Mjolnir obeys Thor’s mental commands, via his bond with the hammer, as though it was alive and, if Thor’s will is strong enough, the hammer can pass through nearly any barrier to reach his hand should he so desire. Once thrown, Mjolnir returns to Thor's grasp when he mystically calls upon it to return to him.

More on Marvel.com: http://marvel.com/universe/Thor_complete_powers#ixzz3MV9WPHNO

:D I hope that help you