- but the fix turned out to be nearly as troublesome.
More foam insulation, including a piece as large as one pound (0.4 kg), dislodged as the shuttle rocketed toward orbit. Luckily, none of the debris struck the shuttle.
NASA has since implemented new manufacturing techniques and removed two long foam windshields that tests showed were not necessary to protect cables and hoses running along the outside of the tank. The ramp-shaped structures had been used since the first shuttle flight in 1981.
Engineers have also reworked wiring for heaters that were installed as part of the first redesign. The heaters replaced foam insulation where the tank is attached to the shuttle.
After Discovery's 2005 flight, NASA discovered that insulation around the wiring had trapped air, which turned to a liquid in the cold and filled tiny voids in the foam. As the pressure changed during launch, the liquid transitioned to gas and popped off pieces of foam.
But the foam debris issue still has not been resolved. And NASA's top safety official, along with its chief engineer, both recently voted against clearing Discovery for its July 1 launch. They were overruled by NASA administrator Michael Griffin and other top managers.
The shuttles have flown 114 space missions, and only two have ended in tragedy. But 14 astronauts died in the Challenger and Columbia disasters and two of the five shuttles built were lost. They cost around $2 billion each and are viewed as irreplaceable.
As engineers learned more about the physics of foam, NASA realized it was not going to be possible to manufacture a shed-free tank.
The foam is needed to prevent ice buildups as the shuttle sits awaiting liftoff with cryogenic propellants in its fuel tank. Ice can be even more dangerous than foam if it breaks off during launch and hits the spacecraft.
"Foam will come off. There is no way around that. It's the very nature of the material and the way that we use it and the way we apply it," said John Chapman, who oversees the external tank project at NASA's Marshall Space Flight Center in Huntsville, Ala.
But Chapman said steps had been taken to minimize foam falling off and prevent the dislodging of chunks large enough to damage the shuttle's heat shield.
The shuttle lifted off under the gaze of dozens of sensors and cameras last year and will have even more this year.
It also has been outfitted with stronger windows, beefed up tires and landing gear and more than 5,000 new cloth fillers between the most critical heat-resistant ceramic tiles on its belly.
During Discovery's previous flight, photographs taken by space station crewmembers as the shuttle approached showed two fillers slightly protruding from the smooth surface.
Engineers grew concerned that the tiny misalignment could cause the shuttle to heat up earlier than expected during its plunge through the atmosphere and possibly damage its heat shield. The shuttle crew conducted an unplanned spacewalk to remove the cloth strips.
Cameras, both aboard the shuttle and on the ground, will photograph and videotape every second of the launch, while sensors aboard the ship will monitor vibrations, speeds, temperatures and other data.
Still, when 2.5 million moving parts catapult through the atmosphere, the odds of something going terribly wrong remain a sobering one in 100, said shuttle program manager Wayne Hale.
"If you're not scared when we fly the shuttle, you're not understanding what's going on," he said.