Much has been written about the announcement that NASA will launch a second Mars Science Laboratory to Mars in 2020. Reactions have ranged from ecstatic to dumb founded depending on where another Mars rover mission fits in each poster's priorities.
One criticism of the decision has been that NASA has made it clear that caching samples for eventual return to Earth is a possibility, but one that will compete with other scientific opportunities. There will be only so many dollars and so much mass and volume available for the science payload. The Decadal Survey report, however, made it clear that another Mars rover was a priority only if that rover the caching element of a series of missions to return samples.
However, it appears that the President's Office of Management and Budget (OMB) also has made it clear that they will not support sample return with its $6-8B price tag. As one report put it, the James Webb Telescope's cost overruns have soured OMB on multi-billion dollars space ventures. In other words, the Decadal Survey's number one priority didn't get the sale. OMB appears to be okay with Mars missions in general, so long as they are no more than modestly expensive. In fact, Mars offers advantages as a destination -- short flight times, lots of developed technology, good science, proven public appeal. (My prediction: sample return will occur only if a rover finds organic pay dirt that strongly hints at life, present or past.)
Why didn't NASA turn to the second ranked priority, a Europa mission? We don't know, but I'll speculate. The latest, many flyby version of the mission has been costed to approximately $2B, approximately half the cost estimate of an orbiter mission from a few years ago. I believe that that cost doesn't include the launch, which would add another 10% or more, taking the total cost to more than $700M greater than the cost of MSL-2020 with launch. NASA's planetary program simply doesn't have the funding for the current version of the Europa mission (or the third priority, a Uranus orbiter). (When asked when a Europa mission will fly, the head of NASA's science program said that the cost would have to come down to the range of MSL-2020 (presumably including the launch)).
Another option would have been to use the money for MSL-2020 for a New Frontiers mission and a Discovery mission, which would cost approximately the same amount (some additional funding probably would be needed for the launches). Here, I believe that NASA faced a strategic management decision. JPL is a unique asset for planetary exploration. It needs a large mission to keep its skills current and its workforce engaged. (If you're good enough to work at JPL, lots of businesses would like your resume.) JPL might or might not win the competitions for the New Frontiers and Discovery mission and the winning missions might not technically challenge JPL.
So, MSL-2020 fits the budget envelope, gives JPL a major project, and will do good science (if not necessarily the top ranked science from the Decadal Survey). In my former career as a strategic planner for a large high tech company, I think I would have advocated for the same decision in an era of declining budgets. As a private citizen, I would have preferred to see the Europa mission fly, but MSL-2020 is a good consolation prize.
The announcement left some key questions open. First, what is the budget for developing the payload for MSL-2020? Technology advances since Curiosity's instrument selection means that some awesome options are in development. Taking them to flight readiness, though, may require a substantial budget. Working in a lab as a breadboard is one thing. Guaranteed reliability on the surface of Mars within a tight mass and volume constraint is another.
The answer to that first question will help the mission's science definition team tackle the second question: What are the scientific priorities for the mission? Take proven instruments to a new location? Deliver next generation instruments? Cache samples? I suspect that it may be a combination of the three. One possibility might be to refly some of the ExoMars instruments (which have the added benefit that they are not paid for by NASA). I'd personally like to see the ExoMars deep drill flown to get samples from well below the surface at a second site to a sophisticated instrument suite.
Then there is the question of what follows MSL-2020? This new rover fits within the budget cap only because JPL has a substantial supply of flight ready spares. Those won't be available for a third MSL. Does NASA fly additional missions to Mars in the 2020's or turn its attention elsewhere? Those decisions will need to be made well before the next Decadal Survey is due around 2022.
And finally, what about the rest of the solar system? By my reading of NASA's projected planetary budgets, MSL-2020 consumes most of the budget once the Mars MAVEN orbiter and InSight landers and the OSIRIS-REx asteroid sample return missions launch. Without a budget increase, follow on New Frontiers and Discovery missions to other targets may be few and far between. I hope that Congress' proposals to increase the planetary budget by $100-150M over OMB's last budget proposal to Congress occur. That small amount per year could breathe new life into these smaller mission programs.
I have found other good commentary (not all of which I agree with, but it's well reasoned and written) on NASA's decision at the following blogs: Vintage Space,