The project SAPERE-STONG (“Sistemi Tecnologie e Ricerche per l’Operatività Nazionale Globale”) aims to increase the Italian spacial operativity with the purpose of improving spacial accessibility and exploration. In particular, the object of the project is to develop an electrical propulsion space tug that is able to catch satellite platform in order to make on-orbit operation of refurbishment and repairng or to transfer cargo and crew. This dissertation focuses on a side of this project: the docking operation. A docking mechanism is a particular device that allows a mechanical coupling between two spacecrafts: a servicer and a costumer satellite. The introduction shows the principal docking mechanisms built from 1960 to nowadays.