Original: The gravity of shepherd moons serves to maintain a sharply defined edge to the ring ; material that drifts closer to the shepherd moon 's orbit is either deflected back into the body of the ring , ejected from the system , or accreted onto the moon itself .
Simplified: Any material that drifts closer to a moons orbit , it can either move back into the body of the ring , deflect out into space , or is added onto the moons surface .