Mission operators at NASA’s Jet Propulsion Laboratory use Science Activity Planners to analyze data acquired by rovers and direct their activities. In designing the SAP for the Mars Exploration Rovers project, developers relied heavily on open source components. They found that using open source software components not only helped keep the project within budget but also resulted in a more robust and flexible tool. When considering an open source component, prospective users should evaluate the project for several characteristics: maturity, longevity, and flexibility. For greatest benefit, OSS users should also build and maintain a strong working relationship with the component’s developers.