Space shuttle astronauts will fly next year without the ability to repair in orbit the type of damage that destroyed the Columbia vehicle in February 2003.
Many more cameras will be trained on the orbiter in future
Flying foam debris on launch punched a 15-25cm opening in the shuttle's left wing, resulting in catastrophic heating of the airframe on its return to Earth.
The US space agency says no method of repair tested so far could withstand the 1,600C temperature of re-entry.
Nasa's answer to the problem is to stop
debris damage occurring at lift-off.
To that end, the space shuttle fuel tanks have been redesigned. The agency believes they should no longer shed chunks of insulating foam of the size that fatally damaged Columbia.
Speaking on the one-year anniversary of the Columbia Accident Investigation Board's (CAIB) report, shuttle programme manager Bill Parsons said Nasa was steadily but surely working towards a "return to flight" in March or April of 2005.
"We've made a lot of progress, but we've not been able to come up with an over-wrap in this timeframe that would allow us to fix a large hole," he told the Associated Press.
"Our expectation is that we'll fix the tank and there won't be anything like that we'll have to deal with."
So far, Nasa has complied with five of the 15 return-to-flight requirements set out by the CIAB.
The remaining 10 must be completed by December for the agency to be clear to launch Discovery to the International Space Station.
As well as an external fuel tank redesign, future shuttles will fly with many more cameras trained on them. They will also only lift off, certainly at first, in daylight.
It is hoped a long inspection boom with lasers on the end, for sensing any gaps on the underside of the shuttle, will be ready in time for Discovery's flight.
The astronauts will have the ability to repair very small cracks but anything on the scale of the Columbia gash would require them to seek sanctuary in the space station until a relief shuttle could be organised to bring them back to Earth.