This might be a thought for later, but ... what happens if there are no slots available when the rover rolls over an object?  I guess right now it gets added to the inventory, but not the UI?  We will probably want the UI to represent the capacity of the cargo hold itself, so that at some point the rover is full and can't pick up and object until it drops another (and that probably means that it can't roll into that space until it drops and object.