"And this is a risk worth taking at this time to get us back on track in space," he said.

NASA chief safety officer Bryan O'Connor and chief engineer Chris Scolese had called for a six-month delay to the launch to redesign foam on the fuel tank.

But the two officials backed the launch after NASA said the astronauts could take refuge on the ISS and wait for a rescue mission should the shuttle suffer irreparable damage.

The space agency has installed cameras on the shuttle and its fuel tank to detect any loss of foam during liftoff.

The shuttle's inspection will continue in space as Discovery will perform a backflip before docking to the ISS so space station crew members can photograph its heat shield.

The images will show whether NASA's fuel tank modifications have succeeded, Griffin said.

NASA's chief has said the Discovery mission was crucial to returning to regular shuttle flights. Griffin wants to conduct four shuttle missions a year to complete the space station by 2010, when the 25-year-old fleet is scheduled to retire.

Finishing the ISS is crucial to US space exploration ambitions, which include returning astronauts to the Moon and eventually sending a manned mission to Mars, he said.

"It's about creating a toehold off the surface of the Earth and using it as a stepping stone to Mars, which is a long-term goal for not only NASA and the United States, but for all mankind," he told Fox News.