This allowed hot air inside the wing, effectively causing it to melt.
But it's still not clear what caused this puncture.
Scientists originally thought that a heat-resistant tile falling off led to the break up of the shuttle.
Space debris
This is Nasa's first serious theory for what might have caused the tragedy.
Their investigations are continuing and could take several months.
Seven astronauts died when the shuttle broke up during re-entry on 1 February.
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