The shuttle missions that have gone to the station so far have been extremely successful. This mission takes the level of complexity up a notch.

If we're going to complete the space station, we're going to need the shuttle. It's going to take a lot of resources.

It goes back to, how likely is it that you'll have a defect? Very low. And what is your test experience? Overwhelmingly positive. Then you look at your design, and your design is to be very safe. And you look at those, and it's a compelling story.

We're satisfied that the necessary precautions have taken place or we wouldn't launch.

We've done all that we can think, as far as testing in a one-G environment and simulating the effects of our (shuttle) maneuvers on the mast. But there is that element of risk that there may be something we didn't get, that we don't quite understand.

We've been extremely successful. There's just much more to do.