Why I Favor EJSM and Focused Exploration

As with all my editorials, I am not trying to convince anyone to my point of view.  No one on the Decadal Survey has ever heard of me, and my opinion counts for no more than any of the readers of this blog.  Rather, I find that reading a good editorial (and I hope my efforts are 'good') helps me focus my own thinking and reach my own conclusions.  So, with that in mind, here is one of the rare editorials on this blog.

In the next few months, the Decadal Survey will have to select missions to recommend for flight in the coming decade (2013-2022).  We have already been warned that sticker shock is coming and that fewer missions can be flown than advocates and scientists would hope.  At $3.2B and perhaps $4B with inflation and cost increases, the Jupiter Europa Orbiter would consume a substantial chunk of that ~$12-13B budget.  (ESA's Jupiter Ganymede Orbiter, if selected, would be paid for out of Europe's budget for its next large science mission.)  Perhaps most damning, funding EJSM would preclude funding for a flagship mission to Titan and Enceladus. 

My first reason for favoring EJSM is that it would explore three classes of important objects: (1) icy moons that may be habitats of life either in our solar system or others, (2) a large gas giant that is our best analogue for the many gas giants found around other stars, and (3) an intense magnetospheres that serves as a surrogate for other such structures in the universe.  The last two points speak for themselves, but I will expand a bit on the first point.  I believe that the ultimate exploration of an icy moon environment will be at Titan, but that world has such active surface processes that untangling its geologic history will prove difficult.  At Jupiter, we have four moons that provide case studies a range of tidally influenced moons (with Io and Callisto at the extremes) without the confusion of active surface processes.  Finally, Europa may be a habitat for life, and we should explore that potential with a highly capable spacecraft.

My second reason for favoring EJSM is that the JEO mission is ready to fly.  A decade of technology development and mission design has brought the mission to a point where risks are low.  We learned last year in the shoot out between EJSM and the Saturn Titan System Mission (TSSM) that the same was not true for concepts to explore the Saturn system (click here for the orbiter and here for the in situ elements). 

My final reason for favoring EJSM is the lesson learned from Mars exploration in the last decade.  At the Red Planet, we have learned that a series of highly capable missions can together bring a deep insight into a world or, in the case of Jupiter, a system of worlds.  NASA's JEO could be just the most capable of a fleet of craft that could also include ESA's Jupiter Ganymede Orbiter, Japan's magnetosphere orbiter, Russia's Europa lander, and possibly penetrators for Ganymede and/or Europa from another space agency.  Together, this flotilla would do for the Jovian system what a decade of missions have done for Mars.  What we learn from the Galilean moons will build towards our understanding of ice-ocean-rock moons including Titan and Enceladus.

Of these three arguments, I personally find the final most compelling.  We have done most of the easy missions for the solar system.  Significantly deepening our understanding of key worlds and systems will require focused exploration.  (Even if JEO turns out to be the only mission to fly to Jupiter, it is capable enough that it would count in my opinion as focused exploration.)  In the coming decade, I favor focused exploration on three and a half targets.  First, there will continue to be Mars which is likely to receive several orbiters from Russia, China, and ESA/NASA, 2-3 rovers, and possibly a network of science stations.  Second, could be the Jovian system.  And third, there could be Venus which could be the recipient of Russian and American landers, a European balloon platform, and several orbiters.  All in all, the next decade, thanks to the combined contributions of a number of space agencies, could see the in-depth exploration of the solar system expand from one target (Mars) to several.

The half target in my scenario would be the Saturn system.  Eventually, we need to return there with flagship class spacecraft.  I found the case laid out for a flagship class orbiter to take the global study of Titan to the next level in the TSSM study compelling.  Not only will a battery of instruments be required, but a high power communications system (which drives the need for a flagship class spacecraft) is essential to return the data stream.  However, there are, I think, a couple of low hanging fruits available for the Saturn system.  The first is Enceladus, for which a New Frontiers-class mission with with advanced instruments should provide a significant advancement in our understanding.  The second is in situ probes for Titan, which is about the easiest place in the solar system to land on or fly or float above.  The proposals in progress for a Discovery-class lake lander and a Discovery-class airplane suggest that in situ Titan probes could be within the budgets and technical capabilites of several space agencies in the coming decade.  The key problem for most in situ probes is the data communications challenge -- there simply isn't room within these probes to house the power systems and antennas to return large amounts of data.  So, I favor a New Frontiers class orbiter that would switch between focused Enceladus studies and relay duties for Titan in situ probes over the course of perhaps a decade or more in orbit around Saturn.

For the past year, I have closely followed the Decadal Survey process as well as the planning processes of other space agencies.  In this blog entry, I lay out the conclusions I've reached.  I hope that the readers of this blog will lay out their own or challenge mine in their comments.

Saturn Atmospheric Probes

A key goal of planetary science has been to understand the formation, evolution, and internal structure of the gas giants.  Galileo and Cassini have studied the outer skin of Jupiter and Saturn, but the data returned has not provided answers to key questions on these issues.  The Galileo atmospheric probe was intended to address many of the compositional questions (which relate to key questions of formation and evolution), but it was skunked by entering in a hot spot that both lacked clouds and water vapor.

In the last few years, plans have been made to address these questions with new and extended missions.  The Juno orbiter to Jupiter will address all three questions by probing the composition and structure of the atmosphere and will study the deep interior through precise gravity and magnetic measurements.  Its close orbit -- cloud skimming on the scale of the Jovian system -- enables these studies.  The final stages of the extended Cassini mission will bring it equally close to Saturn for precise gravity and magnetic measurements.  (Alas, Cassini lacks Juno's microwave radiometer for deep probing of the atmospheric structure and composition.)

Juno and Cassini will not be able to address all the key composition questions.  Those lines of inquiry require the precise measurements that can only be made from within the atmosphere.  This is considered so important that the original concept for what became Juno had both the orbiter and new atmospheric probes for Jupiter.  Unfortunately, the technology to build and test heat shields for the extreme heating encountered in a Jupiter entry has been lost.

Fortunately, the challenges for entering Saturn's atmosphere are much less severe, and scientists are proposing an atmospheric entry mission for that planet.  While Saturn is not identical to Jupiter, it is similar enough that measurements made there will answer questions relevant to both planets.  (Even if new Jovian probes were programmatically possible, scientists would want probes for Saturn to compare the two worlds.)

Two presentations at a recent Outer Planets Panel for the Decadal Survey addressed the science rational and possible approaches for a Saturn probe mission.  A fundamental challenge of the mission is that reaching the depths where water vapor will be present (a key measurement) requires operation to depths of 50 - 100 bars.  (A bar is the pressure of Earth's atmosphere at sea level.)  Not only does this require a sturdy pressure shell, it is difficult to maintain adequate communication rates with a relay spacecraft, and the battery would have to be large to provide power for an extended descent.

Several solutions to the problem of studying the deep atmosphere were proposed:

• Have the probe piggyback a microwave radiometer that would measure water abundances prior to entry from just above the atmosphere as Juno will do for Jupiter.  The radiometer would be jettisoned just before entry.  Alternatively, the carrier craft could carry the radiometer, although that would require a flyby or orbit that passes just above the atmosphere.  
• Have a two stage probe where the larger, more instrument laden probe falls slowly on a parachute while a smaller probe with just an instrument or two falls quickly to the necessary depth
• Forgo the deep measurements and focus on multiple shallow probes to study several locations in Saturn's atmosphere

While no cost estimates were provided, one of the presentations states that a probe mission "may exceed" the cost of a New Frontiers (~$650M) mission and recommends a new class of $1.2-1.5B missions.  (This sounds like a strong hint that a probe mission is likely to be closer to $1B than $650M). 

Editorial Thoughts: In-depth (literally) exploration of the gas planets likely will be a continuing priority for decades.  After Juno, the Cassini end of mission, and an eventual probe mission to Saturn, the science community is prioritizing missions in the same class to Uranus and Neptune.

In theory, a Saturn probe does not require a dedicated mission.  Any spacecraft traveling to or passing by Saturn could drop off a probe.  It's possible that Saturn may be a busy place in the coming decade with the proposed Argo mission passing through on its way to Neptune and the Kuiper belt, a possible Titan lake lander, and a small orbiter to continue the exploration of Enceladus and possibly Titan.  Celestial mechanics may make piggy backing difficult in some of these cases.  The Argo craft, for example, would need to thread a narrow path to get the gravity boost, and that path may not allow a probe delivery and relay. Even if celestial mechanics cooperate, carrying the extra weight of the probe and the communications relay equipment will add costs to a mission.

I personally would like to see ~$2B budget to continue the exploration of the Saturn system in the coming decade with a Titan lake lander, a small orbiter, and an atmospheric probe or two (in that order of priority).  With approximately $7B of the coming decade's expected $12B budget for planetary missions, committing these funds would mean many other planetary targets would not be explored. (I'd also like to see $2B dedicated to Venus studies and $1B or so to small bodies studies...)   It will be interesting to see where Saturn falls in the Decadal Survey's priorities.

Resources

Presentations to the Outer Planets panel (images in this post taken from the first presentation)

Probes, Sushil Atreya, Univ. of Michigan


Probes Technology, Tony Colaprete, NASA Ames Research Center