The shuttle, rolled on to a launch pad on Friday, is being prepared for liftoff in July on Nasa's second and final shuttle test-flight since the fatal accident, when Columbia disintegrated on re-entry into the atmosphere.
   
A successful flight will allow Nasa to resume construction of the half-built International Space Station and possibly extend the life of the 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 the mission would probably bring a premature end to the shuttle programme and disrupt Nasa's plans to keep its workforce intact while a replacement spacecraft is being developed.
   
Wayne Hale, Nasa's shuttle programme manager, recently told reporters: "If we go and fly and have another accident that will be the end of the programme".
   
Nasa faces one big hurdle before Discovery is cleared for flight: it must determine if the shuttle's newly redesigned fuel tank is safe to fly.

Delicate

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 February 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 lift-off and hitting the ship's delicate heat shield. Before Columbia, Nasa never imagined that lightweight foam could be just as deadly.