Weeks-to-months long crewless missions in the marine domain present unique challenges for autonomous systems. At these mission lengths, the platform must take on self-assessment and mission planning tasks as well as short-term autonomy tasks such as navigation. The results of a four-prong research program to explore these challenges are reported here. Existing vessels were reviewed and analyzed and a new rating system was constructed. Interviews with human crew members were carried out to identify assessment and planning approaches. A simplified simulation and an STPA-inspired analysis were used to identify areas where research would be beneficial. From these investigations, it is clear that existing planning systems rely heavily on humans to integrate different sources of information. Humans make decisions today based on shared experience and implicit criteria. Automating this process is a significant challenge. Three test cases are proposed to explore the identified research needs, one on fuel management, one on design and operational support of machinery systems, and one on adapting and updating risk criteria in service.