In January 2004, two NASA rovers, named Spirit and Opportunity, successfully landed on Mars, starting an unprecedented exploration of the Martian surface. Given the limited duration of this mission, the project prepared an aggressive plan for commanding both rovers every day. This paper discusses MAPGEN, which is used as part of the process for generating the daily command loads. MAPGEN provides engineers and scientists an intelligent activity-planning tool that allows them to more effectively generate complex plans that maximize the science return each day. The key to the effectiveness of MAPGEN is an underlying artificial intelligence planning and constraintreasoning engine. In this paper we outline the design and functionality of MAPGEN and focus on some of the key capabilities it offers to the mission.