NASA has undertaken major safety changes on its shuttle fleet since the February 2003 crash in which seven astronauts died, but the space agency acknowledges that flying into space will always carry risks.

"Returning the space shuttle safely to flight and resuming flight operation is the first step in the vision for space exploration," NASA administrator Michael Griffin told a congressional committee in late June.

"The risk won't be zero," he cautioned. The Discovery space shuttle is scheduled to blast off July 13.

Despite the uncertainties surrounding the safety of manned flights, President George W. Bush has set out an ambitious agenda for intergalactic exploration.

Bush announced in January 2004 a plan to return to the Moon by 2020 followed by the first manned mission to Mars.

In the short term, the space shuttles are needed to finish construction of the International Space Station.

Research conducted on the ISS and the shuttle flights are tied to future Moon and Mars missions, according to the National Aeronautics and Space Administration.

Thanks to the ISS, "We're learning the answers to basic questions about how to keep humans alive and healthy for longer and longer periods of time in space, and how to improve spacecraft for those trips," said former ISS commander Leroy Chiao.

NASA plans to retire its three remaining space shuttles in 2010, once the ISS is finished, and replace it with a new spacecraft, the Crew Exploration Vehicle.

The new shuttle, which has yet to be invented, will have to be able to carry six astronauts to the Moon and later to Mars. It will also need the capacity to dock on the ISS for low Earth orbit missions.

But the implementation of the plan depends on the success of the return to flight of the old shuttles, first launched in 1981.

The 2003 crash and the 1986 Challenger disaster, in which seven astronauts died in an explosion shortly after blast off, has left NASA with three shuttles - Discovery, Atlantis and Endeavour.

"The launch is an absolutely essential step to creating forward momentum for the space program," said John Logsdon, a member of the panel that probed the Columbia shuttle disaster and director of the Space Policy Institute at George Washington University.

Investigators found that the Columbia crash was caused by damage sustained by the spacecraft after it was hit by chunks of insulating foam that broke off during liftoff.

The Columbia Accident Investigation Board also blamed NASA culture for the disaster, saying the space agency's managers were "as much a cause" of the crash as technical failures.

NASA's prestige took a hit after Columbia caught fire and broke apart over Texas as it re-entered Earth's atmosphere on February 1, 2003, said former astronaut George Nelson, a Western Washington University professor.

"Certainly, during the early sixties and seventies, Nasa was really the symbol of the country, in terms of technological prowess," he said, adding that it was no longer the case.

NASA has lost ground to major rivals Russia and China, both of which have flown manned missions to space since the Columbia disaster, Griffin noted.

The US space program "has fallen behind," he told US lawmakers.

"Our shuttle is grounded. Two nations have flown people in space since we last did it," he said.

"We (at NASA) and America need to work hard to regain pour pre-eminence in space."

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