primary mission

The term primary mission is often used for an initial set of observations that a scientific space mission is to accomplish, with a given time period and defined set of goals. Typically, a primary mission is declared and funded at some point during the mission's development, often substantially shorter than what is possible, e.g., some spacecraft with the potential to be viable for ten years is given a defined primary mission lasting only three years. Perhaps a reason is to increase the opportunities to declare success, and undoubtedly a reason is avoid over-funding: the development and initial portion of the mission is funded, but funds are not committed to further-future operation that may have proven to be impossible or less than useful. When the primary mission is complete, if the spacecraft is still viable and there is useful work that it can carry out, extensions are generally funded to make good use of the initial investment.