(CBS) NASA Administrator Michael Griffin has cleared the space shutting Discovery for launch on July 1, reports CBS News correspondent Bob Orr, over-riding warnings from his own chief engineer and top safety officials.

"If we're going to fly, we need to accept some programmatic risk and get on with it," Griffin said Saturday.

The risk involves foam on the shuttle's huge external fuel tank. NASA safety officials are worried that foam could break off from brackets securing pressurization lines and damage the shuttle's heat shield.

In their risk analysis, officials label that failure as "probable/catastrophic," meaning it is "probable" that sometime in the final 17 shuttle flights, foam will be shed with "catastrophic results."

A large piece of foam doomed Columbia in 2003 when it punched a hole in the wing. Another large piece just missed Discovery when it took off last July.

Griffin insists this time there's no risk to the crew, as only small pieces of foam can break off the brackets. Even if the shuttle is hit, cameras and sensors will spot the damage. If the astronauts can't repair it, they'll scramble to the International Space Station and await a rescue by Shuttle Atlantis.

NASA Chief of Safety and Mission Assurance Officer Bryan O'Connor and Chief Engineer Chris Scolese gave the following statement to CBS News:

"Crew safety is our first and most important concern. I believe, as does Chris, that our crew can safely return from this mission.

"Chris and I both feel that there remain issues with the orbiter — there is the potential that foam may come off at time of launch.

"That's why we feel we should redesign the ice frost ramp. We do not feel, however, that these issues are a threat to safe return of the crew. We have openly discussed our position in the Flight Readiness Review — open communication is how we work at NASA. The administrator has heard all the different engineering positions, including ours, and has made an informed decision and the agency is accepting this risk with its eyes wide open."

Griffin admits he feels pressure to finish the International Space Station before the shuttle is retired in 2010."I don't want to get us into a situation whereby being more cautious than I think technically necessary today, we wind up having to execute six flights in the last year," he says.

But "schedule pressure," says CBS News Space analyst Bill Harwood, is another red flag.

"Scheduling pressure was a contributing factor to both the Columbia and the Challenger disasters," says Harwood. "It's clearly at work here. In a perfect world, they would fix this problem before they go fly."

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