The remaining shuttles have undergone major safety changes since then and the US space agency has been forced to undertake major reforms following the disaster, which cost the lives of Columbia's seven crew members and severely dented US space prestige.

"Based on a very thorough, very successful review, we are currently (ready for a) launch of Discovery on July 13," the earliest date in the shuttle's 19-day launch window, said NASA Administrator Michael Griffin.

"It is my assessment from the technical review that has been held over the past weeks and months (that) the causes of the loss of Columbia have been addressed; many other things that could have been of concern have also been addressed," said Griffin, who has been at the helm of the US space agency since April 15.

"We honestly believe this is the cleanest flight we have ever done. The only other flight that will be cleaner is the next flight," he added.

Though he acknowledged that "it's risky," Griffin stressed that NASA has done "what we can do to minimize it (based on) the state of our knowledge."

He said the Discovery mission "is a test flight; ... all the shuttle flights are test flights."

"We should never lose sight of the fact that space flight is risky," he stressed.

NASA experts held a final "flight readiness review" on Wednesday and Thursday, after engineers last week finished a detailed examination of modifications carried out to the shuttles since Columbia caught fire and broke apart as it re-entered Earth's atmosphere on February 1, 2003.

The disaster was blamed on damage sustained by the spacecraft after it was hit by chunks of insulating foam that broke off during liftoff.

The new launch will go ahead despite a specially appointed task force saying this week that the National Aeronautics and Space Administration had failed to "totally meet" three of the 15 conditions set by an official inquiry for the resumption of flights.

It said NASA experts had failed to eliminate the possibility of pieces of foam and ice breaking off the external fuel tank and striking the shuttle during liftoff.

But members of the committee, which includes two former astronauts, said NASA should still be allowed to resume the flights, as it had clearly boosted safety.

Discovery has a new external fuel tank, while a new heater has been added to the fuel feedline pipe. The heater is designed to minimize potential ice and frost buildup on a part of the pipeline that carries liquid oxygen to the shuttle's main engines.

The new tank has also been fitted with temperature sensors and accelerometers to gather information about the tank's performance and measure vibration during flight.

Columbia's crew was unaware of the damage that had been sustained during blastoff. Some experts said that if the damage had been detected, an emergency repair operation could have been staged.

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