The space shuttle Discovery is set to return to Earth Saturday with a clean bill of health on its heat-deflecting tiles. NASA has been ultra-careful about inspecting the heat tiles after reaching orbit so as to avoid a repeat of the Columbia accident in 2003 that cost the lives of seven astronauts. Columbia's wings were damaged by a fragment of insulation during launch, and the spacecraft was destroyed during re-entry.

NASA has not always been that careful. A 1988 flight of the shuttle Atlantis - the second mission after the Challenger disaster - nearly ended in disaster after 700 of the heat tiles were damaged during launch (left). One was kocked out entirely.

The crew spotted it, but were unable to communicate their worry - fear - to mission control in Houston because of restrictions imposed by the Department of Defense. They were flying a classified spy satellite mission and were barred from sending clear photos of the damage.

The crew knew it looked bad - likely fatal. But the guys on the ground couldn't see it. They gave the crew a green light to come home.

So the crew crossed their fingers and headed home. They made it, by a whisker. Everybody was astonished by the damage they found after landing. If they had burned up on re-entry just two flights after Challenger, it likely would have ended the shuttle program.

It's a helluva yarn, told by CBS's Bill Harwood, and posted online by SpaceflightNow.com