It's established within the MCU that Mjolnir can sit on a shelf in the helicarrier and can be lifted by an elevator.   (Someone else can edit this question to include references, if they like.)   And obviously it can stick to the ground.   Plus, it is used to pin Loki without crushing him.   So it has the incredible power to "freeze" itself (for lack of a better term)  to various objects and vehicles (elevator, helicarrier, planet).   It other words, it can stick to anything that is stationary in some normal frame of reference for people. 

**Can Thor stick it to objects that we don't normally think of as a frame of reference, or in a non-vertical orientation?** For example, could he stick it to a baseball bat?  A flagpole?   The ceiling?  Hulk?   If not, when does something stop being horizontal?  When does the floor stop being the floor and start being a wall or a pile of rubble?