First, some background. Unlike Congress, which only appropriates money on an annual basis, budgets proposed by the President’s budget officers (in consultation with NASA’s managers) project out five years, with the first year the actual request for the next year’s funding from Congress and the subsequent years being notional, but indicative of the agency’s planning. For NASA to issue the multi-year contracts needed to develop a mission, there has to be a clear, long-term commitment from the agency that is reflected in the official budget request. It is very rare for a spacecraft to successfully come to fruition without appearing in the official President’s budget.

We will soon see if there is agreement for the Planetary Science budget to increase to accommodate this new mission. The President’s FY2017 budget request will be released in early February—we will see if it contains the larger proposed funding to include a Europa mission launch in the early 2020s.

But if the overall budget of planetary science doesn’t increase, there are several alternatives that the Administration could pursue:

Delay the launch of the Europa mission to the mid-2020s to push out its spending bulge well after the Mars program needs

Delay the smaller Discovery and New Frontiers missions and use that funding for Europa, which would result in an unbalanced planetary program with just two Flagship missions launching in the next decade. And this wouldn’t provide all the funding needed for the two Flagship missions.

Take the additional funding from elsewhere in NASA’s budget (which would result in either hurting the human spaceflight program that has strong political backing, or hurting one of NASA’s other science programs such as the Earth Science program (the latter of which has been proposed by various members of Congress, but which I oppose—we are rapidly modifying our planet and need satellites to identify and monitor the changes))

Any of these alternatives represent solutions typical of those made in budget negotiations, which assume a flattish overall budget with the individual line items being traded off.

I’m hoping that this year represents a new possibility. The public has repeatedly shown its interest in planetary exploration through its avid following of missions in the press and the internet. Congress has noticed that interest and been willing to support increases in the NASA’s planetary program for several years. Several key members of Congress also are personally interested in planetary exploration specifically and space science in general and have consistently added money for Europa over the past few years. Congress has also consistently increased NASA’s budget since 2013, providing a surprising (and welcome) 7% increase in 2016.

This seems to be the year to attempt to create a political consensus for a new higher spending rate for NASA’s planetary program. The set of proposed missions is compelling. Congress is willing. For the next year, before the next President changes the players with the resulting delay in dealing with new policies, we have stable management teams in the President’s budget office and at NASA. And public interest groups like The Planetary Society have shown that they can demonstrate the public’s support for increased funding and build the political case for the needed funding.

These budget issues aren’t unique to the Europa mission. They occur with any large mission as NASA’s budgets are planned. In a zero-sum game, something has to give. In recent years, though, Congress has shown its willingness to increase the size of the budget to match the vision. Perhaps it will be possible to have a dream line of up missions in development: Mars 2020, Europa, two or more Discovery missions, and a New Frontiers mission. It’s worth working for.

Appendix: Estimated Europa mission costs

I have seen just one official cost estimate for the Europa multi-flyby mission (previously called the Europa Clipper) in a mission definition update. That estimate was for $2.1B without the launch (and was made by the Aerospace Corporation, which NASA uses to provide independent mission cost estimates). To estimate total spending that must be done before a possible 2022 launch, we need to add in the possible costs of the newly required Europa lander and the launch vehicle. We should also subtract the costs of post launch operations (which seem to run $50M to $70M a year for Flagship missions) and money already spent or appropriated through FY16.

Here’s what the budget swag looks like:

+$2.1B Multi-flyby spacecraft

+$0.5B SLS launch vehicle*

+$0.7B Europa lander*

-$0.3B Post launch operations*

-$0.4B Already spent/appropriated

This back of the envelope calculation results in approximately $2.6B remaining spending before launch. If launch is in 2022, then that leaves six years after FY2016, for an average spending rate of $440M per year. More likely, there will be a higher peak spending rate for a couple of years with lower spending in the beginning and end of this period (based on spending patterns of other missions). A possible average, though, is as far as I can push this thought experiment.

*SLS cost estimate from another Europa mission presentation; lander costs from a press account and may not be firm; operations costs assume a five year prime mission at a swag of $60M per year.