FOX NEWS SUNDAY' HOST CHRIS WALLACE: For more on the state of the (space shuttle) program, let's bring in Florida Senator Bill Nelson (search), who, back in 1986, was a payload specialist on board the Shuttle Columbia (search), and he joins us from Jacksonville, Florida.

Senator, welcome. Good to have you with us.

SENATOR BILL NELSON, D-FLA.: Thanks, Chris. That was a terrific interview that you just did with the crew.

WALLACE: Well, thank you. I have to say, as a child of the space program, it's pretty exciting talking 220 miles into space.

But there are some tough questions, and let's get to them.

The NASA administrator, Michael Griffin, predicted on Friday that they are going to be able to fix this external fuel tank problem quickly and get back to flying perhaps this year.

The question is, given the fact that they grounded the shuttle for two and a half years and didn't fix the problem, why the optimism?

NELSON: Well, they fixed most of the problems. You heard Eileen tell you that they're just amazed that the shuttle has very few dings on it. So this shedding-of-foam problem that has clearly been a problem in the past, they have fixed most of that.

The big mistake that Michael Griffin (search) and you heard Eileen say this morning, is that they decided that they didn't have a problem with this PAL ramp and so they didn't go in. Well, NASA now knows it has a design problem, and it's going to fix that.

WALLACE: Yes, we should point out incidentally — and you probably could do it a lot better than I can — the PAL ramp is a strip along side part of the external fuel tank which protects some of the cables and other systems there. And that's got a foam covering, and that's where the piece fell off.

But let me ask you about this, Senator. The accident board that was formed after the Columbia disaster (search) in 2003 in part blamed what it called the culture of NASA, that it had become too complacent about safety.

And the question I guess is, has NASA really changed that culture?

Let me ask you, because the Return to Flight Task Force issued its final report just two weeks before this launch in late June. And let's put up part of what it had to say: "Despite diligent work, it has proven impossible to completely eliminate debris shedding from the external tank. The hard fact of the matter is that the external tank will always shed debris, perhaps even pieces large enough to do significant damage to the orbiter."

Question, Senator: They knew they hadn't fixed the problem, so why did they still go ahead and fly?

NELSON: Well, you've got about 10 questions there, but let me get back to the culture of safety. I believe that culture has changed.

It's true, it was not safe enough back at the destruction of Challenger in 1986 and then two and a half years ago. And it was not, because NASA management was not listening to the engineers on the line. That's what caused the destruction of Challenger (search), and that's what caused the destruction of Columbia.

WALLACE: But, Senator, my question specifically is, here was the Return to Flight Task Force, which was formed to see how well -- this is a NASA task force -- to see how well NASA had complied with the recommendations of the Columbia accident board. And here they are, and the quote that I just gave you specifically saying we haven't eliminated foam shedding and there could be pieces off that could come off big enough to do significant damage to the orbiter.

That doesn't sound like they've learned their lesson.

NELSON: Chris, space flight is risky business. You can never make it 100-percent safe. You heard Eileen Collins say just a minute ago, there are 2.5 million parts up there that are working perfectly.

Now, the question is, you get your risk down to an acceptable risk. Is a chunk of that foam falling off the size that just went off of this PAL ramp acceptable? Absolutely not. So they've got a design problem there.

But the rest of the fixes seem to have worked as a result of them seeing the inspection of the orbiter.

WALLACE: Senator, I want to step back and take a larger look, because there are serious questions at this point as to whether or not the shuttle should be retired. And let's look at the history of this spacecraft.

The shuttle has flown 114 missions since 1981. It's had two catastrophic accidents in which 14 astronauts were killed. It's been grounded twice for a total of more than five years. And more than $3.5 billion has been spent on upgrades, and it still, as we say, hasn't entirely been fixed.

So the question is, the president wants to retire the shuttle by 2010 anyway. Should we do it now?

NELSON: No. The space program needs to continue to build and complete the international space station so that it can be used as the experimental laboratory that we designed it for.

And at the same time, we ought to get about the business of developing a replacement vehicle for the space shuttle. But you can't make that happen overnight.

I think NASA is now going to accelerate the replacement vehicle so that it's ready by 2010. And I feel very confident, Chris, that they're going to be able to redesign this PAL ramp, any other problems. We'll fly until the end of this decade, and then hand off with a new vehicle.

WALLACE: Senator Nelson, we want to thank you so much for joining us today. And now, I'm sure you join us in saying let's get those astronauts back home safely.

NELSON: And they'll be home very safe.