Some suspect the tile mounting is the least of Columbia’s difficulties. “I don’t think anybody appreciates the depths of the problems,” Kapryan says. The tiles are the most important system NASA has ever designed as “safe life.” That means there is no back-up for them. If they fail, the shuttle burns on reentry. If enough fall off, the shuttle may become unstable during landing, and thus un-pilotable.
from an article written in 1980 and republished in 2001.