The external fuel tank is the biggest piece of the space shuttle, a rust-colored torpedo 154 feet tall that weighs 1.6 million pounds fully loaded. For two years, its fatal flaws have drawn NASA's attention like nothing else under the agency's purview.
Judgment day, NASA hopes, will come as early as May 15, when a three-week launch window opens for a shuttle to lift off from Kennedy Space Center for the first time since Columbia disintegrated on reentry over Texas on Feb. 1, 2003.
The new external fuel tank for the shuttle Discovery -- the product of an extensive redesign -- is wheeled out of the barge at Kennedy Space Center.
(Peter Cosgrove -- AP)
Sensors, cameras and eyes will be fixed on Discovery's external tank to see if two years of fixes have done the job. Will the force of launch rip pieces of foam insulation from the tank? For sure. Will they be big enough to cause serious, and perhaps fatal, damage, such as that from the chunk that breached the leading-edge heat shielding on Columbia's left wing? Almost certainly not.
Yet even today the external tank remains a vexing and in some respects intractable piece of equipment, saddling engineers with 1970s shortcomings that are impervious to modern solutions. Despite millions of computer simulations, NASA's new toolbox still includes a hand sprayer, a putty knife and a swab.
NASA's inability to guarantee the external tank's safety lies at the center of the debate over plans to resume human spaceflight, complete assembly of the international space station and eventually move ahead on President Bush's initiative to explore the moon and Mars. For the future of space exploration, let alone the safety of the astronauts, the fixes need to work.
Some have urged caution, and perhaps a delay, in launching Discovery to buy more time to work on safety, but NASA in recent months has had some success making the point that "space is hard," and the space shuttle, even after 113 flights, is an experimental program.
"If you want certainty, you're in the wrong business," said Wayne Hale, deputy space shuttle program manager. "We are doing our level best to provide a maximum degree of confidence, but certainty is something we don't have a lot of."
The external tank has dictated almost everything that NASA has done for two years under the rubric of shuttle "Return to Flight." Because of the Columbia accident, engineers altered the tank's design to minimize foam loss during launch and eliminated foam from the bipod that connects the tank to the forward part of the orbiter. Bipod foam was the source of virtually every large debris chunk that ever hit the shuttle undercarriage.
Then the engineers changed foam application procedures, so the workers who spray it on would be periodically tested for expertise, videotaped as they worked and watched by colleagues to see whether their technique was sound.
And to make sure engineers can analyze foam damage during launch and in orbit, NASA has installed new cameras and sensors on the ground and on the shuttle, built a 50-foot sensor boom so the orbiter can examine its underside, and ensured that the agency can get photos from a spy satellite if they need them. And, at least for the first flights, the shuttle will launch only during daylight, so the new equipment can see everything it needs to see.
Discovery will carry five experimental kits to repair foam damage during orbit -- if there is any. Two will be tested during a spacewalk and one inside the orbiter. The other two will be used only in an emergency.
Finally, if repairs to a damaged shuttle cannot be satisfactorily completed in space, NASA has devised a last-ditch plan for the crippled orbiter's seven-person crew to seek "safe haven" aboard the space station for at least 45 days, until a rescue shuttle can be sent to retrieve them. Atlantis will have this role initially and will be ready to fly by June 14, should Discovery run into trouble.
"We don't want to exercise that option," Hale said in a telephone interview from NASA's Johnson Space Center. "If we have to do that, it means we've failed."
The external tank, loaded with liquid oxygen and liquid hydrogen, delivers propellants to the shuttle's main engines even as it absorbs most of the thrust during launch. Once the main engines shut down, the tank separates from the orbiter and falls into the sea.