NASA administrator Michael Griffin acknowledged there was concern over the launch but insisted that the second flight since the Columbia disaster was crucial to US ambitions of sending astronauts back to the Moon and eventually to Mars.
He said he did not expect a large chunk of foam to peel off Discovery's fuel tank during the launch which is scheduled for 0548 AEST Sunday.
Columbia was doomed by a piece of foam that fell off its external fuel tank and pierced its protective heat shield. Debris fell off Discovery in last year's launch but missed the spacecraft.
"We are not expecting that. I do not feel vulnerable," Mr Griffin said.
"In fact, I worry that we spend so much time worrying about foam that we won't worry about other things which could get us.
"Foam is a concern, but I very strongly feel that we are not risking the crew for foam in this case, or I wouldn't feel comfortable launching.
He added: "Quite frankly, I think if we are unable to complete the project that we have before us, the space station, we will have a certain lack of credibility in encouraging others to join us in the exploration of Mars.
"So it's important to finish what we have started.”
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, who had dissented in a flight readiness review, backed the launch after NASA said the seven astronauts could abandon ship and take refuge in the International Space Station (ISS) should the shuttle suffer irreparable damage.
The Discovery astronauts will deliver critical equipment and supplies to the ISS and drop off European Space Agency astronaut Thomas Reiter of Germany, adding a third crewmember to the space station.
Bad weather has threatened to delay the launch by at least a day, but Mr Griffin said the 115th shuttle mission would liftoff as scheduled if the sky is clear.
"The vehicle is remarkably clean, as clean as I certainly have seen," he said. "We are looking forward to good weather, and when we have it we will be ready to go."