CAPE CANAVERAL, Fla. (Reuters) - NASA rolled the space shuttle Discovery onto a launch pad on Friday hoping to leave behind problems exposed by the 2003 Columbia disaster and begin a final round of flights before the shuttle fleet is retired.
Discovery was being prepared for liftoff in July on NASA's second and final shuttle test-flight since the fatal accident, when Columbia disintegrated on reentry into the Earth's atmosphere.
A successful flight will allow NASA to resume construction of the half-built International Space Station and possibly extend the life of the beloved Hubble Space Telescope, which has allowed humans to peer into far galaxies.
But with the shuttle fleet due to retire in 2010, any serious problems during July's mission likely would bring a premature end to the shuttle program and disrupt NASA's plans to keep its skilled work force intact while a replacement spacecraft is being developed.
"If we go and fly and have another accident that will be the end of the program," Wayne Hale, NASA's shuttle program manager, recently told reporters.
NASA faces one major hurdle before Discovery is cleared for flight -- it must determine if the shuttle's newly redesigned fuel tank is safe to fly.
The tank, which holds cryogenic propellants consumed by the shuttle's three main engines during the eight-minute climb to orbit, has been modified twice since the loss of Columbia and of its seven-member crew on Feb. 1, 2003. Total repairs will cost the agency $2 billion.
Ever since the shuttle first flew 25 years ago, pieces of the tanks' foam insulation have broken off during launch.
The insulation is needed to prevent ice from building up on the tank, breaking off during liftoff and hitting the ship's delicate heat shield. Before Columbia, NASA never imagined that lightweight foam could be just as deadly.
But it was. Columbia's wing was hit by a piece of falling foam during launch, and the ship broke apart as it flew into the Earth's atmosphere 16 days later for landing.
NASA changed how and where foam was applied to the tank and launched Discovery in July 2005 to test the new design. It failed, though this time NASA was lucky and the chunks of debris flew harmlessly past the shuttle. The fleet was grounded for a second time for tank repairs.
For this second test flight, NASA has made the most radical change in the tank's design yet. Technicians removed two long, hand-sprayed foam blocks that served as wind shields to protect underlying cables and pressurization lines.
NASA managers next month will discuss the results of several wind tunnel tests and decide if the tank is safe.
"We don't have a facility on Earth where you can take an entire shuttle and run it through the entire flight profile and see how it does," said Discovery commander Steve Lindsey.
"You can test pieces of it, but eventually you've got to say, 'OK, this is everything we can do on the ground. It's time to go fly.'"
All that work and effort comes as NASA tries to jump-start a new program to return astronauts to the moon. The agency hopes a new ship, called the Crew Exploration Vehicle, will make its first test flight within two years of the shuttles' scheduled retirement.
"NASA needs to get out of the business of flying the space shuttle. Everybody agrees on that," said Howard McCurdy, professor of public policy at American University in Washington.
NASA wants to uphold its agreement to build the space station, a multi-nation project, before switching its workforce to the lunar exploration program.
If it fails, says McCurdy, instead of heading out to their Florida launch pads, the shuttles will be moving into museums.