The U.S. space agency NASA has formally set July 13 as the date for the first launch of a shuttle since the loss of Columbia in 2003. NASA's chief says the agency has done everything possible to make the mission the safest ever.

A two-day NASA review of flight readiness has resulted in the decision to end the two-and-a-half year moratorium on shuttle flights and fly the orbiter Discovery to the International Space Station in about two weeks.

The decision is the culmination of a long investigation into the causes of the Columbia tragedy in 2003 and an even longer process of correcting them. NASA's new administrator, Michael Griffin, says that, while space flight is risky, the Discovery mission will be the safest in the shuttle's 24-year history.

"My assessment is that the proximate causes of the loss of Columbia have been addressed, many other things which could have been of concern or would have been of concern have also been addressed," he said. "We honestly believe this is the cleanest flight we have ever done. The only other flight that will ever be cleaner is the next one."

The shuttle Columbia disintegrated in 2003 after a piece of hard foam insulation peeled off the huge external fuel tank during launch and punched a hole in the wing. The hole allowed extremely hot atmospheric gases to penetrate and destroy the orbiter during re-entry and kill its seven astronauts minutes before landing.

Earlier this week, an expert advisory panel said NASA has fallen short of accomplishing three of 15 safety modifications recommended by accident investigators, but concluded the shuttle was nevertheless safe to fly and the failures should not prevent a launch.

The experts said the space agency has significantly reduced the chances that launch debris could harm the shuttle and has vastly improved its ability to detect damage. But the advisors noted that NASA has not yet been able to eliminate the risk of falling debris completely or devise practical techniques to have astronauts repair punctures in the shuttle's thermal skin in orbit.

Mr. Griffin says these problems are not now solvable.

"We are being as smart about this as we know how to be, but we are up against the the limits of our human knowledge," he said. "If someone wants more, they are going to have to find smarter humans."

When Discovery launches, it will carry a crew of seven to the space station with badly needed spare parts. Of particular importance is a new gyroscope to help stabilize the outpost. Only three of the four gyros on board are working and electrical circuit difficulties have threatened the operation of one of them.