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.
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.