The loss of Columbia was caused by a piece of ET foam that impacted and resulted in a breach of the left wing
This surface cross section of the space shuttle's external tank reveals the half inch thick cork/epoxy layer covered with one to two inches of spray-on foam that prevents the super-cold cryogenic fuels inside the tank from forming ice on its outside. The polyisocyanurate foam also safes the tank's aluminum skin from aerodynamic heat and keeps its internal propellants at an acceptable temperature.
