The past few days, I've read through the Astronomy and Astrophysics Decadal Survey report released last week. If you are interested in these fields, I encourage you to look through the report. While it is long (225 pages), the 17 page executive summary and 32 page overview of the state of the field are worth reading. You can download the complete report. or read good summaries of the recommendations at the journal Nature or at Spaceflightnow.
The recommendations of the Survey have been widely reported, and I won't rehash them here. Instead, I'll look at the report for clues as to how the Planetary Decadal Survey may approach its recommendations. Before doing that, there are three key differences in the astronomy and planetary programs that must be acknowledged. First, the astronomy Survey covers both space missions and ground facilities. The planetary Survey also can prioritize ground facilities, but in general, the trend is has been to ensure funding for access to facilities already built. Second, the NASA astronomy program is expected to continue to spend the bulk of its funds on the James Webb Space Telescope through the middle of this decade. As a result, the proposed space portion of the astronomy program appears modest with just a single major mission firmly recommended. And third, the incremental cost of meaningfully increasing our knowledge in planetary exploration appears to be greater than in astronomy. The report notes that the three highly successful Explorer astronomy missions of the past decade together cost $560M. The budget for a single mission in the equivalent planetary program, the Discovery missions, is $425M. (Astronomy missions can be expensive, though. Two ground based telescopes recommended by the Survey would cost $465M and ~$1.1-1.4B and two lower priority space missions would cost several billion dollars.)
The astronomy recommendations are based on conservative budget assumptions that assume flat spending for the NASA portion of the budget. It has been common in reports of this nature to request budget increases. With this report, the optimistic scenario is that NASA astronomy and astrophysics budgets will increase for inflation and the pessimistic scenario is that they won't.
The astronomy recommendations are built around two recent key advances in our understanding of the universe: the discovery that dark energy is the major constituent of the universe and the rapidly expanding family of known extra solar planets. These two areas have radically changed our perceptions of the universe and how solar systems form. I don't believe that there has been equivalent discoveries in the planetary program in the past decade. We have much stronger evidence that Mars likely preserves its ancient history and possible record of life and that water was and is abundant, but those discoveries could be viewed as expected. We have also learned that Titan is a more fascinating world than expected and the geysers of Enceladus were truly unexpected. These discoveries argue for continuing heavy funding for Mars exploration and perhaps shifting missions from the ice-ocean moons of Jupiter to the ice-ocean moons of Saturn rather than switching, for example, from a Mars focus to a Venus focus. (If you disagree, please leave a comment!)
The astronomy Decadal Survey chose to make its highest recommendations for general purpose instruments -- as space borne Wide Field Infrared Survey Telescope (WFIRST) and an Earth-based Large Synoptic Survey Telescope -- that can address both the two focus topics and a variety of other questions. In the planetary field, the trend is to more specialized missions that investigate one body in depth (e.g., a single comet) or a particular topic (trace gases at Mars or the lunar gravity field). Proposed missions to explore wide ranging phenomena that include geology, atmospheric sciences, and fields and particles have become relatively rare and expensive. Two examples of proposed missions that would be wide ranging are the Jupiter Europa Orbiter that would explore the entire Jovian system and the Venus Flagship mission that would explore the surface and atmosphere of Venus both remotely and in situ. Both missions have price tags greater than $3B compared to the $1.6B WFIRST.
The astronomy Survey made clear choices between fields. Survey instruments received preference over higher resolution instruments. Infrared astronomy received preference over high energy (X-ray and gamma ray) astronomy missions that were ranked lower and would fly only if US funding permits and international cooperation occurs. I believe that we are likely to see similar clear preferences from the planetary Survey. Fiscal credibility will, I believe, be a major criteria for the planetary Decadal Survey. Steven Squyres, head of the Survey, as warned of sticker shock, which could result in only one no large (>$2-3B) missions being recommended. (See Sticker Shock and Decadal Survey Showdown.)
The astronomy Survey made as its second priority for space missions an expansion of the small PI-led Explorer program, with a request for funding increases from $40M per year to $100M per year starting in 2015. Similarly expanding the planetary PI-led New Frontiers (~$650M each) and Discovery (~$450M each) missions from 2 and 3 per decade, respectively, likely would require a decision to forgo either the Jupiter Europa Orbiter or the Mars sample caching rover.
The astronomy Survey made international cooperation a key element of several of its recommendations. WFIRST could be combined with a less ambitious ESA mission with similar goals. The third and fourth priority space missions apparently would fly only with substantial ESA contributions and, in the case of one mission, Japanese contributions. On the planetary side, NASA already is collaborating with ESA on 2016 and 2018 Mars missions and discussing collaboration with ESA to explore the Jovian system. (All three ESA large class missions in competition for selection involve collaboration with NASA. One is the Jupiter Ganymede Orbiter and the other two are the astronomy Survey's third and fourth space mission priorities.) The planetary Survey may also prioritize missions that require substantial international collaboration. Beyond the missions already in discussion, the planetary survey might, for example, prioritize a small scale orbiter to continue studies of Enceladus and Titan while also suggesting carrying along one or more foreign probes for in situ exploration of Titan.
The astronomy Survey prioritized development of technologies for key missions desired for the decade (2020s) following this coming decade. The planetary Survey is considering similar recommendations. Technologies that have been discussed for development in this coming decade to prepare for the next include Mars sample return, technologies for Titan exploration, and cryogenic sample return from a comet. At the same time, the planetary Survey will have to decide whether or not to make use of key technologies developed in the past decade to allow an Europa orbiter and develop and operate rovers on Mars. A recommendation not to use those investments may mean that the expertise may be lost if the supporting engineering teams are disbanded for the lack of an immediate flight opportunity.
To sum up, I would expect the planetary Survey to select clear winners and losers among the big ticket missions in response to a tight budget forecast, recommend a continued (and possibly expanded) program of small missions, strongly urge international cooperation, and recommend technologies for development to enable key missions to fly in the 2020's.