What powers Mjolnir to return to the hand of Thor? These two questions do not answer this, but do provide some insight,
- Was Thor unworthy during “The Avengers”?
- Why doesn't Thor's hammer return to his hand on Earth? (Avengers - The movie) [duplicate]
The hammer has two properties relating to movement. When it is deliberately thrown by Thor, it will return to his hand despite any intervening obstacles or distance, even traveling through planets to return to Thor. When it has been dropped or set aside, it takes a fixed position, from which it cannot be moved except by a 'worthy' individual.
Even though it states why it returns, it does not state what force is working to return it to Thor. Is there somewhere else that states how it comes back to Thor?