Friday, April 8, 2016

Defining the Missions for the Ocean Worlds

One of the major revolutions in planetary science that I’ve seen in my lifetime is the discovery that the solar system contains not just one ocean world – our Earth – but several ocean worlds.  Unlike or planet, which has its oceans on the surface, these other worlds trap their oceans beneath a surface layer of ice (or in the possible case of the asteroid Ceres, beneath a rocky shell).  For several of these worlds such as Jupiter’s moons Ganymede and Callisto, the oceans appear to be locked between layers of ice and therefore would be unlikely candidates for abodes of life.  For two of the moons, Jupiter’s Europa and Saturn’s Enceladus, the oceans appear to lie directly on top of a rocky core that would provide key elements needed to support life as well as energy from possible hydrothermal vents.  Saturn’s moon Titan is a unique case, with seas of liquid ethane, methane, and propane on the surface and a water ocean in the interior that may or may not be in contact with the rocky core and occasionally interact with the surface.  (This article and poster give more background on these worlds and their oceans.)

NASA’s managers, at the direction of Congress, have begun to put together an Ocean Worlds program to explore Europa, Titan, and Enceladus.  At a recent meeting of an advisory group for NASA, the Committee on Astrobiology and Planetary Science (CAPS), Jim Green, the head of NASA’s Planetary Science Division, and Barry Goldstein, from the Jet Propulsion Laboratory, provided updates on plans to explore these worlds.  In this post, I’ll report on the highlights of their talks (the presentations will be posted to this site (scroll down to the March 29-31 meeting) sometime in the future).

Europa Multi-flyby Mission

The only currently approved mission in the Ocean Worlds program is the Europa multi-flyby spacecraft.  This mission, estimated to cost ~$2 billion, will orbit Jupiter and will take approximately 45 toe dips into the high radiation belts surrounding this moon to make close flybys.  In between flybys, the spacecraft will have time to transmit the volumes of data it collected up close back to Earth.  (This presentation gives a good overview of the mission design and science goals while this presentation summarizes the instrument payload.)

The mission is well into its design phase.  At the CAPS meeting, the project manager, Barry Goldstein with the Jet Propulsion Laboratory, updated the committee members on refinements to the design.

Until recently, NASA’s managers had hoped that the main spacecraft could carry a 250 kilogram free flying daughter spacecraft to conduct complimentary studies.  Ideas ranged from a simple Europa lander, to a spacecraft that would divert to flyby the volcanic moon Io, to a spacecraft dedicated to flying through any plumes ejecting material from the surface of Europa.  NASA had invited the European Space Agency to propose (and pay for) a daughter spacecraft.  In addition, a group at NASA’s Goddard Space Flight Center had developed a proposal for a free flyer that would swoop even lower to the surface than the main spacecraft to fly through any plumes while carrying a mass spectrometer more tuned to identifying bio signatures than the main spacecraft’s instruments.

Unfortunately, it appears that NASA has decided to drop the idea of a daughter spacecraft.  I’m told that ESA’s managers determined that they had no way to fund such a spacecraft on the timeline for the Europa mission.  NASA’s managers may have also decided they lacked the funding to build their own daughter spacecraft.

Dropping the daughter spacecraft opens up new possibilities for launching the multi-flyby spacecraft to Jupiter.  NASA’s primary plan for sending this spacecraft on its way is the Space Launch System (SLS) that would enable a direct launch.  This extremely large booster could launch the spacecraft directly to Jupiter with a flight time of 2.1 to 2.5 years.  It will have sufficient heft to give the project a 33-35% mass margin, providing a cushion should the actual spacecraft as finally implemented weigh more than its designers currently think it will (which usually happens). 

The SLS, however, is still in development and its reliability will only be proven through one or more future flights.  In addition, this program is something of a political football, and so assuming it will be funded through development and into the time period of the Europa launch is a risk.  It’s also unclear what an SLS launch would cost and whether or not the planetary science program could afford it.  The project’s managers, therefore, are designing the spacecraft to also be capable of being launched on a less powerful commercial launch vehicle. 

Currently that backup would be either an Atlas V 551 or Delta IV Heavy booster followed by three Earth and one Venus flybys to receive gravity assist boost that would enable the final flight to Jupiter (known as the EVEEGA trajectory).  This extended looping flight would take 7.4 years to reach the Jovian system.

Dropping the 250 kilogram free flyer (plus supporting equipment on the main spacecraft) opens up an alternative launch plan.  By enlarging the spacecraft’s propellant tanks to allow a large deep space maneuver to set up a single Earth gravity assist (known as the ∆v/EGA trajectory), a Delta IV Heavy vehicle could deliver the Europa mission in just 4.7 years while still providing a healthy mass margin of 34%.  (For Falcon Heavy fans, NASA’s managers will consider this booster, too, once they have its final specifications, but they believe it would have similar performance to the Delta IV Heavy.)

This new launch backup is not yet an official plan as engineers and NASA’s managers examine it in more detail.  If they decide they can adopt it, the net savings in flight time if the SLS launch is unavailable is 2.7 years.

The current baseline plan for launching the Europa multiple flyby mission is the Space Launch System.  A new backup plan under consideration could reduce the alternative flight from over seven years to less than five years. Credit: NASA/JPL

Titan and Enceladus

The Ocean Worlds program now includes Saturn’s moons Titan and Enceladus as target worlds.  Previous mission proposals were for either for expensive Flagship missions (with estimated costs of ~$1.5 billion to ~$6 billion) or the inexpensive  Discovery missions (~$450 million).  The former doesn’t fit within NASA’s budget and the latter appears to be too little to reach and explore these distant moons.  In the past few months, NASA’s managers have opened up the intermediate cost (~$850 million) New Frontiers mission class to explore these worlds.

Science objectives for Enceladus and Titan presented by Dr. Green.  Credit: NASA
At the CAPS meeting, Green presented draft science objectives for a possible New Frontiers mission to Enceladus and/or Titan along with example goals for measurements that would meet those objectives.  For Enceladus, the goals relate to understanding the composition of the material within the plumes erupting from the moon’s southern pole.  What are the organic molecules in the plume detected by the Cassini spacecraft, but which its instrument lacked the sensitivity to analyze in detail?  Do these compounds suggest possible present life or a geological origin from hydrological activity?  Does the chemistry suggest that the ocean below the icy crust has the necessary chemicals to support life?

The goals for Titan mission broke into two sets.  The first related, as with Enceladus, to questions of chemistry.  How are complex organic molecules created, modified, and stored in the upper and lower atmosphere and in the surface lakes and seas?  Do any of these compounds suggest possible pre-biotic or even biotic origin?  The second set of goals focus on the structure of the interior ocean (for example, is it in contact with the silicate core that would provide many of the elements needed for life?) and whether material from that ocean may have reached the surface (as evidenced by past resurfacing).

Previous studies have looked at a number of mission concepts to continue the exploration of these two moons following the Cassini mission.  At the high end – and almost certainly outside the cost cap of a New Frontiers mission – were Titan and Enceladus orbiters and Titan balloons. 

In the past two Discovery competitions, three missions to Enceladus and Titan were proposed (but not selected to fly).  By law, NASA’s managers can’t reveal the results of their evaluations of these proposals – that’s proprietary information for the proposing teams who may well propose future missions in these competitive selections.  However, comments by managers in public meetings have said their science was compelling and that missions to the Saturn system don’t fit within the cost cap of the Discovery program.  The implication is that the higher mission costs allowed by the New Frontiers program could enable a mission to the Saturn system.  These past proposals for Discovery-class missions suggest possible New Frontiers-class missions to these worlds.

Two of the Discovery proposals replaced orbiters with spacecraft that would – like the Europa multiple flyby mission –that would study these Saturnian moons with multiple flybys.  The Enceladus Life Mission (ELF) would have flown through Enceladus’ plumes with two cutting edge mass spectrometers that would have studied the chemistry of the ocean’s volatiles and silicates.  The Journey to Enceladus and Titan (JET) would have carried a mass spectrometer to study the volatiles in the plumes and Titan’s upper atmosphere.  It would also have carried a thermal imaging camera that would have imaged Titan’s surface at up to an order of magnitude higher resolution (as fine as 25 m) than the Cassini spacecraft has done.  The imager would also have imaged the sources of the plumes on Enceladus’ surface in much higher resolution than Cassini has.

A possible New Frontiers proposal might combine the ELF and JET proposals by carrying ELF’s mass spectrometers and JET’s thermal imager and conduct multiple flybys of Enceladus and Titan.  Such a mission could address the composition questions posed by Green for Enceladus and Titan’s upper atmosphere.  The thermal imager could address the questions of whether Titan’s surface morphology indicates that the subsurface ocean has interacted with Titan’s surface. 

Possible enhancements to this type of mission might include an ice penetrating radar to study the subsurface structures of their icy shells.  Or the thermal imager could be enhanced by adding an imaging spectrometer that would search for variations in the surface composition of Titan.  Both of these latter ideas have been included in previous, Flagship-class mission proposals and would address the goal to better understand the structure of the interior oceans and their interaction with the surface.

Both Discovery proposals included high speed flybys of Enceladus.  While these flybys are relatively easy to set up, the velocity (typically ~4 kilometers per second) could destroy any highly complex organic molecules as they impact the mass spectrometer instruments.  One mission option would instead use a number of flybys of the moons Rhea, Dione, and Tethys to lower the orbit over two years to enable Enceladus flybys at ~1 kilometer per second.  Affording the additional costs of two years of mission operations likely is hard in a Discovery mission proposal but might be an option that could fit within a New Frontiers budget.

The third Discovery proposal, the Titan Mare Explorer (TiME), would have landed a probe to float on one of the moon’s large polar seas.  These lakes are believed to be stews that absorb and release gases into the atmosphere, receive a rain of complex organic molecules created in the upper atmosphere, and interact with the ices forming the shores and bottoms of the lakes.  A future mission could replicate TiME’s goal to study Titan’s chemistry.  The TiME proposed mission focused tightly on science conducted on a lake. A plusher New Frontiers mission might add instruments that could enhance atmospheric composition measurements as the probe descends to a lake landing as was proposed for a Flagship version of this mission several years back.

There is no assurance that an Ocean Worlds mission will be selected as the next New Frontiers mission, which will launch in the mid-2020s.  These missions are selected through open competitions.  The other missions on the candidate list – a Venus lander, lunar sample return, comet sample return, a Saturn atmospheric probe, and Trojan asteroid tour – are scientifically compelling in their own right and several may be less risky and expensive to implement.  We should learn which mission is selected in 2019.

On a side note, not discussed at the CAPS meeting (at least while I was listening), is the question of international cooperation in exploring Titan and Enceladus.  An obvious idea would be to combine the Titan lake lander with the multi-flyby spacecraft that could act as carrier and data relay in addition to its own scientific duties.  Fitting both within a New Frontiers budget seems unlikely to me.  However, other space agencies, particularly ESA, are also interested in exploring these worlds.  It may be possible that NASA would provide one craft within a New Frontiers budget while another space agency provides the complimentary craft.  Timing the funding for cooperative missions can get tricky (as shown by the inability of ESA to pay for a Europa mission free flyer on NASA’s schedule), but it is an obvious idea that I’m sure will be explored.

Examples measurements a New Frontiers mission to Enceladus or Titan might make to meet the science goals.  Credit: NASA
An Ocean Worlds Lander

In addition to providing an update to the Europa multiple flyby mission, JPL’s Goldstein provided the first public look at the current concept for a Europa lander.  In the normal progression of exploring a world, NASA would not look at detailed plans for lander until the results from a mission orbiting that world (replaced with multiple flybys for Europa) were in.  However, Congress has directed NASA to add a lander to the currently planned Europa mission.

JPL’s engineers have decided to make the lander an entirely separate spacecraft from the multi-flyby spacecraft.  To find the spot on this moon that best combines scientific value and landing safety, the multi-flyby spacecraft must first complete its examination of the surface.  As a result, a landing would come at least two to three years after the arrival of the multi-flyby spacecraft.  The lander spacecraft could either launch with the multi-flyby spacecraft and park itself in Jovian orbit while waiting for the reconnaissance to be complete or the lander could be launch later.  (I’m betting on the latter.  NASA’s Green described the current design state of the lander concept as “immature” and it’s not clear that NASA will receive sufficient time or funding to mature the design in time for launch with the multi-flyby spacecraft.)

A Europa lander, whose design could be used for landing on other ocean worlds, would consist of four major elements, a carrier craft that would also relay the lander's data, a solid rocket motor to slow the lander, a sky crane descent stage, and the lander itself.

The lander itself would look much like and be about the size of the Mars Pathfinder that landed on the Red planet in 1997 (but without the Pathfinder’s small rover).  The lander would be encased in petals that would deploy, allowing the lander to right itself if necessary after touchdown and that could also act as “snowshoes” in case the landing is on a soft surface.  A mass spectrometer and a Raman spectrometer would study the composition of the surface material, panoramic and microscopic cameras would provide context and close up images, and a geophone would provide seismic measurements.  The lander would include an arm that could scoop or drill samples from the surface to deliver to the instruments.  Batteries would power the lander for up to 21 days.

The proposed Europa lander would look much like and be roughly the same size as the Mars Pathfinder lander (bottom).  Credit: NASA/JPL

 While the initial target for this lander design is Europa, Goldstein pointed out that the design could be used to land on a number of ocean worlds including Enceladus and Jupiter’s Ganymede.  (As discussed above, a Titan lander will need enter and descend through a thick atmosphere and then float on a sea.  Its design is likely to be quite different.)  Perhaps, if the funding gods are kind, we could see both multiple flyby missions to these moons and landers for these moons launch in the next decade or two.

Additional Material

Current launch plans for the Europa multiple flyby mission.  Credit: NASA/JPL.
The proposed multiple flyby tour of Titan and Enceladus for the Journey to Enceladus and Titan (JET) mission (top) and the ground tracks below each flyby (top panel of bottom slide) and the imaging resolution for Titan and the height of the flyby through the plumes for Enceladus.  Credit: JPL

Thursday, March 17, 2016

Challenges to Enabling a Richer Planetary Exploration Program

This week has brought to the fore two challenges for NASA’s managers as they try to enable the richest possible mix of coming planetary missions.  At stake are whether the agency will be able to select two (the default is one) Discovery missions from the current competition, and whether there will be the possibility of a mission selected for Enceladus and/or Titan in the next decade.

When I write these blog posts, it is always more fun to write about the exciting science or clever engineering solutions than to write about policy.  Often, however, the richness of NASA’s planetary science program hangs on questions of management and policy.  Can NASA’s managers find the flexibility in spending to allow the selection of two Discovery missions?  Can they find a way to incorporate new scientific discoveries and a changed political landscape (a new Congressional mandate) in between Decadal Surveys?  This is not glamorous stuff, but it is the essential background to what missions we’ll see in the coming decade.

The Discovery question probably is the simpler because it likely “only” involves solving a $150M or so problem.  For this low cost ($450M) program, the space agency is currently conducting a competition among five finalist mission proposals.  These competitions are expensive to conduct and arduous for both the agency and the proposers.  As a result, NASA’s managers would like stage the competitions no more frequently than approximately every four years.  To increase the number of missions flown, the agency would like to select two missions that would be launched two years apart, allowing a total of five launches in the coming decade.  (This plan assumes that each competition provides at least two proposals that after extreme scrutiny are scientifically compelling, technically feasible, and can be done within the cost cap.  It also assumes that the currently planned future budgets are provided by future Presidents and Congresses.)

In the current Discovery competition, NASA’s managers will select one or two missions from among three asteroid and two Venus mission proposals.  Credit: NASA
The wrench in this plan is the two-year delay in the previously selected Discovery mission, the InSight Martian geophysical station.  As you’ve probably heard, the key instrument for this mission wasn’t ready in time to allow the planned launch this year.  Instead, under a just announced new plan, the mission will lift off in 2018 when Earth and Mars next align for launch.  As a result of the delay, the costs of launch and operating the mission that had been planned for the next two years are pushed out to 2018 to 2020.  This is when NASA wants to start development on the next one or two Discovery missions.  In addition, a substantial group of engineers and scientists need to be paid from now until launch in 2018 to fix the instrument problem and retain the core group needed to prepare the mission for launch and flight.

When NASA’s director for its planetary science program, Jim Green, announced the new InSight plan at a recent meeting, one of the earliest questions was whether or not this meant that NASA would still be able to select two Discovery missions.  An article on the journal Nature’s website poses the same question.

NASA has not formally released an estimate of the additional costs required under the new plan, saying that those figures will be available this coming August.  At a recent meeting, the InSight mission’s principal investigator stated that flying the mission later would cost $150M.  It’s not clear from his statement what all is included in that figure.  NASA had ~$150M budgeted for InSight for the next two years to cover launch and mission operations.  Those costs will need to be shifted out two years.  There will also be new costs for the 2016-2017 period to fix the instrument and to keep the core team together, and it’s not clear if this spending is included in the $150M estimate.
Because the federal budget operates on a cash basis, any money not spent in the current fiscal year returns to the treasury.  So NASA can't simply bank the money it had planned to spend in 2016-2017 and spend it two years later.  The agency’s managers may have some flexibility, though, subject to many rules that I don’t pretend to understand in any detail:

NASA may be able to sign long term contracts with an outside company or organization (which would include the Jet Propulsion Laboratory, which is developing and will later operate the InSight mission).  In some cases, money committed to a contractor counts as spent by the government when the contract is signed but the contractor can spend the transferred money later. 

NASA’s managers may be able to shift money among projects.  For example, they might be able to spend money in the next two years originally earmarked for InSight on the Mars 2020 rover project.  Then in 2018-2019, the agency might be able to spend money originally planned for the rover on InSight.

The $150M is less than half of what NASA is projected to be spending on Discovery mission development per year by the end of the decade.  In theory, delaying the start of the next Discovery mission(s) by around six months might solve the cash flow problem and allow the selection of two Discovery missions.  Jim Green has said that mission timing will be crucial, and all the teams currently in the Discovery competition have been asked to identify alternative launch dates.

I am convinced that NASA’s managers would like to fly InSight and select two new Discovery missions if fiscally possible.  The kinds of options that I've mentioned above probably just scratch the surface on the kinds of ideas they are exploring.  We will know more when NASA’s managers release the final budget plan for InSight this August and when they announce in late this year whether or not they selected one or two new Discovery missions.

In the meantime, veteran space reporter Jeff Foust has tweeted that Science Mission Directorate head John Grunsfeld (Jim Green’s boss) told him that the option to select two missions is still on the table.  We can hope that he and his managers find a solution to select two.

The other problem, how to enable missions to Saturn’s moons Titan and Enceladus may not have such a clean path to a resolution.  A spate of new discoveries have raised interest in exploring these worlds as possible abodes of life.  These discoveries caught the attention of key members of Congress who have directed NASA to establish an Ocean Worlds program to explore these two moons along with Europa.  (There are other ocean moons in the outer solar system, but they are less likely to be abodes of life or would be harder to explore.)

The Congressional mandate directed NASA to explore these worlds through a mixture of low-cost (Discovery program), medium cost (New Frontiers program, ~$850M), and high cost (Flagship, >$1B) missions.  Planning for a Europa mission with a Flagship mission is underway (although there’s no agreement on when the mission should launch).   NASA is left to find a way to send new missions to Titan and Enceladus to fulfill the Congressional mandate.

Unfortunately, neither of these moons was highly ranked as targets by the most recent Decadal Survey, which represents the consensus of the scientific community on exploration targets.  (Europa was highly ranked.)  The Survey examined missions to land on the lakes of Titan and decided that the scientific return for the estimated approximately $1B cost was not high enough.  A mission to orbit Titan, land on a lake, and fly a balloon was estimated at an astronomical cost of $6.7B.  Mission concepts, primarily orbiters, were examined for Enceladus, found to be expensive at $1.9B and were not a priority at that cost.

Fortunately, mission concepts such as multi-flyby spacecraft instead of more expensive orbiters have been more fully developed, giving mission planners lower cost options.  In recent competitions for Discovery missions, two teams have proposed multi-flyby missions to fly through Enceladus’ plumes to search for clues to habitability and in one of the proposals to also map Titan’s surface at higher resolution.  A third team proposed a lander for a Titan lake.  All of these proposals were for missions at a fraction the cost of the Decadal Survey cost estimates for similar (but to be fair, more capable) missions. 

Unfortunately, per a comment by Green at a recent meeting, while these Discovery missions proposed compelling science, they were judged by reviewers of being unlikely to fit within the Discovery cost cap.  Green implied that similar missions could fit within the New Frontiers budget cap (further implying costs substantially lower than their Decadal Survey cost estimates).

Given NASA’s current budget level and missions in development, the agency has no ability to add another major Flagship mission to the queue of missions before the mid-2020s.   That leaves NASA’s New Frontiers program as the only potential home for an Enceladus and/or Titan mission in the next ten years or so.  But there’s a problem using this program, too.  While Discovery proposals can target any world in the solar system except the Earth and sun, candidate missions for the New Frontiers program competitions were pre-selected by the Decadal Survey.  The list was carefully examined to ensure that the missions are scientifically compelling, affordable, and balance the wide interests of planetary scientists.  Because NASA is mandated to follow the Decadal Survey in setting its priorities, the list of recommended missions carries weight.  One purpose of the list is to prevent teams from lobbying outside the Decadal Survey for their favorite New Frontiers candidates to NASA’s senior managers and Congress.  (There’s no mechanism to prevent Congress from imposing its favorites on NASA, though.)

For the fourth New Frontiers competition that is in its initial stages, the Decadal Survey reports said that a mission should be selected among these candidates.  Credit: NASA
For the fifth New Frontiers competition, currently planned for the early 2020s, the Decadal Survey recommended that two additional missions be added to the list of candidate missions.

NASA has just begun the process of starting the competition to select the next New Frontiers mission from among the candidates, with a target launch in the mid-2020s.  The agency’s managers decided to add a mission to Enceladus and/or Titan to the list of candidate missions.  Many scientists within the planetary community are not happy with this addition.

Last week, I listened in on the first day a meeting (the Planetary Science Subcommittee) of senior scientists discussing the issue with Jim Green.  Their main objections boiled down to several points:

The New Frontiers current list was developed by the entire community and NASA’s managers should not arbitrarily change it.  At a minimum, the addition should be vetted before the mid-term Decadal Survey review planned for next year.  Doing otherwise undermines the credibility of the Decadal Surveys.  Green’s response was that given the timing of the Congressional mandate and the planned start for the next New Frontiers competition, a discussion in the mid-term review wasn’t feasible.  It was either now or wait for yet the next New Frontiers competition that would start in the early 2020’s.  In Green’s words, the timing of the mandate sucked because it limited his ability to consult with the broader scientific community before having to tell the community what missions they could propose for the competition.

Many are concerned that rather than being an open competition, an Ocean World’s selection is pre-determined.  There’s no point to teams proposing the other candidate missions.  Green’s response was that NASA will run a clean competition.

Why these two moons and not Pluto or the asteroid Ceres where there have also been exciting new discoveries?  Green’s response was that there are no credible New Frontiers-class follow up Pluto missions and teams have yet to propose cost-capped Ceres missions, but he expects them in the future.

At this meeting, scientists reported on discussions in analysis groups representing specific planetary science disciplines.  Perhaps predictably, the report for the outer planets group was enthusiastic about the addition while reports for the small bodies and Venus groups – which have candidate New Frontiers missions – were troubled to upset by the addition.  The Mars group – which has no New Frontiers candidates – was silent on the topic.  The report from the lunar group – which has two candidate missions – was also silent on this topic, but this group meets the least frequently of the analysis groups and simply may not have had a chance to caucus and decide either way.

As the discussion on adding Enceladus and Titan to the New Frontiers candidate list continued, it became more nuanced.   A few of the scientists wondered if an Enceladus/Titan proposal would be competitive given the lack of a mature list of scientific questions and design concepts.  The Decadal Survey carefully defined the scientific goals for the missions on its candidate list.  In addition, most of the missions on the New Frontiers candidate list have been proposed before or have similar Discovery proposals to draw on.  Green noted that Enceladus/Titan proposals would also have Discovery proposals to draw on.  He said that NASA was working on a plan to define the scientific goals for an Enceladus/Titan mission for this competition.

Green also talked more about NASA’s choices in response to the Congressional mandate.  NASA’s managers could decide to consider a mission to these moons to be a strategic mandate outside of the Decadal Survey’s recommendations.  The agency is planning for a Mars orbiter in the early 2020’s as a strategic mission to provide a communications relay to future missions, follow up on scientific questions raised since the last decadal survey, and prepare for future human missions.  In a similar vein – and this is my speculation and wasn’t raised by Green – the agency could decide to substitute the selection of a planned New Frontiers competition in the early 2020s for a strategic Ocean Worlds mission.  Instead, the agency has chosen to add a mission to the Saturn’s moons to the New Frontiers candidate list where it must beat out five other candidates on compelling science, feasibility, and cost.

NASA’s managers take the advice of the planetary science community seriously.  They are actively seeking the response to their decision to add Ocean Worlds to the New Frontiers program (and at the same time selling their proposed solution).   No one in the meeting I listened to stated that they questioned the scientific importance of Enceladus and Titan.  However, several would appeared to believe that these two worlds should compete in the next Decadal Survey (to be released in the early 2020s) for a place as prioritized missions.  NASA's managers are arguing that the time is now to recognize their priority.  
This is a conversation likely to take at least a few weeks if not months to play out. 

As I said at the beginning of this post, these issues aren’t glamorous.  They are simply the nuts and bolts issues NASA’s managers face as they work to create the richest possible program of planetary exploration within the resources the President’s budget office and Congress provide them.

Appendix: At the Planetary Sciences Subcommittee meeting, NASA's Jim Green presented these slides to make his case that and Enceladus/Titan mission should be added to the New Frontiers candidate list.  The final slide is from the Small Bodies Analysis Group (SBAG) arguing that any addition should be made follow a review by senior members of the planetary science community.

Congressional language directing the creation of an Ocean Worlds program that would include Enceladus and Titan.
Key discoveries at Enceladus since the priorities for the last Decadal Survey (in its Visions and Voyages (V&V)) priorities were set.
More Enceladus discoveries
Titan discoveries.
SBAG's statement on how new candidate missions should be added.