The Planetary Science decadal survey or, simply, “the decadal,” is a report prepared by the National Academy of Sciences, Engineering, and Medicine at the request NASA every ten years. The National Academies convenes a committee of leading planetary scientists to write the report, which aims to produce a consensus on the most important scientific goals and missions for the upcoming decade.
The current decadal survey is Vision and Voyages for Planetary Science in the Decade 2013-2022. A new decadal survey is being prepared for the 2023 - 2032 period.
The decadal provides an underpinning to all discussions about the priority of missions and the commitment of resources within the scientific community, NASA, the White House, and Congress. It is used as both "sword and shield": a means to rally the community around new projects and investigations, and also to defend current priorities against budget cuts or other threats.
Lawmakers and NASA take the decadal survey seriously, but they are not bound to it: the report is only a recommendation. Budgetary limitations or other political priorities inevitably disrupt the total fulfillment of goals of the report, though generally NASA and Congress will support the very top recommendations.
Writing the decadal requires significant effort over the course of a year and a half. Dozens of scientists serve on panels reviewing the scientific opportunities for major destinations and themes in the solar system, and thousands of scientists submit papers and input into the process as well.
Recommendations of the Current Decadal Survey, 2013 - 2022
The best way to understand the recommendations in the Decadal Survey is to read the executive summary in the report [pdf]. There are many different types of recommendations, and it’s good to get a sense of the complexities.
Briefly, though, the current decadal recommends a balanced program of solar system exploration. No one program or destination should grow too large at the cost of others.
Planetary Science within NASA is broken down into several major programs: Flagship (big) missions, New Frontiers (medium) missions, Discovery (small) missions, research and analysis, and technology development. All of these programs are crucial to the success of NASA's goals.
The decadal gives various recommendations given the budgetary environment that the division could face, but, generally, it prioritizes support to the scientific community (research funding), a steady pace of small missions (Discovery), two additional medium-sized missions (New Frontiers), and a Mars rover that will cache samples of Martian soil for eventual return to Earth followed closely by a spacecraft to investigate the subsurface ocean of Jupiter’s moon, Europa (the top-two flagship missions).
There is a lot of generalization in the above statement, so again, reading the executive summary in the report [pdf] is highly recommended.
The Planetary Society stands by the overall goals and priorities stated in the Decadal Survey and is working hard to ensure that NASA has the resources to fully pursue them.