The journal Science has two articles on Decadal Surveys that are just getting under way, one for astronomy and one for planetary exploration. These documents are where American scientists fight it out and agree as a community on the priority projects they would like to see funded.
The last Surveys for both disciplines each had a major failure, with one mission ended up goobling the budget in each discipline. The James Web Space Telescope (JWST) was supposed to be $1B and is now $4.5B, and the Mars Science Laboratory (MSL) was supposed to be $650M and is now, per Science, around $2B. I actually judge both of these as cases not as costs run wild (although there was probably that, too) but failures in the process. The then NASA administrator, Goldin, claimed the JWST could be done for $1B because he could manage the agency to do things better, cheaper, and faster. Of course once the real issues were known, JWST's real costs became known and ate up NASA's astronomy budget. For MSL, the Decadal Survey called for a cost-capped rover that would be modestly more capable than the MERs. The Mars advisory committee, on the other hand, was calling for a much more capable mission that would never fit within that cap. NASA chose to go with the latter recommendation and in doing so made missions that the Decadal Survey thought were higher priority impossible to start. (See this blog entry for more on the history of MSL.)
Science gives the results of the last planetary Decadal Survey (released in 2003) mixed grades. For smaller missions, five Discovery missions have been approved and two New Frontiers missions. A regular program of these small, principle investigator (i.e., lead scientist) led missions was a high priority. However, neither of the two priority large missions, a Europa orbiter and a Mars sample return have been approved. Earliest launch for the former is now 2020 (but it is locked in a competition with a Titan mission) and for the latter probably the late 2020s.
New Decadal Surveys have been kicked off with the public release of the planetary report due in 2011. This time, both Surveys will put a heavy focus on developing real cost estimates instead of using management goals or best guesses. The good news for the readers of this blog is that the scientific community is very open in sharing the preparatory material. We will have a feast of proposed missions to think about, although only a few of them are likely to be high enough priority to make the final cut and even fewer may actually make into the budget and eventually to a launch pad.
You can download the previous Decadal survey here.