code: def _soi(self, orb):
          
  
          for body in self.alt:
              soi = self.SOI[body.name]
              sph = orb.copy(frame=soi.frame, form='spherical')
              if sph.r < soi.radius:
                  active = body
                  break
          else:
              active = self.central
  
          return active
docs: Evaluate the need for SOI transition, by comparing the radial distance
          between the considered body and the spacecraft
  
          If therer is no body in sight, default to central body.