The request goes on to list the challenges of implementing the mission. "The primary challenges facing any mission to Europa involve the harsh radiation environment and planetary protection requirements...Planetary Protection requirements for Europa are very strict and involve ensuring that the probability of introducing a viable Earth organism into Europa is [less than one in 10,000]."

The request's details make it clear that proposers must do a solid amount of science and engineering analysis to show that they have a credible concept that could cost less than $1B (not including the launch costs) and make their case in 15 pages.

It is common for government agencies to issue these "Requests for Information" to learn whether an idea is credible and worth pursuing. This request doesn't commit NASA to any follow up studies, but if its managers judge any of the proposals to be credible, it presumably would follow through with more detailed analyses.

Is a $1B mission idea credible? I did a thought experiment in a previous post and concluded that technically it likely is. The Juno spacecraft that will orbit and study Jupiter cost ~$700M. A mission to fly by Jupiter's moon Io, deeper in Jupiter's radiation field, six or so times has been estimated to cost ~$1B. The European's JUICE mission will reach Jupiter next decade, flyby Europa twice, and then orbit the moon Ganymede for ~$1.2B.

If the goal simply is to fly by Europa a few times with a spacecraft with a small number of instruments, then by analogy with these other Jupiter missions, it likely can be done for ~$1B. The bar, though, for a scientifically credible mission is higher. A follow-on mission has to substantially enhance our scientific understanding of Europa to justify a cost of $1B to $2B. Most of the key studies identified by the science team require numerous flybys distributed across the globe.

However, Europe's JUICE 2020's JUICE mission to the Europa system is committed to two flyby of Europa with a highly capable spacecraft and instrument suite. To be justified, a NASA mission must produce significantly better science than the already funded JUICE mission will. (While the JUICE mission, which is still in design, it has committed to just two flybys of Europa. I suspect that if the engineers conclude it is safe, the mission's managers will consider one or two additional flybys closer to launch.)

So is there hope for a $1B mission that is scientifically compelling? Color me skeptical (and several of NASA's managers are reported to have said they are skeptical, too), but if there is, I suspect that it will come in one of two forms:

In a few months, we are likely to learn whether NASA received any proposals it considers worthy of further study. They key, though, will be whether the science community agrees that the mission meets the core requirements for understanding Europa. If it doesn't, then the community seems likely to recommend waiting until budgets allow the right mission to be flown. Missions to each outer planet or their moons occur only every couple of decades. Why do a sub-par job on the next mission to Europa and then have to do it over a decade or two later to get it right?

The Politics

In the introduction to this post, I said that NASA (and the President's budget office that writes NASA's budget requests) and Congress disagree on whether a Europa mission should begin now or wait to begin development several years from now.

The root of the disagreement, as in so many relationships, is money. Jupiter's harsh, electronics-frying radiation belts, make any mission that does more than a handful of flybys a technically challenging – read expensive – proposition. More than a decade's worth of technology development and mission studies has provided the solutions to most of the technical challenges. JPL's scientists and engineers have developed a killer proposal for a dedicated multi-flyby Europa Clipper mission. At ~$2B, this mission would be cheaper than the Curiosity rover mission currently exploring Gale crater on Mars.

Unfortunately, NASA's budget is oversubscribed. The only way to fit the Clipper mission into the budget is to either increase NASA's budget by several hundred million dollars a year for several years (which I would support as a US taxpayer!) or take the funding from other NASA programs.

We are left with this strange waltz in which Congress, which ultimately sets NASA's budget, has not increased the overall budget enough to fully fund the Clipper mission, but over the last two years provided $150M for advanced development work. This year, the House of Representatives is proposing to put another $100M in the pot for next year. If the Senate continues its previous support, it is likely to substantially match this funding.

At the other side of the dance floor, the President's budget managers and NASA's managers have made it clear that they don't want to commit to any Europa mission this decade because of the funding constraints. They also seem reluctant to commit to any $2B-class science mission because the last two large science missions (the James Webb Space Telescope and the Curiosity rover) went well over budget, which caused substantial harm to the overall science program.

As a result, today NASA is spending $150M because it's legally required to (Federal budgets in the US are laws) to advance a mission its senior managers don't want to do, at least for this decade.

The House of Representatives has released details of its proposed budget for next year. Where NASA proposed to spend $15M to study $1B mission concepts, the House is proposing to spend $100M. Under the House's bill none of the funding could go towards a $1B mission (which it doesn't see as credible) but only towards the full Europa Clipper mission.

We will have to wait for several weeks to see what the Senate proposes. It will be several months before we learn what the two houses of Congress ultimately compromise on for next year.