Enceladus Mission Options

Enceladus' plumes.  Credit Cassini Imaging Team, SSI, JPL, ESA, NASA

Enceladus is back in the news following a science conference dedicated to the latest results hosted by the Enceladus Focus Group at the end of last month.  The journals Nature and Science (subscription or library access required) summarized latest findings presented at the conference.  


Two results of the conference stand out: First, the evidence is mounting that Enceladus contains a liquid ocean beneath at least a portion of its icy crust, and second, that that ocean is in contact with the moon's rocky core.  The combination of liquid water and minerals from rocks could provide the ingredients that are believed to be needed to enable the formation of life.  


While the evidence is mounting (Science quotes scientists as believing the likelihood of a liquid ocean is, " 'pretty likely,' 'most likely,' or 'almost inescapable,'), it is still circumstantial in part because the Cassini spacecraft's instruments are not ideally suited to studying the composition of the plumes.  After all, no one expected in the 1990s when the mission was designed to find a moon ejecting plumes of its interior into space.  A return mission is needed to confirm these interpretations of Cassini's data and to examine whether the building blocks for life may have formed.


Several Enceladus mission concepts were considered by the Decadal Survey, and one was a runner up recommended for flight if more money than then planned became available for developing planetary missions.  We now find ourselves in the opposite situation where less money than expected was planned.  The $1.9 billion Enceladus orbiter described in the Survey's report now seems unlikely.  


At the Focus Group's meeting, Nathan Strange, a mission architect at JPL, presented a history of investigations into Enceladus missions and options for a new mission to this moon.  Since the discovery of the plumes, a number of teams have looked into follow-on missions:


2006 GSFC NASA Academy EAGLE Study
2006 NASA “Billion Dollar Box” Study
2007 NASA Enceladus Flagship Study
2007 ESA Titan and Enceladus Mission (TandEM)
2007 JPL Enceladus RMA Study
2008 NASA/ESA Titan Saturn System Mission (TSSM)
2010 PSDS Decadal Survey Enceladus Studies
2010 JPL “JET” Discovery Proposal (PI: Christophe Sotin)


The 2006 Billion Dollar Box study (goal: find compelling Enceladus or Titan missions at or less than $1B, it didn’t find any) and the Decadal Survey looked at the greatest number of options.  One set of missions identified would have a spacecraft orbit Saturn and perform multiple flybys of Enceladus:

• Multi-flyby missions where the spacecraft's orbit crosses the orbits of Titan and Enceladus, resulting in 10-20 encounters with both moons.  At Enceladus, the encounters occur at the relatively high speed of 4 kilometers per second.  (The Cassini spacecraft uses orbits of this type for its Enceladus encounters as would the proposed JET mission.)  These are the cheapest missions, estimated in the neighborhood of $1.5B by both the Billion Dollar Box and Decadal Survey studies.
• Early flybys of Titan followed by a leveraging tour in which many (20-50 of each moon) low-speed flybys of Rhea, Dione, and Tethys for gravity assists enable 50 or more Enceladus flybys per year at speeds ~1 km/s.  These missions incur greater mission operations costs from both the longer mission time and the larger staff needed to manage the many encounters.
• Plume sample returns in which icy particles are captured during plume and E-ring (composed of material from the plumes) flybys in a manner similar to the Stardust comet sample returns.  These missions require technology to capture and preserve volatile material and to prevent release of the returned samples into Earth’s biosphere even in the event of a crash landing of the return vehicle.  The authors of Decadal Survey report found these issues to be significant impediments to flying this mission.

At the next level of complexity are Enceladus orbiters.  These missions would use the leveraging tour to lower the spacecraft's Saturn orbit to allow orbital insertion at Enceladus.  Orbital missions incur the cost of the longer, more complex tour with the cost of larger fuel tanks for the insertion burn.  Because Enceladus is a tiny gravity well deep in Saturn's much deeper gravity well, polar orbits would be unstable.  As a result, polar orbits would be unstable, making the study of the plumes, which are found at the south pole, difficult.  The plumes would either have to be studied in the low speed flybys prior to insertion or from brief excursions from stable orbits around Enceladus.  An orbiter would allow detailed studies of the surface and interior of Enceladus up to latitudes of ~65 degrees, providing the opportunity to study the extent of any interior oceans and study the surface history of this moon.  The Decadal Survey studies concluded that a simple orbiter would cost ~$1.9 billion, or about half again (with launch costs included) the costs of a New Frontiers mission.


Beyond these simpler missions would be an entire range of missions that include landers (hard to plan for with our current knowledge of the surface properties) and even missions in which the spacecraft would orbit both Titan and Enceladus.


Nathan Strange concluded his presentation with a list of the mission options that, in his opinion, might be possible to squeeze under the cost caps of the Discovery and New Frontiers programs:


Within the lower mission cost Discovery program:

• Plume sample return
• Titan-Enceladus flyby mission
• Icy-moon leveraging tour (many moon flybys resulting in low speed Enceladus flybys)

Within the medium mission cost New Frontiers program:

• Enceladus orbiter or orbilander (a spacecraft that first orbits and then lands on Enceladus)
• Icy-moon leveraging tour
• Plume sample return
• Enceladus impactors

Strange emphasizes that these are ideas to explore and that these are concepts that might be "feasible with innovation and creativity."  However, "Currently, there is no obvious solution for a low-cost mission above the science floor [minimum requirements]. We must innovate to both lower the cost and increase the science value of concepts."


Editorial Thoughts: If you are a regular reader of this blog, you've probably noticed that I've spent quite a bit of time exploring options, post Decadal Survey and lower projected budgets, relating ideas for continuing exploration of the icy-ocean moons.  I believe that exploration of these moons should be a priority in the next decade if the science community and mission architects can develop feasible concepts.  I wish Strange and his colleagues the best of luck in finding missions that thread that intersection between costs, feasibility, and compelling science that will enable one or more of these missions to launch in the next decade.


Additional Resources:


Nathan Strange’s presentation to a Decadal Survey meeting 


The end of Nathan’s presentation has references for additional reading 


Two Decadal Survey Enceladus mission concept study reports can be found here


NASA Enceladus Flagship Study Report, NASA Goddard Space Flight Center, 2007


Titan Saturn System Mission Final Report on the NASA Contribution to a Joint Mission with ESA, Jet Propulsion Laboratory, 2009.

An Europa Discovery Mission?

As I mentioned in my previous post, I recently heard that an Europa Discovery mission has been proposed for the current mission selection.  Assuming that I correctly heard the quickly made remark while listening to a meeting on the phone, it's fun to speculate what such a mission might look like.  My guess is that an orbital mission with the necessary radiation hardening and the complex (and expensive) mission operations for many gravity assists to enable final entry into orbit probably is not likely.

That would leave a multiple flyby mission as the likely proposal.  If a multiple Io flyby mission is compelling, then why not a multiple Europa flyby?  Such a mission would not provide all the information needed to select landing sites for future missions and would not come close to replicating the depth of science a Flagship Europa orbiter would provide.  During each flyby, however, such a mission could image the surface in greater resolution and coverage than the crippled Galileo orbiter was able to enhance our understanding of the processes shaping the surface.  It could carry a modern infrared spectrometer to analyze surface composition in greater spatial and spectral resolution to look for locations where subsurface ocean material may have been carried to the surface.  A radar sounder could measure the depth of the icy shell along the ground track below each flyby path.  A magnetometer would seem a likely instrument to measure the interaction of the ocean with Jupiter's magnetosphere and continue measurements begun by Galileo.

Two types of orbits around Jupiter might be considered.  The first might be a highly inclined orbit such as the one proposed for an Io multi-flyby mission that would avoid Jupiters equatorial radiation belts except for the brief time of each Europa flyby.  The second would be a Galileo-style equatorial orbit that just touches Europa's orbit at each perijove.  If this orbit was chosen, the spacecraft could also do multiple flybys of Ganymede and Callisto as an extended mission.

The kind of mission I've described would not replace dedicated orbiter for either Europa or Ganymede.  The option to continue study of the Galilean icy moons on a Discovery budget, however, might be compelling and could mean that we don't ignore these worlds in the coming decade.  The following charts from the Jupiter Europa Orbiter planning documents give an indication of the types of coverage that might be possible with a multi-flyby mission.  (The JEO mission is no longer feasible given its cost and NASA budgets.)  The JEO encounters with Europa were designed to pump the orbit around Jupiter down to enter orbit around Europa.  As a result, the encounters occur over the same equatorial real estate.  A mission designed to maximize science from flybys presumably would vary the encounter geometry with coverage more like what JEO would have done at Ganymede and Callisto.

Example of coverage from multiple flybys for four Io, six Europa, six Ganymede, and nine Callisto flybys from the planning for the Jupiter Europa Orbiter (JEO) mission.  The imager on a Discovery mission might be less capable than that planned for JEO and might image smaller portions of the moons at these resolutions.  All images from http://www.lpi.usra.edu/opag/feb2010/presentations/Senskev8.pdf

Example image resolutions for Europa from JEO flybys.

Example image resolution for Ganymede JEO flybys.

Editorial Thoughts: If a multi-flyby Europa Discovery mission were proposed, I worry about whether the science would be compelling enough to compete against missions proposed for other destinations.  A multi-flyby mission would advance our understanding of Europa, but might not answer the fundamental questions the science community has.  If not, then an eventual orbiter would still be needed.  The same arguments could be made about a multi-flyby Io mission or the proposed Journey to Enceladus and Titan Discovery mission.  In the case of Io, the radiation levels are so high that a follow on orbiter mission is all but inconceivable.  The question, then, is when to fly a multi-flyby mission.  In the case of JET, it's two instruments provide measurements that fill gaps in the Cassini mission's measurements and neither instrument requires global coverage to add significantly to our knowledge.  Even so, I worry about whether review teams will consider it compelling enough for a $6-700M mission (with launch costs).

One concern for a Europa multi-flyby mission would be acquiring coverage of both hemispheres.  To minimize radiation exposure, the mission likely would have its perijove, and hence maximum radiation exposure, at the orbit of Europa.  (If radiation was not an issue, the perijove could be inside the orbit of Europa and have encounters on both the inbound and outbound legs of its orbit to image Titan on both hemispheres.)  This makes it difficult to change the encounter geometry over a reasonable mission lifetime.  Gravity assists could be used to walk the perijove around, but that would entail additional costs for a mission operations staff and a longer flight.  (Editorial note: I was surprised at how expensive mission operations for the multiple gravity assists for an Enceladus orbiter would be -- it would be a substantial portion of the Principal Investigator's budget for a Discovery mission.)  Perhaps mission designers have a good solution to this problem.

If I can see issues with a possible mission, then a team of experienced scientists and mission planners will have seen them, too.  If an Europa Discovery mission has been proposed, it likely has clever solutions to these issues.

If ESA's Jupiter Ganymede Orbiter is selected for flight, then I hope it's mission would be enhanced to include a number of Europa flybys.  One mission designer I've talked to says that the additional cost likely would be low.  In this case, the mission is justified by the in-depth, global measurements at Ganymede.  Flybys of Callisto and Europa would make nice bonuses.

Discovery Icy-ocean Moon Missions?

The best hope for NASA missions to the icy-ocean moons of Jupiter and Saturn in the coming decade may lie in the low cost Discovery mission program.  In past posts, I've described two proposals that I understand were proposed for the current Discovery mission selection competition: the Titan Mare Explorer (TiME) lake lander and the Journey to Enceladus and Titan (JET) multiple flyby mission

In addition, I understand that the Io Volcano Observer (IVO) multiple flyby mission was also proposed.  As I listened to the recent Planetary Science Subcommittee meeting, I heard of a fourth proposal for a Discovery Europa mission mentioned in passing, but no details were given.

On one hand, it would seem that the prospects for Discovery missions to the outer planets are poor.  The Decadal Survey considered equivalent missions to TiME, JET, and IVO, and estimated their costs at $1.4 to ~$2B, including the launch vehicle.  That is well above the approximately $6-700M equivalent budget for Discovery missions.  The missions studied by the Survey generally were more capable than the Discovery proposals and in the short time available for the studies were not optimized to fit within a cost target.  So can more focused missions designed to cost make up the difference and allow outer planet exploration within a Discovery budget?

NASA's conditions for the current Discovery mission selection may be enabling for outer planet missions.  While the principal investigator's (PI) budget for the spacecraft, instruments, operations, and analysis is similar to previous selections ($425M for this selection), NASA now pays for the intermediate class launch vehicle outside of this cost.  This is an a significant boost to Discovery mission budgets.  (I don't follow launch costs, and so I'm not sure of how much is this adds to the effective budget.  $100-200M?)  The PI for IVO has said that this change changed the budget from tight under the old rules to doable under the new rules.

In addition to the launch vehicle, for this Discovery selection, NASA will to provide a plutonium ASRG generator to power the craft at no cost to the PI.  For missions beyond Jupiter, this would be an enabling technology.  For missions to Jupiter, an ASRG may be enabling (solar power is an option there), but could simplify spacecraft design.  For example, an Io multi-flyby mission with solar panels would require a scan platform to allow the solar panels to remain sun pointing and the instruments to point toward Io.  With an ASRG, the craft can skip this expense and use the entire spacecraft to point the instruments as the Cassini spacecraft does.  NASA previously has said that it intends to make ASRGs an option for every other Discovery selection, although I've not heard what they expect to do under the new post Decadal Survey plan to select Discovery missions more frequently (every two years instead of every 3-4 years).

Proposals for outer planet Discovery missions face two hurdles.  First, they must be credible and convince the review teams making the selection that they are technically and financially feasible within the Discovery budget.  Second, they must be compelling and offer better science than proposals to other destinations.  Here, the narrow focus and the long flight times (with operations costs of $7-10M per year, I've heard) may hurt.  JET, for example, carries two instruments that would significantly advance our knowledge of Titan and Enceladus.  Would a mission to, say, Venus or a comet with five or six instruments provide more compelling science for the dollar?

In the next few weeks, we should learn which three of the 28 proposals (including the outer planet proposals) will become finalists in the current selection for further study to eventually select the winning proposal.  If an outer planets mission is among the selections, that would suggest that outer planet Discovery missions are possible.  Unless an outer planets mission is eventually selected for flight, however, we won't have a conclusive answer to whether outer planet Discovery missions can clear these two hurdles.  The team that has proposed the OSIRIS-REX asteroid sample return New Frontiers mission, originally proposed a similar Discovery mission that was a finalist in a previous selection.  The team has reported that while they received top science scores (i.e., the mission was compelling) in the final analysis the mission was judged to be too expensive for the then Discovery mission budget limit.

Editorial Thoughts: I am encouraged by the teams that have proposed outer planet Discovery missions.  They include a long list of credible, experienced researchers in planetary exploration who have been around the block a few times.  If they are willing to put the time and energy into these proposals, then I feel there's reason for optimism balanced with the observation that outer planet missions inherently carry more costs than equivalent missions to the inner planets.  We may know more soon when the current finalists are announced.

Rethinking Icy-ocean Moon Missions - Part 1

Several weeks before the release of the Decadal Survey’s report, a special meeting of the Outer Planet Analysis Group (OPAG) was scheduled for the week following the release.  It was the only one of the analysis groups (there are also ones for Venus, the moon, Mars, and small bodies) to schedule a special meeting.  It was a strong hint that the report was not going to have good news for the outer planets community. 

For several years, the exploration of the icy-ocean moons Ganymede, Europa, Titan, and Enceladus has been the highest priority of the outer planet research community.  The Survey attempted to enable a vibrant outer planets program with three flagship missions on the recommended list (Europa and Enceladus orbiters and a Uranus orbiter and probe mission) and Io and Saturn probe missions on the New Frontiers candidate list.  In plusher budget times, the outer planet program would have been a big winner.

In the new leaner budget times, if there was a category of missions that was loser in the collision between he Decadal Survey’s recommendations, and the new budget realities for NASA’s planetary program, it was the icy-ocean moon missions.  The flagship missions now appear unaffordable.  The Io and Saturn probe missions do not address the icy-ocean moons.  Now, further exploration of these moons rests on the European Space Agency’s (ESA’s) possible Ganymede orbiter that is in competition with two good astronomy/astrophysics missions for selection.

The OPAG report from the meeting following the Survey report may be a first step in asking NASA to look for new approaches to enabling the exploration of icy-ocean moons.  The official report strongly endorses the Decadal Survey recommendations.  (You can read the report at http://www.lpi.usra.edu/opag/mar2011/meetingReport.pdf).  The closest OPAG came to suggesting a re-examination was a request to pursue non-Flagship outer planet missions and to consider technology improvements that might enable Titan missions.

I listened to a good portion of the meeting, and there were several requests to see if it would be possible to add an icy-ocean mission to the list of New Frontiers candidate missions mid decade.  (Unlike the Discovery missions that can be proposed for any target, New Frontiers missions are selected from a candidate list.  Proposals for the ~$650M-$1B New Frontiers missions are so expensive to prepare that the list is kept short so that proposing teams can focus their resources.)  There is precedence for such a request to change the New Frontiers list.  The previous 2003 Decadal Survey recommended four candidate missions, and the list subsequently was expanded mid decade.  In addition, the new Survey report states that possible New Frontiers class missions to the icy-ocean moons were not prioritized (and Ganymede was dropped from the previous list) at least in part because of the priority given to a now unlikely Europa flagship mission. 

Any decision to re-examine the New Frontiers candidate list properly belongs several years in the future.   In my next post, however, I’ll start looking at tactics and missions that might enable continued exploration of these moons in the coming decade.

JET - Cost Capped Titan-Enceladus Proposal

JET Titan and Enceladus orbital tour

Following the release of the planetary Decadal Survey, I’ve discussed the need for missions in the coming decade to be highly focused to fit with fixed cost caps.  Nowhere will that need be greater than for outer planet missions to explore the icy-ocean moons.

For me, there were two major surprises in the Decadal Survey.  The estimated costs of missions, especially the Flagship missions, stunned me.  As I’ve had a week to think about the Survey, though, the bigger surprise has been the lack of focus on the icy-ocean worlds.  Perhaps there is no way to meaningfully explore Europa given limited budgets and its location deep in Jupiter’s radiation belts – perhaps only the surface of Venus is a more hellish location for spacecraft.  Titan and Enceladus are comparatively easy to explore, and their major drawback for further exploration is the many years needed for a spacecraft to reach them.  (I heard in a conference this week that operating a spacecraft during cruise costs $7-10M a year, and Saturn can take up to seven years to reach.)  Cassini’s discoveries at these two moons, in my opinion, however, put them on the short list of the highest priority worlds to explore.

I believe that the Survey’s members concluded that the cost of incremental missions to these moons for the science they could return would be too low to make them priorities.  They prioritized a $2.9B Uranus mission to begin the in-depth exploration of the ice giant worlds over a $1.9B Enceladus orbiter.  As budget projections stand, we are likely to get neither.

A Discovery mission to the outer planets would set a new cost-return point that might be quite attractive.  At least two Discovery-class missions have been proposed for missions to Titan or Titan and Enceladus.  The proposers are attempting to implement missions that would be approximately half the cost or less of equivalent missions briefly studied by the Decadal Survey.  However, several observers have pointed out that the Decadal Survey mission concepts tended to have rich payloads, weren't optimized to reduced costs, and were made under conservative design and cost assumptions.

I don’t know if the Discovery budget (perhaps ~$800M with launch) can enable missions to explore the outer solar system.  While the proposers of the three outer planet Discovery missions I know about (Io Volcano Obersver, Titan Mare Explorer, and Journey to Enceladas and Titan) have a great deal credibility, mission champions have also been known to be too optimistic.  I hope they succeed.  It is instructive, however, to look at the tradeoffs that might enable an outer planets Discovery mission.

The Decadal Survey examined an Enceladus orbiter that would also make a number of Titan and other moon flybys.  That mission would carry five instruments and involve a complex 4-4.5 years of moon flybys and orbital operations.  By comparison, the Journey to Enceladus and Titan (JET) Discovery proposal would have just two instruments and one year of operations.  The Principal Investigator for the proposal, Christophe Sotin, was kind enough to give me permission to reprint his team’s abstract on the mission from the just completed Lunar and Planetary Science Conference.  This abstract both shows how Discovery missions to the outer planets must have limited goals and shows how they may be able to still address interesting science.

Example of the improved imaging JET would offer for Titan.

JET: JOURNEY TO ENCELADUS AND TITAN
C. Sotin1, K. Altwegg2, R. H. Brown3, K. Hand1, J.I. Lunine3, J. Soderblom3, J. Spencer4, P. Tortora5, and the JET Team, 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, 91109, Pasadena, CA, 2University of Bern, Switzerland, 3University of Arizona, Lunar and Planetary Laboratory, Tucson, AZ, 4Southwest Research Institute, Boulder, CO, 5Universita di Bologna, Italy.

Introduction: The Cassini-Huygens mission has demonstrated that Enceladus and Titan represent two crucial end-members in our understanding of planet/moon formation that might have habitable environments. Enceladus is a small icy world with active jets of water erupting from its surface (Fig. 1) that might be connected to a subsurface water ocean. Titan is the only moon with a dense atmosphere and the only object besides Earth with stable open bodies of liquid on its surface (Fig. 2). An organic-rich world, Titan has a methane cycle comparable in atmospheric and geological processes to Earth’s water cycle.

Fig. 1. Jets emanating from faults on Enceladus form a plume that provides access to internally processed material that can be studied in situ. 

Fig. 2. Specular reflection at 5 μm from Jingpo Lacus, Titan [1] demonstrates the lack of scattering in Titan's atmosphere at 5 μm (Credit: NASA/UA). JET exploits this unexpected “window.” 

The questions that JET would answer directly respond to the wealth of discoveries made by the Cassini- Huygens mission. High-resolution mapping of Titan surface is required to determine what processes have shaped and are shaping Titan. High-resolution mass spectroscopy would permit assessment of the astrobiological potential of Enceladus and Titan.

Concept: The JET mission has been proposed in response to the 2010 NASA Discovery Announcement of Opportunity. In order to achieve JET’s rich science return within the Discovery cost cap, a planetary orbiter with a simple, two-instrument, but powerful payload would make a total of 16 flybys of Enceladus and Titan. The only new technology is the NASAprovided Advanced Stirling Radioisotope Generator (ASRG) that would be validated as part of the Discovery Program’s engineering goals.

Along with making new observations of Enceladus and Titan, JET would fill a critical temporal gap in our understanding of the Saturnian system. Enceladus and Titan have near-equatorial orbits around Saturn, and Saturn has an inclination of 26º relative to its orbital plane around the Sun. Consequently, Saturn and its moons have seasons. Voyager briefly observed Saturn and its moons during the Saturnian Vernal equinox. Cassini observations extend from winter solstice to the summer solstice. JET would observe Titan during autumnal equinox, an opportunity that will not arise again until 2054. Observations during the Autumnal equinox are critical to understanding the fate of lakes and seas, Titan’s complex meteorological cycle, and the fate of organic molecules.

Science: The three goals of the mission are to determine the processes that have shaped and are shaping Titan, to assess the astrobiological potential of Enceladus and Titan, and to investigate the formation and evolution of Enceladus and Titan. These three goals are then detailed in eleven science objectives and thirty-one science questions that cannot be answered by the Cassini mission. Three examples are given. First, the Cassini mission has discovered that mass 28 in Enceladus’ plume has possible CO, N2 and/or hydrocarbon components. JET would confirm what fraction is CO vs. N2 vs. hydrocarbons. Second, the Cassini-Huygens mission has discovered that rivers and valleys are carved into plateaus and mountains. JET would search for sedimentary layering in valleys to determine the history of the flows. Third, Cassini has discovered that heavy molecules (m>100 Da) are produced in Titan’s upper atmosphere. JET would determine the nature of these molecules. This list of questions includes ten of the major discoveries of the Cassini mission that await a more capable mission to unveil the geological history and astrobiological potential of these two unique moons in the solar system. JET’s payload—a camera (TIGER [Titan Imaging and Geology, Enceladus Reconnaissance]) and a mass spectrometer (STEAM [Spectrometer for Titan and Enceladus Astrobiology Mission]—would provide the capability for achieving these science goals.

Instruments: The payload is limited to two powerful instruments and the radioscience investigation. The total data volume would be on the order of 120 Gb during the one year nominal mission.

STEAM: The reflectron time of flight mass spectrometer [2] is the Rosetta flight-spare of the Rosina mass spectrometer. It would characterize elements and molecules including complex organic molecules with a 10× larger mass range, 100× higher resolution, and 1000× better sensitivity than Cassini (Fig. 3). It would resolve fundamental issues related to the chemical composition of Enceladus’ jets and their relationship to the structure and evolution of Enceladus’ interior. STEAM would also characterize Titan’s organic-rich upper atmosphere.

Fig. 3. STEAM (right) has 102 better mass resolution and 103 better sensitivity than Cassini/INMS (left). Mass 28 and 40 will be definitively characterized. 

TIGER: This high-heritage IR camera exploits four IR windows through Titan’s haze and would image the heat of Enceladus’ fractures (Fig. 4). This camera uses the 5 μm window [3] to provide 10× better imaging resolution of Titan’s surface than Cassini, yielding 50 m/pix images of 15% of Titan’s surface. At each Titan flyby, an area twice as large as France would be mapped at 50 m/pixel in addition to Titan full disk at 500 m/pixel. The 50 m/pixel resolution is achieved at 2250 km from Titan’s surface. Currently, only ~10-6 of Titan’s surface area was imaged at this resolution by the Huygens probe. JET would deliver a five order of magnitude increase in coverage of highresolution imagery of Titan’s surface. Similarly, TIGER would provide 10 m/pix images of selected Enceladus’ tiger stripe fractures, permitting detailed thermal modeling.

Fig. 4. Cassini/VIMS spectra of Titan revealed a major surprise: the surface is visible in several atmospheric windows. High resolution surface imaging requires a careful balance between spectral regions of high transparency and low absorption, matched appropriately with detector sensitivity. The red curve shows the transparency of Titan’s atmosphere. The black curve is a typical reflected spectrum of Titan where high values show low absorption. The four TIGER channels (C1 to C4) shown in grey are optimized for these critical parameters. C3 and C4 provide thermal emission maps of EnceladusJets emanating from faults on Enceladus form a plume that provides access to internally processed material that can be studied in situ. 

Mission: The mission would be launched in February 2016 and would be inserted into Saturn’s system in May 2023. The nominal one-year mission would start with 12 Titan flybys with the closest ones at 900 km from Titan’ surface. This distance would vary from one flyby to the other in order to sample different layers of the upper atmosphere. Both Saturn and anti- Saturn hemispheres would be mapped. The Titan phase would be followed by 4 Enceladus flybys to sample the different jets of the South pole. The nominal end of mission would be a Dione disposal. A 6- year science enhancement option would permit to get into Titan orbit, offering the opportunity to further test the ASRG and to relay data from any element present on Titan’s surface or in its low atmosphere at that time.

References: [1] Stephan K. et al. (2010) GRL, 37, L07104. [2] Scherer S. et al. (2006) Int. J. Mass Spectrometry, 251, 73–81. [3] LeMouelic S. et al. (2011) LPS XLII.

JET: Journey to Enceladus and Titan

At the just completed AGU conference, I had a chance to talk with the PI, Christophe Sotin from JPL, for a Discovery proposal to continue the exploration of Enceladus and Titan.  The JET proposal would send a small Saturn orbiter to explore those two moons within a constrained ($425M FY10 PI cost), which is substantially less than the estimated costs of the Enceladus missions under consideration by the Decadal Survey (equivalent PI costs of ~>$1.1B FY15 and up depending on option selected).  As you may guess, the JET proposal makes some tough choices to fit within the Discovery program -- there's no magic wand.

The biggest compromise is that JET would fly just two instruments: a mid-infrared camera/thermal imager and a mass spectrometer.  The most minimal of the proposed Enceladus payloads considered by the Decadal Survey would add a medium angle camera, a magnetometer, and a dust instrument.  Other versions of the the Decadal Survey concepts would add up to another nine instruments beyond that minimal list.

Even flying two instruments on JET requires using an already built Rosetta mission mass spectrometer plus other hardware contributed by other nations that wouldn't be counted towards the NASA cost cap.  Without those contributions, and with the mission cost estimates shared by the PI, JET would be unable to fit within a Discovery budget.  Even adding a simple instrument like a magnetometer would push the JET proposal close to the cost cap, and the PI held firm against what he described as many requests to add an instrument.

Even within these limitations, the JET mission would considerably extend the measurements that the Cassini mission has been able to make in key areas.  The JET camera would take advantage of spectral windows in Titan's atmosphere to image the surface at up to 25 m per pixel, 40 times better resolution than the equivalent imager on Cassini and up to 12 times higher resolution than the Cassini radar.  The camera would image 15% of Titan in the nominal one year mission at resolutions of 50 m or better  The cameras would take images in mid-infrared bands, possibly enabling some compositional studies if the surface materials differ in their mid-IR spectrum.  At Enceladus, two of the bands would allow imaging of the distribution of heat sources associated with the tiger stripes and vents at up to 5 m resolution.  The higher resolution at both moons would reveal details not seen by Cassini and allow a better understanding of the processes that may have created those structures.

The mass spectrometer would both extend the range of compounds that could be measured by sampling larger molecules and provide greater resolution within ranges of atomic weights.  Combined, this mass spectrometer would allow detection of a wider range of organic molecules in the upper atmosphere of Titan and (if present) in the jets of Enceladus.

Here are some examples of the specific studies enabled by JET's instruments from Sotin's poster:

• Search for sedimentary layering in Titan valleys resulting from erosion of plateaus and mountains
• Map the distribution of solid organics and organics in Titan's small lakes
• Measure the energy output and lifetime of Enceladus' jets by high resolution mapping
• Inventory organic and heavy molecules (mass >100 Daltons) in Enceladus' plumes (if present) and Titan's upper atmosphere
• Determine what molecules (CO, N2, hydrocarbons) make up the mass 28 in Enceladus' plume that has been identified by Cassini

In the nominal one year mission at Saturn, JET would encounter Enceladus several times and encounter both the pro- and anti-Saturn facing sides of Titan on opposite sides of Saturn.  This will allow mapping of both hemispheres of Titan while they are illuminated by the sun.  For comparison, the Jupiter Europa Orbiter would encounter the Galilean moons on just a single hemisphere of each in its flybys, limiting studies to that hemisphere.

Two extended mission options (which would require additional funding as the prime mission nears its end) would be particularly exciting.  JET would be powered by ASRG plutonium power sources, and NASA would like to have a full 14 year test of those power sources.  To fulfill that desire, JET would need to continue for approximately seven years after entering Saturn orbit (although further science observations aren't required for the engineering life test).  During that time, JET could act as a data relay for any in-situ craft that might land or fly above Titan.  This could enhance the data return of a mission such as the Titan Aerial Explorer or AVIATR plane by many times what they could return direct to Earth using their own antennas.  Complimentary to this first option (but not required for it), JET could spend three years pumping down its Saturn orbit using Titan flybys and eventually enter orbit around Titan.  The orbit could be high, 2500 km, well above the atmosphere and would allow continued observations of Titan's surface for years.

Editorial Thoughts:  JET is a good example of the trade offs necessary to conduct Discovery missions in the further reaches of the solar system.  Assuming that the Discovery review panel agrees with the PI's cost estimates, Discovery missions to the Saturn system are possible if you can get friends to contribute some hardware and instruments and stick to minimal payloads.  JET's two instruments would provide valuable science in key areas of study, though.  I would prefer to see one of the more capable Enceladus missions described in the Decadal Survey studies fly over JET -- more instruments are better.  (The mass spectrometer and thermal imagers of those missions should fulfill JET's Titan goals.)  However, if the Survey doesn't recommend one of those missions, then I believe that JET would be an exciting mission to fly.  Better a simple mission than no flight to these worlds in the coming decade.

Let's Add an Instrument

I think many of readers of this blog read about a proposed mission and imagine how much the mission might be improved with just another instrument or two or another goal or two.  I certainly do.
I learned in my life in a high tech company, though, that "simple" additions often turn out not to be simple and could drive up cost rapidly.  For those of us outside of the planetary mission design world, it's hard for us to understand which additions might truly simple additions and which would be unacceptably complex and costly.

The recently published Decadal Survey mission concept studies offer a peak into some of these tradeoffs.  Several reports explicitly explore several versions of missions with a variety of goals and instrument costs.  In this blog entry, I'll look at examples of the cost tradeoffs for possible missions to Ganymede and Enceladus.  The concept studies don't cover all options I would have liked to see discussed.  For example, how much would it drive up the costs of an Enceladus orbiter to have an instrument or two to study Titan during several flybys?  Or, what would be the design and cost impacts of adding several Europa flybys for a Ganymede orbiter?  Still, these reports offer insights that often aren't available to the public.  They also were all carried out under the same ground rules and using the same FY15 dollar costs, making comparisons between them reasonable.

The following table list a number of Ganymede and Enceladus mission options.  The Ganymede missions differ both in the number of instruments and in the length of time in Ganymede orbit.  The Enceladus missions would all orbit that moon for 12 months (except for a multiple flyby mission), but differ in the number of instruments.  Several of the Enceladus options were also ranked for relative science value.

Option numbers are from the reports listed at the end of this blog entry.  Click on image for a larger version.

The Ganymede mission options range from $1.3B to $1.7B.  Looking at the charts in the table, most of the difference in costs appears to be driven by costs associated with building and operating additional instruments rather than the longer time in orbit.   This is dramatically shown by adding up the costs to build and test the instruments suites for the Ganymede orbiter.  They are $62M, $96M, and $190M for the three options (reserves do not appear to be included in these numbers, so the real costs probably could be higher by ~50%).  Additional instruments also require additional operational costs and additional teams of scientists to plan instrument usage and analyze the results.  (Instrument costs given do not have reserves included; total mission costs do.)

The Enceladus orbiter missions range from $1.6B to $2.7B.  The costs of individual items wasn't detailed in charts, but by examing the graph showing relative cost elements, it appears that this difference again is largely driven by the costs of building and operating different instrument suites.

Costs of individual instruments vary considerably.  A magnetometer is only a few million dollars.  A mass spectrometer, radio and plasma wave package, or a subsurface radar isseveral tens of millions of dollars.

Even for a specific instrument type, costs can vary considerably.  The simpler Ganymede mass spectrometer apparently would cost ~$25M to build while the more sophisticated Enceladus mass spectrometer would cost ~$57M.  (Note: For some items, I'm making educated guesses to determine which instrument costs in the reports go with which instruments.)

Neither the Ganymede nor Enceladus mission reports spell out the cost of a narrow angle camera (NAC), which would be useful for exploring their target worlds and really useful for observing the rest of the Jupiter or Saturn systems.  However, it appears that instrument #10 in the Ganymede report at ~$13M is the NAC and the Io observer NAC is listed at ~$17M.  These are simple cameras compared to those proposed for the Titan and Europa flagship missions that proposed NACs costing ~$54M and ~$43M respectively.  A lot of capability is given up to keep the costs of these Decadal Survey concept missions below the Flagship mission costs.

Editorial Thoughts: My favorite instrument to add to an Enceladus mission would be a 2-micron imager that would use a spectral window in the atmosphere for high resolution imaging of Titan.  The specific costs of that instrument wasn't spelled out in the reports.  However, assuming that its cost might be similar to that of a NAC (similar optical and mechanical design but a different sensor, I think), then the final cost to the mission might be $35-40M with design, fabrication, testing, operation, analysis.

I would hate to see a mission go to Enceladus and not carry this instrument.  However, it might be that adding this instrument would  bust the budget and jeopardize approval of the mission.  In my experience, engineers are very creative at exploring all the options.  Then they become hard nosed about what has to be left out to fit within the financial, manpower, expertise, and weight restrictions.  If this instrument can be flown within those restrictions, I expect that it will be.  In the meantime, I can imagine what such a mission might do.

Source reports:

Reports can be downloaded from: http://sites.nationalacademies.org/SSB/SSB_059331

Ganymede Orbiter Concept Study

Enceladus Flyby & Sample Return Concept Studies*

Enceladus Orbiter Concept Study

*Despite its title, this study examined a range of missions from multiple flybies to several flavors of orbiters to samples return and landers.

New Frontiers to Ganymede and Enceladus

In my previous post, I listed missions to explore the diversity of icy ocean moons such as Europa, Ganymede, Titan, and Enceladus as my third and fourth picks for my most compelling missions.  This post begins a series that looks at approximately New Frontiers-class missions to these worlds based on mission concepts examined by the Decadal Survey.

From my examination of the mission concept studies from the Decadal Survey, it appears that there were two classes of missions examined.  The first were flagship class missions costing over ~$2B.  These were the three missions composing a Mars sample return, the Europa Jupiter System Mission, and the Titan Saturn System Mission. (There were also some Flagship-scale options in other reports.)  In addition, the Survey commissioned a number of concept studies for missions that might fit within the New Frontiers class of missions (~$650M for principle investigator costs; ~$1.2B for NASA's fully burdened costs).  All costs in the studies were for FY15 costs, when the burdened costs of a New Frontiers mission (assuming 3% inflation per year) would be ~$750M for the PI costs and ~$1.4B for the fully burdened costs*.  (The Survey reportedly will recommend specific Flagship and New Frontiers-class missions; it will not recommend specific missions for the lower cost Discovery missions.)

Several New Frontiers class missions were studied for the icy moons of Jupiter and Saturn:

• Several incremental flavors of a Ganymede orbiter that would also conduct several flybys of Callisto
• A number of variations of Enceladus missions that included flybys, orbiters, landers, and flyby sample returns along with flybys of other moons of Saturn
• Four variations of probes to float on or descend into one of the polar lakes of Titan

Option numbers are taken from the reports listed below.  Missions in the Enceladus Flyby & Sample Return Concept Studies report were ranked by the relative value of the science they would be expected to return.  Click on table for a larger image.

In this post, I'll begin looking at the orbiters of Ganymede and Enceladus.  Unfortunately, the concepts studies for Enceladus landers and flyby sample returns determined that these missions are premature.  For landers, we don't understand the nature of the surface (fluffy snow or rock hard ice? gentle plains or steep slopes?) and for sample returns there are uncertainties associated with the design of the sampling mechanism (a derivative of the Stardust aerogel collector) requiring "a significant technology development" as would issues of ensuring sterilization of the return probe for planetary protection.  The studies concluded that for Enceladus, an orbiter represents the most attractive target for the next mission (after Cassini) to this moon.

In several ways, a mission to Ganymede and Enceladus have similar requirements.  Both must travel to and operate in the outer solar system.  Both would study icy ocean worlds, and hence their list of desired instruments are similar.  However, there are also important differences.  A Ganymede orbiter is close enough to the sun that solar panels could be used.  An Enceladus orbiter is far enough from the sun that the safe bet is on plutonium-powered spacecraft (ASRG's).  An Enceladus orbiter would study a tiny moon so close to its dominant planet that a polar orbit would be unstable.  Instead, the southern polar geysers and terrain would have to be studied during a series of flybys prior to orbit insertion.  The final orbit could not exceed 60 degrees latitude to ensure a stable orbit.

While missions with costs in the $1.3-1.6B range are possible for both moons, the capabilities of the instruments suites would differ considerably.  For $1.6B, the Ganymede orbiter would carry the full suite of desired instruments.  For the same cost, the Enceladus orbiter would have to forgo desirable instruments such as a narrow angle camera, an imaging spectrometer to study the composition of surface materials, and an ice penetrating radar to directly detect the presence of a subsurface ocean.  These instruments could be added to an Enceladus orbiter, but only by increasing costs to almost twice that of a New Frontiers-class mission.

Either mission, however, would substantially expand on the knowledge of their target moons.  A Ganymede orbiter has been a priority mission for NASA for several years and is currently in contention as an ESA mission.  The discovery of active geysers at Enceladus have made it a priority since it provides our only near term option to directly sample the composition of an icy moon's ocean in the next two decades.  In lieu of a Flagship mission to study the moons of either Jupiter or Saturn, these seem to be worthy missions.

In my next post, I'll look at the impact of instrument costs on options for exploring these two moons.
 
*Note: The budgets for New Frontiers missions are something of a mystery to me.  The PI budget is stated in the Announcements of Opportunity, and the fully burdened cost can be derived from NASA's New Frontiers budget line and include some obvious big ticket items like launchers.  The Decadal Survey studies seem to be giving cost estimates somewhere in between these two numbers.  As near as I can determine, the budget for a New Frontiers mission using the items included for FY15 would be between $1.0B and $1.1B.
Source reports:

Reports can be downloaded from: http://sites.nationalacademies.org/SSB/SSB_059331

Ganymede Orbiter Concept Study
Enceladus Flyby & Sample Return Concept Studies*
Enceladus Orbiter Concept Study

*Despite its title, this study examined a range of missions from multiple flybies to several flavors of orbiters to samples return and landers.

Enceladus Sample Return Mission Concept

I'm finding that the subject of Enceladus missions intriguing, and thought I'd follow up on Wednesday's brief blog on possible Enceladus missions with a fuller description of a possible sample return mission.

I'll start with a report on the limitations of the Cassini mission to explore Enceladus (copied from the presentation, 2007 Enceladus Flagship Study):

• 22 flybys total, if XXM [Soltice mission] is funded [now funded] and executed successfully
• However, due mostly to pointing limitations, only one prime science goal per flyby (e.g., remote sensing, in situ sampling, gravity) - 
• Only three south polar remote sensing flybys between 2009 and 2017
• Limited instrumentation 
• Remote sensing instruments not optimized for high-resolution, wide-area coverage from close range
• Very limited hi-res remote sensing coverage
• Mass resolution and range of Cassini mass spectrometers prevents identification of complex molecule
• Limits understanding of organic chemistry
• No ability to detect biosignatures such as chirality
• High speed impacts prevent detailed molecular analysis of plume gases
• Can’t image plume particles
• No ability to measure tidal flexing
• No subsurface sounding

The proposed LIFE (Life Investigation For Enceladus) mission addresses some of these limitations by flying a Stardust like mission to Enceladus.  Stardust, you'll recall, flew through the the coma of a comet to capture samples of dust that were successfully returned to Earth.  The LIFE mission would adopt a similar mission architecture and mission design philosophy.  The latter focused on designing a mission that fit within the $164M cost commitment (not including launch vehicle).  The Stardust project remained within that cost cap by refusing to accept mission creep.  Worthwhile science investigations were not added if they did not contribute to the core requirement of collecting and returning samples to Earth.  The proposal team does not give a cost for LIFE, but state that they believe it can come in under the previous $1.3-1.8B estimates for a sample return mission.

From http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat%202%20Tsou-Enceladus.pdf

The LIFE mission would enter orbit around Saturn after an approximately eight years flight from Earth.  The team proposes to encounter Titan one to two times for sampling of its upper atmosphere.  (It's not clear if the sampling would be with in situ instruments or by the collection of material to return to Earth.)  LIFE then would encounter Enceladus' plumes and the E-ring that contains material ejected from the plumes several times at speeds of 3-4.5 km/second (compared to Stardust's 6 km/s).  After collecting the samples, the spacecraft would depart from Saturn on a five year return trajectory to Earth for a total mission duration of 13.5 years.

Samples would be collected by an aerogel collector modified to soften the impact for solid particles plus a 'volatiles trapping and sealing deposition' to collect samples of the gases.  (Aerogel is one of the least dense materials known.  Think of it as super styrofoam that can soften the impact of particles so that they remain intact for analysis on Earth.  The LIFE mission would use a version of aerogel modified to better capture fragile material than was used for Stardust.  I'm not sure what technologies would be used to trap the volatiles.  A volatile collection device was proposed for Stardust but later dropped for reasons that I don't know -- perhaps cost.)  

From http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat%202%20Tsou-Enceladus.pdf

To preserve the integrity of the volatile samples, the collection system would be kept below -123 degrees Celsius.  However, the presentation points out that even samples maintained at room temperature would still include important dissolved salts, dissolved organics, and insoluble particulates.

To contain costs, the instrument payload would be kept to a minimum.  In addition to the sample collection devices, a mass spectrometer would measure the composition of the gases released from Enceladus and a magnetometer would help measure the internal structure of Enceladus by looking for induced changes in the magnetic field of Saturn.  (It's not clear what the magnetometer would add over what Cassini's magnetometer can already measure.)  Apparently cameras and a dust counter would be considered, but only if they fit within the cost target.  (The presentation is not clear on this.)

Editorial Thoughts:  I like the core of this mission concept.  However, given the long flight times to reach Saturn, I don't think that flying the minimal mission makes sense.  (The tightly focused Startdust mission made sense -- comet flyby mission opportunities are abundant and fairly inexpensive.)  Even if LIFE were constrained to New Frontiers mission costs, the total mission would be over $1B once launch costs and other overhead were included.  I think that the mission needs at a minimum to also carry an ice penetrating radar to directly measure the extent and depth of the ocean believed to lie within Enceladus and be the source of its plumes.  I also think that a minimal remote sensing instrument suite consisting of a visible and thermal imager should be carried.  The former can study the surface structure and pinpoint plume sources while the latter can measure the 'hotspots' that are the connection between the suspected ocean and the surface.  The remote sensing instruments  could be mounted so that they can study the surface during each flyby while samples are collected.

International involvement could help lower the costs of the mission.  One agency, for example, could provide the spacecraft bus while another could provide the sampling system and return capsule.  Instruments could be paid for by several nations.

If the mission is implemented by several nations, there might be a simple way to enhance the mission.  The mission could be implemented as a two spacecraft system.  A primary spacecraft could deliver the entire system to Saturn and carry the instruments.  A return spacecraft would house the sampling system and carry the return capsule.  Once the samples were collected, the return spacecraft would depart for Earth while the primary spacecraft would remain at Saturn for additional studies of Enceladus and perhaps Titan.  (The Russian Phobus GRUNT mission uses a staged spacecraft system similar to this.)  Different space agencies could implement each piece of the mission.

Look at this blog entry for a description of what kinds of studies a carrier craft could do for Enceladus.

Enceladus: A Rising Star

The latest results from Cassini's flybys of Enceladus increase the likelihood that this small moon has a liquid oceans and the conditions for life.  (See BBC story among many.)  That in turn increases the priority of Enceladus as a target for exploration in the coming decade.  At the moment, the Decadal Survey is studying possible architectures for missions ranging from returning samples from the plume, to multiple flybys, an orbiter, or a lander.  Unfortunately, missions to the Saturn system tend to be expensive, and even a minimalistic multiple flyby mission will probably exceed $1B.  More ambitious missions such as landers or sample returns may well cost much more and would suffer from technical immaturity.  (Which is what kept the Saturn Titan System Mission from being selected as the Flagship mission for the coming decade over the Jupiter Europa System Mission.)

While we wait to see what priority the Decadal Survey gives to Enceladus here are links you may want to follow:

An Enceladus New Frontiers Proposal (solar powered multiple flyby)

Mission Architecture Options for Enceladus Exploration (new astrodynamics techniques make low speed flybys, orbiters, and landers possible)

Enceladus Sample Return (a la Stardust, with samples gathered during flybys through the plumes.  I really like this idea, but think the Decadal Survey may find that cryogenic cooling is required to preserve the ices and any hydrocarbons, which would raise the technical difficulty)

An Enceladus New Frontiers Proposal

Cassini's surprise discovery of active plumes on Enceladus has made that moon a priority target for future exploration.  The key question is whether their is an internal ocean that might -- like an internal ocean might within Europa -- harbor life.  Even if Enceladus turns out to be lifeless, the existence of the plumes provides an inexpensive opportunity to sample the interior of an ice world.

Two Decadal Survey White Papers address science goals for future Enceladus exploration.  The first, The Case for Enceladus Science, lays out the key scientific questions that future missions would address.  If the Saturn Titan System flagship mission (~$3B) eventually flies, these questions will guide the planning of its Enceladus encounters.  However, a flagship mission will not arrive at Saturn for at least another 15 to 20 years.  Several of the authors of the science paper contributed a second paper, The Case for an Enceladus New Frontiers Mission to propose a mission that might launch in the coming decade. (New Frontier missions cost ~$650M.)

This mission would use solar power and batteries to avoid the costs of a plutonium power supply.  Just four instruments would be flown:

• "Mid-IR Thermal Instrument (MIRTI): The mid-infrared thermal mapper measures temperatures in the vent region, and can search for other regions on Enceladus that may be warmer than their surrounding areas. [This instrument would have both finer spatial and spectral resolution than Cassini]
• "Ice Penetrating Radar (IPR): An ice penetrating radar system provides the best single measurement to determine Enceladus’ sub-surface structure, and unlike direct seismometry, does not involve touching the surface with its implications for planetary protection.  [Cassini lacks this instrument]
• "Enceladus Mass Spectrometer (EMS): Thus, to study all biologically interesting amino acids, while also studying bulk composition and high-order hydrocarbons, requires a mass range up to a minimum of 300 Daltons, with a mass resolution (m/Dm) sufficient to resolve molecular isotopes (m/Dm of 500).  [Cassini's mass spectrometer measures only to 100 Daltons; a Dalton is another term for an atomic mass unit.]
  
• "Imaging camera for Enceladus (ICE): The imager is a multi-spectral camera, capable of pushbroom imaging and high spatial resolution (5 m) to resolve the polar vents and other surface structures.  [Resolution would be as fine as 5 meters.]
• "Gravity Experiment: While not strictly a science instrument per se, it is highly desirable to further refine our knowledge of the Enceladus gravity field by performing multiple gravity passes.  Passes at different sub-spacecraft latitudes will help constrain the interior structure."
  

The White Paper goes on to specify a number of mission requirements to fulfill the science goals.  For example, the Enceladus Observer (to give this mission a name) would need at least 12 enounters at 100 - 200 kim altitude at various latititudes including both poles and the equator to determine core size and crust thickness.  Pointing accuracy of the cameras would need to be 2 mrads to accomplish the imaging goals.  Several flights through the plumes would be required.

After a 9.4 year flight to Saturn, the mission begins with four Titan encounters that set up the Enceladus encounters, which would typically occur at 4 km per second every 6.85 days.

Editorial Thoughts: I suspect that many of the readers of this blog imagine missions they would like to see fly.  I'm no exception, and this is a mission I would like to see fly -- with a few modest enhancements.  First, I'd like to see a near IR imager added that would be optimized to image Titan's surface through the spectral windows in it's clouds.  And second, I would enhance the mission's satellite tour.

For the latter, I would include more Titan enounters to study its surface and upper atmosphere.  (The White Paper states, "Titan science could also be accomplished with a spacecraft studying Enceladus, during many Titan flybys.")   Then there would be a series of Enceladus encounters to fulfill the core Enceladus science much like those outlined in the White Paper.  An extended mission could significantly enhance the science return.  New astrodynamics studies have found that two years of encounters with Rhea, Dione, and Tethys would lower the Enceladus encounter speeds to ~1 km per second for around 50 encounters per year.  Encounters with the other moons of Titan with this instrument set would allow comparative studies among the medium sized moons of Saturn.  It's possible that the mission could end with the spacecraft entering orbit around Enceladus, again thanks to astrodynamic methods developed in the last few years.

And while I'm day dreaming, I'd like to see the mission enhanced to the small Flagship class (~$1B) by carrying a Titan lake lander that might be supplied by ESA.  While I understand why Jupiter-Europa were prioritized over Saturn-Titan-Enceladus for the coming decade (technology readiness), I would like to see both an observer-class orbiter and a lake lander fly in the next decade.  Twenty years is too long too wait.

Resources:

The Case for Enceladus Science
The Case for an Enceladus New Frontiers Mission
Mission Architecture Options For Enceladus Exploration