The Saturnian moon Enceladus with its extensive water bodies underneath a thick ice sheet cover is a potential candidate for extraterrestrial life. Direct exploration of such extraterrestrial aquatic ecosystems requires advanced access and sampling technologies with a high level of autonomy. An innovative exploration concept has been developed as part of the research consortium Enceladus Explorer (2012-14). It is based upon the minimally invasive melting probe IceMole, which enables to travel along a curved trajectory and allows obstacle avoidance as well as target selection maneuvers. These maneuvers, however, necessitate a sophisticated on-board navigation system capable of autonomous operations including in-ice attitude determination, localization, and forefield exploration, which are integrated through a high-level sensor fusion. The Enceladus Explorer Initiative (EnEx), established by the DLR Space Administration in 2015, addresses these challenges and contributes to developing key technologies for a future exploration mission to Enceladus.