The journal Nature has an article about how Mars has lost its special place in NASA's planetary program. For the last decade or so, the Mars program has received approximately 50% of NASA's planetary exploration budget. That has enabled a steady stream of missions that -- and I don't think this is hyperbole -- has revolutionized our understanding of the Red Planet. However, the Mars program has no special place in the Decadal Survey. In addition, small Mars missions must now compete against other planetary targets in the Discovery program instead of having their own Mars Scout line item.
A couple of good quotes:
"Before the Mars programme office was set up in 1994, individual missions had to round up Congressional support — the reason, says Christensen, why no Mars missions flew between the 1976 Viking landers and the 1992 Mars Observer, which failed to reach Mars. With a dedicated programme, missions can plan for the future in smaller steps."
"But the Mars community might have itself to blame for the tight budgets that have led to the current quandaries. The $2.3-billion Mars Science Laboratory — the super-sized rover scheduled for launch in 2011 — ended up being the mammoth, bells-and-whistles mission that a stepwise Mars programme was supposed to help avoid. The mission also ended up chewing through hundreds of millions of dollars in its budget overruns — more than enough to fund a Mars Scout."
Editorial Thoughts: I believe that the real decision of the Decadal Survey in progress will be whether to prioritize several medium to big missions to a number of planetary targets or to have a focus program that chews up a large portion of the budget leaving money for only a few modest missions to other targets. There are a number of high priority missions that fall into the approximately $1B class that is too small to be a Flagship mission but too large for the New Frontiers program. A comet sample return, a Mars Network mission, or an advanced Venus orbiter with a high resolution radar are all examples. The survey could prioritize three of these missions and still fund the Discovery ($425M) and New Frontiers ($650M). (All of this analysis assumes flat budgest adjusted for inflation.)
Or the Decadal Survey could prioritize one location. A continuation of a well funded series of Mars missions is one possibility. Funding the Jupiter Europa orbiter would be another. (My analysis of the budget says that both cannot be funded; I hope I'm mistaken.) In addition to this one focus target, the Discovery and New Frontiers programs could target modest missions to other locations in the solar system.
In a way, the champions of a continued focus on Mars have a speed bump in their path. The program is largely justified on exploring the past and current potential for life or habitable locations. The Curiousity rover (MSL) and ExoMars will directly test that possibility at two sites. Arguing for continued high levels of funding requires faith that the answers delivered by those rovers will not be a resounding, 'No'.
I personally believe that Mars exploration as a prioritiy can be justified by simply learning about another terrestrial world in depth. Another decade of missions will greatly expand our knowledge.
On the other hand, a continued focus on Mars seems to preclude either the Jupiter Europa orbiter or the Discovery and New Frontiers missions.
It will be interesting to see how the Decadal Survey choses among these options.
Resources: the Nature article: http://www.nature.com/news/2009/090605/full/news.2009.551.html#comments