Afternoon thunderstorms delayed work to load the shuttle Discovery's fuel cell system today, but engineers expect to make up the lost time later this evening and mission managers said the orbiter will be ready for launch Saturday, weather permitting, on the second post-Columbia mission.

"I'm very happy to report that we just had our launch-minus two-day mission management team review and other than some questionable weather, we have no constraints to launch," said John Shannon, chairman of the mission management team. "It's been a long year, with a lot of hard work by all of the team members to get to this point and I just want to say I'm extremely proud of the team and we are ready to go for Saturday and do what NASA does best."

Liftoff from pad 39B at the Kennedy Space Center is targeted for Saturday at 3:49 p.m. EDT. But forecasters continue to predict a 60 percent chance of afternoon showers and electrically charged anvil clouds that could delay launch tries Saturday, Sunday and Monday.

Launch Director Mike Leinbach said NASA's strategy will be to make back-to-back attempts Saturday and Sunday, if necessary, take a day off and then try again Tuesday and Wednesday. After that, the launch team would stand down for two days to reload fuel cell hydrogen. Launch managers are hopeful the weather will cooperate before that point rolls around.

"I'm very confident the hardware we've put on the pad is the best hardware we have and I'm just looking forward to a great flight," Shannon said.

Discovery's flight is somewhat controversial because of NASA Administrator Mike Griffin's decision to press ahead with launch over the objections of his chief safety officer and the agency's chief engineer. Both men were no-go for flight because of concern about a recently discovered failure mechanism that could result in foam debris falling away from so-called ice-frost ramps on the external tank. The debris poses an impact threat to the shuttle's fragile heat shield.

The ice-frost ramps are officially classified by NASA as "probable/catastrophic" in the agency's integrated risk matrix, meaning that over the course of 100 flights, there is a 50-50 chance the foam could break away and lead to a catastrophic failure.

But Griffin argued the risk is to the shuttle - not the crew - because in a worst-case scenario, the astronauts could move into the international space station to await rescue by another shuttle crew. Given a 2010 deadline to finish the space station and retire the shuttle fleet, Griffin said he decided to accept the risk posed by the ice-frost ramps and to proceed with assembly.

Shannon defended Griffin¹s decision, saying the objections of Bryan O'Connor, director of Safety and Mission Assurance, and Chris Scolese, NASA's chief engineer, had been "mis-characterized a little bit" in the media.

"The way I would put it, and I have attended every one of these meetings, when people are asked in your specific area, which is engineering or the safety group as well, would you change the ice-frost ramps? The answer came back yes," Shannon said. "But whenever Mike Griffin and (spaceflight chief) Bill Gerstenmaier go through the rationale for flight and you start really poking into the numbers for the ice-frost ramp and the real risk that it poses, I think we had just about 100 percent agreement that yes, we understand.

"As engineers, we would not fly with this condition ... but we have looked at all the data, it does not look to be something you have to go fix right now, we understand the rationale to go fly. We kind of handicap people by asking them just to vote for their specific area. I think when you lay out the whole story, the whole picture, we got almost 100 percent understanding."

But it was NASA's own engineering community that classified the ice-frost ramps as "probable/catastrophic," the only red-level threat in the risk matrix. In fact, Discovery's flight is the first in shuttle history to be cleared for launch with a system officially deemed an unacceptable risk.

"I have no idea if it's really yellow or red," Shannon said. "What I'd say is we don't have enough information to very accurately characterize that. Some people think it's in the red because they don't know, and that's one way you might characterize it.

"I am very comforted by the fact that we have not seen foam losses that would be catastrophic, regardless of what time they're released. I'm comforted by the fact that the geometry of the underlying metal under the foam seems to be such that a mass loss of that size is not possible. So those things make you feel better, but is that real science that you can hang your hat on and say that's really guaranteed or not? We're in a learning environment."

Shannon said the open debate over the issue showed NASA has changed its culture since the Columbia disaster. But in this case, engineers do not have enough solid information to reach solid conclusions.

"So then you're left to opinions," Shannon said. "The debate is the important piece and I think that was fulfilled in spades."

NASA has been criticized in the past for proceeding with flights in the absence of solid engineering data about troublesome systems.

Astronaut Stephen Robinson, who flew aboard Discovery for the first post-Columbia mission last July, said he agree the ice-frost ramps need fixing, but he said he tends to worry more about the high-speed rotating machinery in the shuttle's main engines and hydraulic system.

Asked how the public should interpret the actual risk posed by the ice-frost ramps, he said "it's difficult to tell the risk of something that hasn't happened before, isn't it? That's the biggest problem in our whole business. How do you characterize the risk of something that's never occurred even once."

"Now we have never had, for instance, a problem with the thing we worry the very most about, which are the big machines that are moving very fast during the launch phase," he said. "These are the engines and all the pumps and compressors and the hydraulic power units. Those we worry about a tremendous amount every single launch. We've been extremely fortunate ... that nothing has ever occurred with those. Worrying about ice-frost ramps doesn't even stack up to those kind of things."

Engines have shut down before launch and once, in July 1985, in flight. But in all cases, the engines shut down safely and in the 1985 abort to orbit, the crew was able to accomplish most of the mission objectives.

"The ice-frost ramp is something we've learned a lot about, it's one of the many things we should worry about, but it cannot keep us from launching, it cannot keep us from reaching orbit successfully," Robinson said. "The absolute worst it could do is shed some foam. We don't think it could shed foam big enough to critically hurt us and not be able to come home. So from a crew safety point of view, I completely back the administrator that it's the correct thing to do.

"From worrying it's something that could hurt the shuttle and maybe the long-term reusability of the shuttle, I completely back our safety community and Bryan O'Connor that that is something to worry about. But you have to put it into perspective. There are many things to worry about in a shuttle launch and we should be worrying about them all. I think you can have confidence that NASA has made the right decision. The dissent you're seeing is absolutely normal. ... Remember how much we talked about culture change after Columbia? This is culture change."

Summing up the ice-frost ramp debate, Robinson said he worries "more about rotating machinery going at tremendous rates, I really do. I'm a mechanical engineer and a pilot and those are the things that worry me the most. But I have a wide spectrum of things to be concerned about when it comes to safety and you have to decide that the risk is minimized. I think the risk for this flight is minimized. It's not gone. We're going to accept the risk we currently know about and we're going to go fly and see how the changes we have made, how much safer they've made us."