After years of flying hundreds of missions, NASA managers admit they don't know everything there is to know about the Space Shuttle .
Some serious questions are hanging over the Shuttle Discovery as it nears its next mission.
Dan Billow reports that some of those questions don't have clear answers.
"There are still some things we don't understand about the Shuttle after 26 years," says Wayne Hale.
The astronauts know it, and they're willing to take the risk. Discovery's wing panels, the very part that failed on Columbia's last fatal flight; the very part that keeps the shuttle from burning up - have some flaws, and engineers don't know how they got there.
So, is it safe to fly Discovery? The forthright answer from NASA's Wayne Hale: no. "I didn't say it was safe and I wouldn't say it. We have an acceptable risk to go fly."
But even in the worst-case scenario, engineers believe the damage would be mild enough that the astronauts would survive. The tiny imperfections in the wings now are almost too small to see with the naked eye, unlike the big hole, more than a foot wide, blasted into Columbia's wing.
And so the astronauts will put their lives on the line Tuesday. Commander Pam Melroy personally helped recover the Columbia astronauts bodies, and she's ready to take the risk.
"This is a very complicated vehicle. It's an old vehicle and there are a lot of loose ends out there. We fly every time without having solved every one of our problems."