While making a low pass by Mun, Mun III's radar altimeter dropped below 2km, which triggered a failsafe that decoupled the heat shield. This in turn separated the capsule fom the transfer stage. This fail-safe is intended for capsule return on Kerbin, in the event that the astronauts are incapacitated after re-entry. The heat shield needs to be dropped to ensure a slow and safe landing. The communications antenna is deployed as well. This trigger was not disabled and its activation led to the crew of Mun III being stranded and awaiting rescue
