81 seconds into flight, a briefcase-sized part of the external fuel tank's foam insulation, which stops ice from forming while the very cold liquid fuel is in the tank, broke away and hit "columbia". such debris strikes had happened before - in particular on sts-27 in which a piece of a booster's forward heat shield broke and hit "atlantis"' bottom-side heat shield - and were seen as an acceptable risk. however, this debris strike hit "columbia"'s left wing head on, punching a hole through the panels on the wing's forward edge.