BepiColombo and Comet Sample Return

The ESA/JAXA BepiColombo mission that once faced possible cancellation because of budget overruns, has been approved for development (at the higher costs) to eventually reach Mercury orbit in 2020.  The BBC has an article on the  (Mercury mission clears key hurdle) approval.  As the article states,

"Dr David Rothery, the lead scientist on Bepi's Mercury Imaging X-ray Spectrometer (MIXS), said the science case for another Mercury mission was exceptional.

'The best way I heard it expressed, very kindly by a member of the Messenger team, was that Messenger is providing the 'hors d'oeuvre' and BepiColombo will be the 'feast'.

'BepiColombo has more instruments and more capable instruments than Messenger does."

I have also reposted the blog entry on why a comet sample return mission is so hard.  I normally clip the images from presentation on my workstation, but that machine has been tied down doing multi-day data analyses.  For the original post, I clipped the images on my netbook.  Lesson learned, clip from a big screen.

Bruce Moomaw has also been digging into the history of presentations on this topic and has some corrections and extensions to my post:

I had written: "The small body science community has ranked a comet sample return as its top priority for the last decade or so.  This mission was included (optimistically, for reasons explained below) in the first set of mission candidates for the New Frontiers (~$650M) missions and remains on that list."

Bruce corrects my poor memory: "Actually, the comet sample return included in the first set of NF mission candidates (as listed in the previous Decadal Survey) was the less ambitious warmer Comet Surface Sample Return, rather than the Comet Cryogenic Sample Return, which was recognized in the first Survey as a mission sufficiently difficult that it was Flagship class.  And it's the Comet Surface Sample Return that was listed among the acceptable candidates in the first New Frontiers solicitation.  The considerably greater difficulty of the CCSR has been clearly recognized from the start."

"Note also the very next page in the CNSR (aka CCSR) presentation that you quote: "If you can’t maintain cold enough temperatures to preserve the ice sample during return, then at least collect the evolved gases.  Must also store them in a way that prevents chemical changes (e.g., using special getters, different chambers filled at different temperatures)."  The Small Body scientific community seems to regard this as a clearly justifiable mission, although of course they'd prefer a cryogenic return if there was the money for it. 

" I do note from Hal Weaver's and Michael A'Hearn's Nov. 2007 advance report that CSSR would use "flasks" (in the plural) to collect those gases, which presumably is the same thing Weaver mentioned in his new CNSR presentation.  Note also from the 2007 mission presentation (pg. 17) that only one out of 11 SDT members thought volatile preservation in flasks was an absolute necessity for the mission, although the others considered it "highly desirable" and it is baselined."

I still think that this is a very complicated mission given the sampling challenges and the necessity to preserve the evolved gases.  I hope that the community comes up with good solutions to the problem.  I want to see this mission fly.

I really appreciate corrections.  Planetary exploration is a huge field, and I follow it part time as a hobby while (supposedly) writing my dissertation in an unrelated field.  Please point out my errors and omissions!

Coming Planetary Budget Cuts?

A number of news reports suggest that the U.S. administration will ask non-entitlement* and non-defense budgets to take a 5-10% budget cut in the Fiscal Year 2011 budget.  A budget cut at the high end of that range carried out over a decade would eliminate a mission roughly the size of a New Frontiers mission.  Additional budget cuts in future years would also be possible.

Currently, NASA's projected budgets (from the FY10 budget package) would provide approximately $12B over a decade to develop planetary missions (http://futureplanets.blogspot.com/2009/09/thoughts-on-scary-messages.html).  There has been talk that NASA would receive an increase next year to put the manned spaceflight program back on track.  Also, there are long term promises to increase spending on science programs of all types.  Given the competing pressures, several scenarios could play out; for example:

• All NASA programs receive budget increases
• The manned portion of NASA's budget receives an increase but the science program remains flat at best or is cut
• All NASA programs receive budget cuts

Editorial Thoughts: Whatever happens to NASA's budget next year, the political pressure to reduce deficits seems strong in both parties.  We may see multiple cuts to the planetary program over the coming decade.  Given this, in my opinion, a key measure of the success of the Decadal Survey is to propose a prioritized set of missions that remains robust even in the face of declining budgets (and likely cost overruns on some early missions in the queue).  It is quite possible that the lower priority missions will never fly because of insufficient funds.  An interesting question is whether the Survey will prioritize smaller missions (support a diversity of targets) or larger missions (in-depth study of one to two targets) as higher priorities.

*Budget Primer: I find the budgeting processes of other nations confusing and suspect that many of this blog's readers may find the U.S. process confusing.  There are four large pots of spending in the U.S. budget: Entitlements (social security and medical funding for retirees, etc.) at 54%, payment of interest on the national debt 8%, military spending 21%, and everything else 17%.  (Breakdown from FY08 from  http://en.wikipedia.org/wiki/United_States_federal_budget)  Interest spending cannot be avoided, and it is politically challenging to cut entitlements and military spending.  Therefore, attempts to reduce deficit spending tend to fall on the everything else category that includes NASA, the FBI, national parks, and many other functions. 

Resources:

Sharpening the budget cleaver

Is There A 10% Budget Cut In NASA's Future?

Why a Comet Sample Return is So Hard

The small body science community has ranked a comet sample return as its top priority for the last decade or so.  This mission was included (optimistically, for reasons explained below) in the first set of mission candidates for the New Frontiers (~$650M) missions and remains on that list.  In the current Decadal Survey, the goal of the community appears to secure funding for serious technology development to enable fhe mission for the following decade.

A recent presentation to the Decadal Survey Small Bodies panel (http://www.spacepolicyonline.com/pages/images/stories/PSDS%20PB2%20Weaver.pdf) illustrates why this mission is so hard.  The first requirement, to keep the ices collected frozen, may be the hardest.  It is the ices and their record  of the early solar system's volatile inventory that makes this mission so valuable.  The samples, however, have to be kept frozen during the collection, then within the sample return capsule for the long flight home, during the atmospheric entry, and finally during the capsule retrieval.

(The freezing temperature of water at sea level is 273 K).

Another challenge is design the mechanism(s) that will collect the sample from potentially meters inside the comet.  A recent examination of technology obsticals for sampling near Earth asteroids (http://futureplanets.blogspot.com/2009/10/parting-look-at-primitive-body-sample.html) put it bluntly: it is not clear how to design a reliable sampling mechanism for near Earth asteroids.  This is probably even more true when your goal is not to sample the surface of a rocky asteroid, but to sample at depth a mixture of ice and rock.

A third challenge is that we know little about the surface of comets.  We know only that they exhibit a wide range of geomorphologies.  Whether that diversity extends a variety of surface and near surface types at the scale at which a lander would sample is unknown.  This simply makes designing the sampling mechanism that much harder.  The Rosetta mission's lander will tell us a great deal about the surface of one comet, but even if we can rely on its data for all comets, the answers could not be incorporated into a mission that would fly before the end of the coming decade at the earliest.

It now seems to be generally accepted that returning a frozen sample of a comet is beyond the scope of a New Frontiers class mission and is probably is a $1-2B mission.  In recognition of this, NASA is now willing to allow a mission that returns a dust sample plus the thawed remains of any volatiles:

"Scientific community interest in a Comet Surface Sample Return (CSSR) mission has been very high for many years. The advantages of such a mission have been stated in many documents including the decadal survey. Flyby missions to comets are fairly simple, and the Deep Space-1, Stardust, and Deep Impact missions have produced remarkable data. Rendezvous missions such as the ESA’s Rosetta mission (Figure 2.3) are more challenging, and a sample return mission can take twice as long as a rendezvous mission, thereby increasing cost and risk. The decadal survey concluded that bringing back a warm (i.e., non-cryogenic) sample was within a New Frontiers mission budget. While cometary science goals make the return of a cryogenic core sample highly desirable, such a mission may not fit within the fiscal limits and programmatic timescale of the New Frontiers Program. The science yield from a warm sample return mission will have to be strongly defended by proposers."

From Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (http://www.nap.edu/openbook.php?isbn=NI000529)

 One Discovery mission proposal would collect only dust samples at low velocity from the comet's dust streams during a rendezvous (http://futureplanets.blogspot.com/2009/09/comet-coma-rendezvous-sample-return.html).

More on Saturn Probes

After my previous post on Saturn atmospheric probes, Bruce Moomaw pointed me to a 2007 presentation (http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/41220/1/07-1820.pdf) that had much more information on trajectories for the Saturn encounter.  In this study, two probes would descend to 10 bars (a shallow depth) for in situ studies while the carrier craft would probe the atmosphere to 100 bar depths with a microwave radiometer.  The latter measurements can only be made from within 60,000 km.  A flyby carrier can easily get close enough with a periapsis of 11,700 km.  An orbiter would come only within 60,000 km on it's initial trajectory.  Subsequent orbits, such as those planned for the Cassini end of mission, would have to be used to get close enough for effective radiometer measurements.

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