This was the transfer stage that carried ComSat Minmus I out of low Kerbin orbit and off to Minmus, where it insterted the probe and was then detached - but not discarded. It remains in a dormant state, awaiting a wake-up signal from mission control that will allow us to regain command and use the remaining fuel to direct the craft into an impact trajectory with Minmus. This will be done to perform siesmic readings while a lander or rover is on the surface
As it is inactive, it is currently classified as debris
