 Complete the following phrase:  Discovery suffered severe damage to its thermal protection tiles in the underwing area. Post-flight analysis showed that the impact of a 12-inch long piece of cork insulation during ascent was the culprit. The origin of the debris was the forward field joint on the right-hand SRB. The damage was such that, during re-entry, the thermal protection tile eroded almost completely. A similar chain of events ultimately led to the 