Wednesday, December 28, 2011

Planetary Missions Up For Review

You may never have heard of the upcoming NASA Senior Review, but it may have as big an impact on planetary exploration in the coming decade as any mission selection except possibly the approval of a Flagship mission to Mars or Europa.

First, though, some background.  When a mission is first approved, its budget includes funds for operating the spacecraft in flight and initial analysis of the scientific data.  The period of flight covered by this budget is known as the prime mission.  Once the spacecraft completes the prime mission, there's usually years of life left in it to go to new targets (such as the flights of the Deep Impact and Stardust spacecraft to second comets) or to continue collecting science at the prime target (such as the rover Opportunity at Mars or Cassini at Saturn).  These mission continuations are called extended missions.

NASA has to budget substantial funds to operate its many planetary and other science missions in their extended missions.  Generally the costs of these extended missions aren't well publicized (likely because NASA and the press reasonably assume few people in the public care).  However, we know that the Cassini mission in its current extended mission costs about $60M per year (probably near the high end) and the EPOXI (formerly the Deep Impact) mission costs about $5M per year (probably near the low end).  Cassini could operate through 2017, so the rest of its extended mission will cost ~$300M (assuming flat budgets).  The EPOXI mission will cost around $45M if its fuel holds out for a 2020 asteroid flyby.  Compare these costs to the $425M dollar cost of a new Discovery mission.

Then consider the list of missions that are in or will enter extended missions in the next two years:

Cassini (Saturn)
EPOXI (possible 2020 asteroid flyby)
GRAIL (lunar orbiters)
Kepler* (planet finding space telescope)
Lunar Reconnaissance Orbiter
Mars Odyssey Orbiter
Mars Reconnaissance Orbiter
Opportunity Mars Rover
MESSENGER** (Mercury orbiter)
*Kepler is an Astrophysics Division mission, but it, too, will be up for review
**MESSENGER's extended mission was recently approved in a special Senior Review

To decide whether or not to continue funding an extended mission NASA holds Senior Reviews.  These panels evaluate the potential scientific return of the extended mission against its costs.  The panel can recommend continued funding as planned, enhanced funding, reduced funding, or mission termination.  As I understand it, NASA provides a budget for of its science divisions for extended missions.  The panel needs to recommend the mix of missions and their funding to fit within the budget.

Early in 2012, a series of Senior Reviews will be held that appears to cover all NASA's planetary, astrophysics, and heliophysics missions that are in or will enter extended missions by 2014.  (I don't know if the Earth science missions will be included in this senior review.)  As backdrop to this review, remember that each of these divisions are projected to face flat or declining budgets in the next several years.  In addition, NASA will need to find funds in its declining budget from these programs to help fund the completion of the James Webb Space Telescope, the over budget successor to the Hubble Space Telescope.

The review panels have a tough job.  Each of these missions continue to gather data for which there will not be a chance to gather again for many years or even decades.  How do you rank, for example, the science that will be enabled by the next 1%* of Mars imaged at 0.3 m resolution by the Mars Reconnaissance Orbiter against the next 40+ planned Titan flybys by the Cassini spacecraft?  (*In it's first four years of operation, the MRO's HiRISE camera imaged about 1% of Mars at this resolution.)

While NASA is still waiting to hear (or be able to publicly talk about) its future planetary program budget, the indications are that the budget for extended missions will be tight.  Dollars reserved for extended missions will not be available to fly and build new missions.

In my next post, I'll look at what is at stake for one extended mission, the Cassini mission at Saturn.

Process for the Planetary Science Division's (PSD) Senior Review from here

Most recent PSD update from here


My Google searches failed to find the website for the Planetary Science Division's Senior Review.  (Either poor searching, or it's posting may have been delayed because the planetary budget appears to be in particular flux.)  If you are interested, however, you can read the guidance for the Astrophysics programs' Senior Review along with results from past reviews.

Each extended mission team has to write a proposal to support its request for continued funding.  The call for proposals includes this description of the process for the reviews:

Instructions to the Senior Review Committee (SRC):

In the following descriptions, “project” denotes a full mission or project in the traditional sense or U.S. participation on a mission led by an international partner. NASA HQ will instruct the Senior Review panel to:

(1) Rank the scientific merit of each project on a “science per dollar” basis (based upon expected returns during 2013 and 2014) in the context of science goals, objectives and research focus areas described in the SMD Science and Strategic Plans.
(2) Assess the cost efficiency, technology development and dissemination, data collection, archiving and distribution, and education/outreach as secondary evaluation criteria, after science merit/usefulness.

(3) Based on (1) through (2), provide findings to assist with an implementation strategy for Astrophysics Division missions in extended operations for 2013 and 2014, including an appropriate mix of:

    Projects continued as currently baselined;
    Projects continued with either enhancements or reductions to the current baseline;
    Project terminations.

Wednesday, December 14, 2011

Titan on the Installment Plan

Without planning it, almost all my posts for the last several weeks have looked at options for modest Flagship-scale missions, whether from NASA (Europa), ESA (Ganymede), or both jointly (Mars).   That's largely by default -- there's little news for either small or medium-cost missions.  For lower cost missions (<$500M), NASA will not select its next Discovery mission for several months and the next competition is likely two years away, ESA's next selection for a medium class science mission (roughly equivalent to a NASA Discovery mission) is years away, and Japan, India, and China continue developing their next missions.  For medium scale missions(~$1B), I've not heard a date for the next NASA New Frontiers competition, and I believe that the agency is waiting to see the budget projections from the FY13 budget proposal before deciding when to select the next medium scale mission.

That leaves the modest cost ($1.2-1.5B) Flagship missions as the current topic of news.  Scientists have identified three high priority worlds for future intensive investigations: Mars, Europa, and Titan.  Fulfilling the identified high priority science goals for each will not come cheaply: $8.5B for a Mars sample return mission, $3-4B for Europa, and ~$4B for Titan (per Decadal Survey estimates).  To help make these costs more manageable, engineers have found ways to explore Mars and Europa on what would be essentially the installment plan. 

The Mars sample return program would be split across four missions and the Europa program would be split across three missions (including an eventual lander(s)).  The four Mars missions would be a 2016 orbiter that provides a communications relay, the 2018 rover that would collect samples, a mission to retrieve and launch the samples into Martian orbit, and a mission to collect the samples from orbit and return them to Earth. 

The Europa missions would be a multiple-flyby spacecraft that would collect high volume remote sensing data, an orbiter that would carry the minimum instrument set for measurements that could only be done from orbit, and eventually one or more landers.

So could Titan be explored in a series of smaller missions?  The Titan Saturn System Mission Flagship proposal called for a highly sophisticated orbiter, a balloon platform for aerial surveys, and a probe to sample the atmosphere and a northern lake.  As many of you already know, mission teams have already taken advantage of this natural division to propose three low cost missions.

Before looking into those missions, it’s useful to look at the advantages and disadvantages of Titan from a mission designer’s point of view (it’s scientific and exploration advantages are well known to readers of this blog).  On the plus side, Titan poses a thick atmosphere that makes entry, flight, and landing easier than on any world except possibly our own.  It is frigidly cold, but for either a short-lived probe or a long-lived probe with a nuclear power supply, that is likely not a problem.  (In fact, the AVIATR plane depends on constant movement to bring cool air past its power units to keep from overheating.)  Unlike Europa, there are no harsh radiation belts to fry electronics at Titan.  And there are no Richter-scale technology developments needed to continue exploring Titan as would eventually be needed for a Mars sample return to launch the samples from the surface and retrieve them in Martian orbit.

On the negative side, Titan is far from Earth.  This results in long cruises, typically around seven years, with mission operations costing $7-10M a year during cruise.  On a $425M Discovery mission budget, that’s a significant piece of change. 

Perhaps more difficult is that at those distances, data rates either must be low if a small antenna that could be carried by a probe or plane is used, or the spacecraft must have the power supply to sustain high bandwidth communications to Earth.  It is for this reason that an attempt to define a lower cost Titan orbiter in 2007 determined that the lowest practical cost for an orbiter would be ~$1.5B.  A less expensive spacecraft couldn’t support the data rate to return a high resolution map of Titan.

Despite these challenges, teams have proposed two Discovery class missions and one that fits between the Discovery and New Frontiers ($1B) mission classes.  The inherent tradeoff for all these missions is that they accept low data rates to fit within allowable mission budgets among other tradeoffs that include smaller instrument compliments.

For the TiME Discovery proposal, which would land a long-lived probe on a Titan lake, the low data rate probably does not incur significant science tradeoffs.  Its composition and physical properties instruments are inherently low data rate.  The probe will carry a camera, but the number of pictures would likely be limited.  (I also suspect that the view from the middle of an arctic lake under a hazy sky far from the sun is likely to be low contrast, which should enable efficient data compression of images.)

The following list compares quoted data return (some calculated on the back of an envelope from data in published documents, so all assumptions may not be the same) for one year of operation for different mission proposals.  I’ve included a Martian orbiter for comparison.

AVIATR Titan airplane (~$700M) - ~1 Gbyte
JET Saturn orbiter with multiple Titan and Enceladus flybys ($425M) – 120 Gbytes
2007 Titan orbiter concept (~$425M)– ~300Gbytes
2009 Titan Saturn System Flagship Titan orbiter (several $Bs)- ~600Gbytes
Mars Reconnaissance Orbiter (MRO) ($720M) - ~1,000Gbytes

In one sense, these numbers may be misleading.  The AVIATR mission, for example, would employ elaborate procedures to allow scientists to determine which data to return.  Each of its bits may be 10X to 100X as impactful as a bit from the Mars Reconnaissance Orbiter.  However, the high data rates of MRO have allowed extensive coverage of terrain types and monitoring that the 1GB of the AVIATR mission would not allow.

The message is that mission costs can be reduced, but there are few magic bullets.  Reductions in cost come at the expense of capabilities, with the amount of data returned a key tradeoff.  Since these missions are justified by the data they collect, this is a significant tradeoff. 

If data rates are a challenge for Titan exploration, the relatively benign environment allows for missions to fly at cheaper incremental rates than for Mars sample return or Europa.  For approximately $1.5B (if the proposer’s cost estimates are correct), NASA or another space agency could fly three missions to Titan.  That is approximately NASA’s contribution for the 2016 and 2018 Mars missions or one of the new lower cost proposed Europa spacecraft.  There would probably be some cost savings if one or more of the Titan missions were combined.  The AVIATR plane mission would be greatly enhanced if it flew with an orbiter such as JET that could relay data back to Earth.  The amount of high resolution imaging from the plane would increase many times over.

Two important caveats must be considered before getting too excited by visions of Titan missions.  First, we haven’t seen independent cost reviews for these missions, and proposers have been known to be too optimistic.  Second, we don’t know how these missions would rank scientifically in a mission selection competition.  Would 1Gbyte of high resolution imaging from a Titan plane provide better return on the dollar than a sample return from a comet or a lunar geophysical network, for example?

NASA currently is committed to supporting the priorities of the Decadal Survey, which after funding the Discovery and New Frontiers programs, prioritizes the 2016 and 2018 Mars missions and if they cannot be flown, a Europa mission, and as third priority a Uranus orbiter.  However, we are seeing a new level of creativity from NASA and the planetary science community in finding ways to continue exploration of the Solar System’s highest priorities.  With the proposals on the table, Titan is in play as a target, either through individual missions or through a relatively inexpensive program for several low cost missions.

Monday, November 28, 2011

Retreat From Mars?

"Without more funding and a firm, renewed commitment to Mars from NASA, "MSL will be the last thing we put down on the ground and MAVEN will be the last orbiter we do unless it comes from the Discovery program," said Jim Green, director of NASA's planetary science division."
Will NASA retreat from Mars after string of successes?
- Spaceflight Now

As I write this, the latest news on the Phobos-Grunt spacecraft is that it remains in orbit, apparently operational, but has not communicated with Earth in the last several days.  In the meantime, the Curiosity Mars Science Laboratory launched successfully and its flight so far has been boringly good.

My hopes are with the Russian engineers and scientists as they attempt to re-establish control of the spacecraft and find another target, potentially an asteroid, now that that mission's window to begin the flight to Mars apparently has closed.

Looking to future missions, the news is also mixed.  The only committed and funded mission to follow Curiosity to Mars currently is MAVEN, which is a Discovery-class orbiter to study the Martian upper atmosphere.  (MAVEN was the last mission in the now defunct Mars Scout program which also flew the Phoenix lander.)  The European Space Agency (ESA) and NASA both have hopes of flying a joint orbiter for 2016 and a joint rover for 2018.  Neither has the money currently committed to do the missions alone and neither currently has the committed funding to implement their planned portions.

However, there are encouraging signs of progress.

First a brief recap on the 2016 and 2018 proposals.  The 2016 mission would fly an orbiter that would carry instruments to study the Martian atmosphere and continue high resolution imaging of the surface.  Most critically, this orbiter would also be in place to act as a communications relay for a proposed 2018 rover.  The rover would carry a highly sophisticated ESA instrument suite to examine Mars for signs of life and habitability and NASA equipment to collect and cache samples for eventual return to Earth.  The 2016 mission would also carry an ESA demonstration lander that would function for up to four days on the surface.

A new option is on the table now, that could be an exciting enhancement to the joint program.  When NASA was unable to commit a launch vehicle for the 2016 orbiter, ESA asked Russia to contribute a launcher in return to having its instruments also fly.  The initial reaction reportedly was, nyet!, but recent news sounds more encouraging.  Scientists at Russia's Space Research Institute, IKI, have put together proposals for a minimum and maximum level of participation.  For the minimum proposal, Russia would contribute unspecified instruments for the orbiter that have previously flown on other Russian Earth and planetary missions.  The maximum proposal would add two to four Russian landers to replace the currently planned ESA demonstration lander.  At least one version each of two different landers would fly, and if technical issues could be worked out, two copies of each version might fly.  The first version would be a penetrator that would carry a weather station, and would be similar to the landers jointly developed with the Finns.  The second would be a soft lander that would open up, petal style once on the surface.  The proposed instrument is a French seismometer (which I believe is also the seismometer planned for the Mars GEMS lander in the current NASA Discovery competition).  Power sources aren't mentioned in the news articles, but the implication is that both landers would be long-lived.  Russia reportedly has at least one flight ready copy of each lander left as spares from its Mars-96 mission that experienced a launch failure.

There is no mentioned whether or not the Russian orbiter instruments would be in addition to the currently planned instruments (most supplied by NASA and one supplied by ESA) or would replace one or more of the planned instruments.

Russian scientists are currently in discussions both with ESA and with the Russian political system to secure approval and funding.

On the European front, ESA currently has 850M of the 1B Euros committed from its member nations that it needs to conduct its portion of the currently planned 2016 and 2018 missions.  In the meantime, European financial problems are causing ESA to plan to reduce costs and Europe to debate whether to reduce funding for its flagship Earth observation program (GMES).  I believe that GMES is a European Union program, rather than an ESA program.  Either way, the news speaks to a tightening of budgets for space programs by European governments.  Dropping the ESA lander from the 2016 mission would help reduce the gap between committed Euros for the Mars program and what is needed.

Then there is the American side, which is possibly the most confusing.  At the moment, the President's Office of Management and Budget (OMB) is refusing to allow NASA to commit funds from future budgets to ESA to support the Mars program.  At the same same time, Congress has just passed NASA's Fiscal Year 2012 budget with full funding for the Mars program and a rather pointed requirement that NASA implement the recently completed Decadal Survey that included the Mars program as its highest priority planetary Flagship mission.  (If this seems crazy, welcome to our system of separate powers in government.  Congress can set direction only for the current year's budget; commitments for spending from future budgets (eventually to be ratified or modified by Congress one year at a time) must be approved by OMB.)

The good news is that NASA has funding to support the development of its instruments for the 2016 orbiter and the 2018 mission remains a priority -- for the remainder of FY12.  OMB has promised to make clear its position on NASA's ability to make future commitments to enable ESA to continue to count on NASA for 2016 and 2018 in its FY13 budget to be revealed in February.  Reportedly, the responsible OMB official has stated that a key concern is that going forward with the 2018 mission is just the down payment on a program of missions to return samples that could cost $8.5B.

Editorial ThoughtsE: The 2016 and 2018 missions are both excellent missions, and I hope to see them fly.  Even without NASA's equipment to collect and cache samples, the 2018 rover mission should fly.  All of this political maneuvering is frustrating as hell, and my hat is off to the NASA officials who are handling this situation as the professionals that they are.

The addition of Russian instruments and landers would only make the 2016 mission more exciting assuming all the technical and managerial issues can be worked out without unacceptable additions to technical, schedule, or budget risk.

I can understand, however, OMB's position.  It is looking at a suite of programs for human spaceflight and the over budget James Webb Space Telescope that it and Congress have agreed are NASA's top priorities and for which the current NASA funding likely is insufficient.  The American budget problems are likely to reduce NASA's funding or at very best keep it flat.  Commitments made in haste may be undone later.

I have come to wonder whether the American political process will approve the start of a Mars sample return program without the discovery of strong signs of life or at least habitability on Mars.  If Curiosity or another mission (such as the 2018 rover) finds complex hydrocarbons consistent with life, for example, I think support for the sample return will be strong.  Without it, it has proven difficult over the past twenty years to convince the political process that bringing back "a bunch of rocks" justifies the expense of many billions of dollars.

So, to address the question in the title of this post, I don't yet know if this will be a retreat from Mars.  The fact that Congress held a hearing on NASA's Mars program budget impasse and required continued spending on it for this year is an encouraging sign.  The scientific community remains committed to supporting the program.  In normal times, I would be optimistic that these problems would be worked out.  Given the budget problems and politics in the US, Europe, and Russia, I am only hopeful that they will be.


Russia's proposals for the 2016 mission article at Russian Space Web and a shortened version at the BBC website.

Aviation Week article on NASA budget situation regarding its Mars program, U.S., Europe, Russia To Talk Mars Mission

Friday, November 18, 2011

" at Thera, and you might be able to taste Europa's ocean!"

Artist's conception of a lake beneath a chaos region on Europa but above the global ocean.
Courtesy of University of Texas at Austin

"So for all of you people who were secretly hoping the thin-icers would win the argument because you are hoping to see humans send a probe onto Europa's surface and maybe even drill through the ice to its ocean, you have a consolation prize... land at Thera [Macula], and you might be able to taste Europa's ocean!"
Emily Lakdawalla, 
The Planetary Society Blog   

As many of you probably know by now, a paper to be published in the journal Nature next week postulates a new theory for the chaos terrain on Europa.  As the nomenclature implies, 'chaos terrains' are jumbled areas on the surface of Europa that appear to be huge blocks of ice scattered across the surface with rifts between them.  This new theory suggests that these areas are caused by the formation of large subsurface lakes beneath the surface.  Initially, the formation of the lake reduces volume beneath the surface (water fills less volume than ice), causing the surface to crack and collapse.  When the lake eventually refreezes, the volume expands pushing the shattered surface up into a dome.  The surface of a chaos region, then, is a mixture of surface ice and water that upwelled from the subterranean lake.  Current imaging suggests that up to 50% of Europa's surface represents chaos terrains ranging from fresh to old.  One area that looks to represent chaos terrain with an active lake with greater volume than all the great lakes of North America combined is Thera Macula.

These chaos regions appear to represent the best locations to sample the ocean that underlies the surface of ice at Europa.  Beneath that ocean lies a rocky core, which is tidally flexed and heated by the gravitational pull of Jupiter.  That puts the ocean in contact with minerals and energy (think of the hot vent plumes beneath the Earth's oceans) that could provide the mixture needed to support life.  In this respect, Europa may be unique among the icy worlds believed to have subterranean oceans.  On Ganymede and Titan, by contrast, the oceans would be sandwiched between layers of ice, removing the oceans from the minerals and heat needed to support life.  (A lively debate continues as to whether or not Enceladus' plumes come from an ocean in contact with the rocky core.)

The NASA press site has a summary of the findings and nice illustrations.  I recommend reading Emily's write up even if you've read this or other press accounts.  She does a nice job expanding on the press release.  

Fortuitously, NASA has also asked a team of scientists and engineers to develop the concept for a minimalistic Europa lander mission.  This follows the definition of minimalistic mission concepts for an Europa orbiter and a multi-flyby mission (see my post, Europa - New Options) that would each cost approximately $1.5B (not including launch).  The hope is that the lander mission will also cost ~$1.5B, but that's not a requirement.

Since the team has yet to formulate it's plan, all details are subject to change.  However, I'll summarize the presentations giving the current thinking captured in the kick off meeting (presentations posted here).  

Europa's surface roughness presents a landing challenge.
Except as noted, all illustrations are from the Europa Lander Forum presentations posted at

The surface of Europa at all scales at which it has been imaged (the best images come from the Galileo mission) looks forbidding to land on.  Jumbled surfaces of ice appear everywhere, especially in the now high priority chaos regions.  For this reason, the current concept includes two landers in the hope that one would make it to the surface intact.  The landers would descend under rocket power, much like the Mars Phoenix lander.  An instrument (lidar) would scan the surface to find the location with the smoothest surface, and the lander would steer itself to land there.

Radiation levels on the trailing side of Europa.  The red line shows the boundary between the high radiation trailing hemisphere (left) and the low radiation leading hemisphere (right).

The jagged surface presents one problem.  The radiation at Europa presents another problem.  A radiation field surrounds Jupiter that becomes more intense closer to the planet.  Europa lies fairly deep within this field and an unshielded spacecraft either would have a short life (~1 month) or would require expensive radiation hardened electronics and a price tag well above $1.5B.  Here, the current concept proposes to use Europa itself as a radiation shield.

Cumulative radiation dose for a spacecraft in orbit around Europa (blue line) and a lander on the leading hemisphere of Europa (red line)

The major moons of Jupiter and its radiation field both rotate counter clockwise when viewed from above Jupiter's north pole.  The radiation field is trapped within Jupiter's magnetic field and so rotates at the same speed as the planet, every 9.9 hours.  Europa, however, orbits at the more leisurely pace once every three and a half Earth days.  As a result, the radiation field slams into the trailing hemisphere of Europa full on, but not its leading side. (Remember that Europa is tidally locked with Jupiter and keeps the same face to the planet throughout its orbit as our own moon does with the Earth.  As a result, the same face of the moon is always the leading hemisphere, and the opposite face the trailing hemisphere.)   On the other hand, the leading hemisphere experiences just a fraction of the radiation of the radiation hitting the trailing hemisphere.  (Thera Marcula lies near the edge between the two hemispheres just on the leading hemisphere side.  )

Concept for the orbiter and two lander stack

The landers would be carried into Europa orbit by a carrier spacecraft.  After entering Jupiter orbit, the combined spacecraft would perform sixteen Ganymede and Callisto flybys before entering Europa orbit.  (The presentation doesn't specify how many Europa flybys would be performed, although the goal would be to get into Europa orbit as quickly as possible to minimize time in the high radiation fields.)  After a few orbits around Europa, the first lander would be released with the second lander released at the same time or a few orbits later.  Once on the surface, the landers might either communicate directly with Earth or use the orbiting carrier craft for data relay.

Concept for the lander

Many details of the mission are still to be worked out.  For example the landers might either be battery powered if they are to last just a few days or carry nuclear ASRG power sources to enable life for several weeks until radiation kills the electronics.  The concept images in the presentation show ASRG's on the carrier spacecraft and on each lander.

The minimum instrument compliment for the landers is one of the decisions to be made by the lander concept team.  The slides suggest that the minimum might a mass spectrometer to measure the surface composition, a seismometer to examine the subsurface structure, and a suite of physical state instruments to measure temperature, radiation, light levels, and acceleration (presumably from surface flexing).  An enhanced instrument suite might have more comprehensive chemistry and geophysical instruments as well as cameras.  A likely key decision for the concept team would be how the samples would be acquired for composition measurements -- arm and scoop, drill, ?

Artist's concept of landing within a chaos region on Europa

Editorial Thoughts:  Given the recent study suggesting ideal landing spots on Europa -- fresh chaos regions -- the idea of a landed mission as our next Europa mission becomes very attractive; hell, it is exciting.  To temper that excitement, however, remember that a simple orbiter with a few instruments or a simple multi-flyby spacecraft with a few instruments would each cost ~$1.5B.  This mission would require an orbiter plus two landers that each would have the approximate sophistication of the Mars Phoenix lander.  That feels tight for a $1.5B budget.

There is also the practical problem that most of Europa has not been imaged or spectrally mapped to determine surface composition at high resolution.  When we had a similar situation with Mars and the MER rovers, we selected a landing site for Spirit that looked at moderate resolution to be a lake bed but turned out to be a lava flow bed.  (Fortunately, Spirit reached hills above the lava flow that retained clear evidence for past watery environments.)  Unfortunately, our highest resolution imaging of Europa is on the trailing hemisphere, leaving the highly desirable low radiation leading hemisphere terra fuzzy.

So, should the carrier spacecraft include a high resolution camera and spectrometer to image Europa during flybys prior to orbit insertion to select the best landing locations?  Orbital mechanics plays a dirty trick on us here.  Galileo imaged the trailing hemisphere at highest resolution because the natural location to encounter Europa at around the 3 o'clock position (again, looking down from above the northern hemisphere of the Jovian system).  At that position, the trailing hemisphere is fully illuminated and the desirable leading hemisphere is in darkness.  A mission that tries to get into Europa orbit as quickly as possible would face a similar problem.  Gravity assists can be used to walk the location of Europa encounters around the clock (so to speak) but at the cost of a longer mission and higher operation costs.

Once the carrier craft is in orbit, should it carry any instruments?  For a truly minimalistic mission, the simple answer is no.  However, much of the cost of the orbital science is getting to Europa and orbiting for a few days to a few weeks.  The orbiter's radio system would allow gravity mapping, a high priority measurement to study the interior of the moon.  I suspect that the temptation to carry at least a camera and a magnetometer (which enables inferences about the subsurface ocean through magnetic induction) would be strong.  (The orbit would dip to just five kilometers above the surface to deploy the probes where a simple camera's resolution might be ~5 m.)  The laser altimeter to measure the flexing of the surface to estimate ice thickness and the Langmuir probe to measure the particle and fields environment (and isolate their impact on the magnetometer measurements) might be too much.  I think a camera to document the terrain of the landing sites both to pick safer spot and to put the surface measurements in context would be strongly desired.

I would not be surprised to learn that the final cost estimates for this mission with any carrier craft instruments deemed necessary or minimal added cost will be above $1.5B.  Given that NASA's projected budgets cannot afford even a $1.5B mission, selling the political system on a, say, $2B orbiter plus two landers is probably not much harder than selling them on a $1.5B orbiter or multiflyby mission.

I'm hoping for good news out of this task force.

Sunday, November 13, 2011

JUICE – Jupiter Ganymede Orbiter Revised Proposal

Current concepts for the JUICE spacecraft.  All illustrations are from the JUICE presentation at the recent OPAG meeting.

This time last year, both NASA and ESA were considering a joint mission to the Jovian system, with the former to concentrate on Europa and the latter on Ganymede.  Today, NASA’s budget outlook has caused it to abandon its plans for the Jupiter Europa Orbiter (although it is looking at ways to reduce costs to enable a Europa mission should budgets become increase).  ESA, however, is still considering its Jupiter-Ganymede orbiter as part of a competition to select is next large science mission.  The new moniker for the mission is JUICE for JUpiter ICy moon Explorer. 

At the recent Outer Planets Analysis Group (OPAG) meeting, two members of the JUICE proposal team provided an update on their proposal.  Ganymede remains the focus of the mission, with the spacecraft studying it from orbit about the moon for the better part of a year with the final orbits just 200 kilometers from the surface.  The original mission also included a number of Callisto flybys that have been retained in modified form in the latest proposal.

The focus of the JUICE mission remains the study of Ganymede from orbit around that moon

In the last few months, the JUICE team has looked at whether the mission could be enhanced to more thoroughly study the rest of the Jovian system, including Europa.  From the point of view of celestial mechanics, adding Europa flybys to the mission would be relatively straightforward.  Unfortunately Europa orbits Jupiter deep within that planet’s radiation belts.  Targeting Europa requires enhancing the radiation hardening of the spacecraft and instruments, increasing mass and costs.

In the end, the team is recommending just two flybys of Europa as a compromise.  The team looked at two options for those flybys.  One would have the spacecraft make its closest approach to Europa at different faces of the moon to maximize geophysical measurements of the gravity (mass distribution) and interaction of the subsurface ocean with Jupiter’s magnetosphere.  The second option would have the spacecraft fly over the same hemisphere and would focus on remote sensing surface composition measurements of high priority areas.  The team has chosen the latter approach both because detailed composition measurements of Europa from the Galileo mission were limited and to simplify the spacecraft design.  The proposal team recognizes that two flybys capture just a tiny fraction of the science that NASA Europa orbiter would have.  However, the team estimates that as many as 50 - 100 flybys would be needed to fully replace a dedicated Europa orbiter, which would require radiation hardening not feasible within the mission’s budget but which would also result in the loss of the rest of the science objectives of the mission, that of Ganymede and Jupiter system science.

Example ground tracks and focus areas for remote sensing during the two Europa flybys.

The team also looked for ways to enhance studies of Jupiter’s atmosphere and magnetosphere.  For both sets of studies, scientists would like to move the spacecraft out of Jupiter’s equatorial plane.  By doing so, the instruments can view the atmosphere at higher latitudes and get a better understanding of the three dimensional structure of Jupiter’s magnetic field and plasma fields.

The team proposes to incline the spacecraft’s orbit around Jupiter by 29 degrees by using the Callisto flybys to first pump up and then pump down the inclination.  There will be a price to pay for these orbital maneuvers.  The previous mission plan had the closest approaches to Callisto well distributed around the moon’s surface.  The new plan fixes the approach geometry so that the spacecraft follows nearly the same ground track for each flyby.  As a result, the instruments will see the same portion of the surface and sense the same area of the gravity field as a measure of internal structure for each flyby.

Inclined Jovian orbits to study the Jupiter atmosphere and magnetosphere at higher latitudes

In the new plan, the JUICE spacecraft would follow nearly the same ground track for each Callisto flyby (left) instead of distributing the ground tracks around the moon as previously planned (right)

Unfortunately, Io remains too deep in Jupiter’s radiation field for the mission to flyby it.  Future studies of this moon will depend on the success of the proposers of the Io missions in future NASA New Frontiers and Discovery mission selections.

Currently three missions, JUICE and two astrophysics missions, are in contention for the single funding slot for the next ESA large science mission.   In February, ESA plans to select either its final choice or to eliminate one of the three with a final winner selected after further study.  If JUICE is selected, it would launch in 2020, with a backup launch opportunity in 2022.  Approximately six years later, the spacecraft would reach orbit.  Another two and a half years would be spent exploring the Jovian system and preparing to enter Ganymede orbit for ten months of orbital science observations before the mission ends.

Editorial Thoughts:  The budget for ESA’s next large science mission is capped, and major changes to the original Jupiter Ganymede Orbiter proposal are not possible.  The proposal team has done a nice job of enhancing the proposal with limited additional studies of Europa, Jupiter’s atmosphere, and the magnetosphere. 

The other two missions in competition for selection by ESA are exciting astrophysics missions in their own right.  The competition is likely to be tough.   Barring a significant budget increase for NASA, however, JUICE appears to be our only option for exploring the icy moons of Jupiter in the next two decades.

My thanks to Michele Dougherty, Professor of Space Physics at Imperial College and Lead of the JUICE Science Study Team, for reviewing a draft of this post.

Saturday, November 5, 2011

Return to Venus

Portion of Venera 14 landing image.  Ted Stryk's website has the nicest versions of these images I've seen.

A bit over a year ago, I wrote that a return mission to Venus would be compelling because "We live on a terrestrial planet, and one on which we are undertaking a grand experiment to see what happens when we dramatically increase the proportion of greenhouse gases in the atmosphere.  As a result, I think that furthering our understanding of Venus as a terrestrial planet and a greenhouse atmosphere carried to extremes is a compelling target for exploration in the next decade."

While NASA has taken a pass on Venus for now, Europe has no missions in the offering, and Japan's Akatsuki mission is struggling to reach orbit, Russia has a mission planned for 2018.

Russia's Venera-D mission has gone through several iterations.  At one time, Russia had hoped that international cooperation might lead to a truly ambitious flotilla consisting of advanced orbiters, multiple balloons, and multiple landers.  The new plan appears to be a Russia-only mission, but one that is still ambitious with a capable orbiter and lander.  It is really three missions in one, with significant capabilities for studying the atmospheric structure and composition, the plasma environment enveloping the planet, the surface geology.

The orbiter will carry a number of spectrometers that will measure the structure and composition of the atmosphere from its top to the planetary surface.  One of its instruments, the interferometer spectrometer, will replace the Planetary Fourier Spectrometer that failed on the Venus Express mission that would have measured the atmospheric structure below the cloud tops.

A second suite of instruments on the main orbiter and a small sub-satellite will extend our studies of the plasma environment.  A key problem with studying plasmas environments is that they are dynamic, and it is hard to separate local conditions surrounding a single spacecraft from more global conditions.  The sub-satellite will provide a second point of measurement to compare results.

The atmospheric and plasma experiments will continue, and significantly extend, studies that have been made by previous missions extending as far back as the Pioneer Venus orbiter in the late 1970s and continuing today with the Venus Express mission and hopefully continuing with the Akatsuki mission if it can reach orbit.

For me, though, the key contribution of the Venera D mission will be a return to the surface.  The lander will explore a different terrain than previous landers, which landed either on the basaltic plains that cover most of the planet or on the fringes of the highlands.  Venera D will be targeted to explore the tessera highlands of Venus that rise, like continents, above the surrounding lowlands.

NASA studied a tessera lander as part of its recent Decadal Survey.  The authors of the report on the mission concept summarized the importance of a mission to these highlands: "The key science driver... is to measure the mineralogy and major elemental composition of tessera terrain, which is distinct from the plains and is yet unsampled... Tesseara terrains appears.. as the oldest material on a planet where the average surface age is ~500 million years.  Thus, the tessera proved the best chance to access rocks that are derived from teh first 80% of the history of the planet, an era for which we currently have no information."

While the probe is called a lander, it is just as importantly, an atmospheric probe.  The NASA report continued, [Previous] "compositional measurements of the atmosphere constrain atmospheric evolution, but to date, very little compositional or physical information has been garnered [below 16 km altitude], which is key to understand both atmospheric evolution and the surface-atmospheric interactions."

In many ways, the lander portion of Venera D is similar to the twice proposed New Frontiers SAGE Venus lander.  A comparison of their instrument lists shows that both missions would have similar capabilities.  In its most recent incarnation, the SAGE lander was targeted toward an area that Venus Express scientists believe may have been covered with recent lava flows.  Should both missions fly, we will have sampled the dominant younger (<500 M years old) plains of Venus with the older Venera landers, possibly the much older surface on a tessera, and possibly the some of the youngest surface with SAGE.

Location of previous Venera landing sites from the Wikipedia article on the Venera landers.

Editorial Thoughts: Venera D is an exciting mission.  I don't know whether it is a fully approved mission, or whether it is still a mission concept that needs to be formally approved by the Russian government.  (If any of you have insight into its status, please post a comment.)  A recent conference abstract (link below) says the mission is in Phase A, which is the earliest phase of a mission's conceptual definition and design.  This would be normal for a mission still six years from launch.


You can read Ted Stryk's summary of the Venera D mission (which provided essential background for this post) at  He also posted a photograph of a poster on the mission from a recent conference.

A Russian space agency website on the mission can be found at  While the website is in Russian, Google Translate produces a reasonable English translation, and likely does well for other languages.  A recent conference abstract on the mission can be found at  My thanks to colin_wilson at the Unmanned Spaceflight Forum for these two links.

I wrote a post on NASA's tessra lander concept, which you can read at can read NASA report on its tessera mission concept at

There is a summary of the SAGE mission proposal at

I wrote a post on previous concepts for international Venus missions that would have been more capable than the current concept for Venera D:

Thursday, October 27, 2011

Whither NASA's Planetary Program?

"Word has leaked out that in its new budget, the Obama administration intends to terminate NASA’s planetary exploration program. The Mars Science Lab Curiosity, being readied on the pad, will be launched, as will the nearly completed small MAVEN orbiter scheduled for 2013, but that will be it. No further missions to anywhere are planned."
- Robert Zubrin

"Rumors of the death of NASA’s planetary science program are greatly exaggerated, according to the head of the agency division responsible for that activity...'“It is not true the planetary program is being killed' [said James Green].

"...this is the most challenging budgetary time of my entire 33 year career."
- Dr. James Green, Director of Planetary Science at NASA Headquarters

"In 1980, the Director of the Office of Management and Budget (OMB) and the NASA Administrator made the decision to shut down planetary exploration in NASA in order to free up funds for the development of the Space Shuttle.... The administration leaders told us, face-to-face, that the planets could wait because soon the cost of access to space would be so cheap that we could fly any missions about which we could dream.  
"We fought back, and they didn’t shut down planetary exploration. However, they did cut it deeply, resulting in a dark decade with no launches to and no data coming back from from other worlds.
"Imagine a NASA that for ten years (say, 2015 to 2025) ceases to explore the solar system and stops looking deep into the universe.  We’re in a similar situation today. "
- Dr. Louis Friedman (former Executive Director of The Planetary Society)  

I know that discussions of budgets are not the favorite topic of some of my readers, nor mine.  However, the recent news and commentary on budgets has been dire enough that returning to this topic seems appropriate.  Without funding, all the great ideas for NASA missions are no more than the paper or webpages they are printed on.  And so, I am publishing one of my few opinion pieces.  Please feel free to agree or disagree in the comments.

Four pieces of bad news, not including the unnamed sources cited by Zubrin, have triggered the concerns:

  1. The President's budget request for FY12 projected severe declines in out year budgets for the planetary program.  These cuts, if enacted in future years, essentially end NASA's ability to carry out Flagship scale planetary missions on its own for the foreseeable future.
  2. The President's Office of Management and Budget (OMB) has refused to allow NASA to commit the approximately $1.5B needed for its proposed joint Mars program with ESA.  No public reason has been given (but I have speculations, see below).
  3. The James Webb Space Telescope (JWST) is severely over budget, and NASA has proposed to OMB that it cover the additional costs by in part transferring funds from other science programs, including, presumably, the planetary program.  No figures have been given, but my back of the envelope calculations suggest that the transfers, if approved by OMB and ratified by Congress, might cost the planetary program a Discovery mission.
  4. Current American law will require automatic cuts to all discretionary Federal programs, including NASA, if the two political parties are unable to overcome their differences on identifying future substantial budget cuts.  A figure published by the journal Nature suggests the automatic cuts could be ~11% for programs like NASA.  Again, that could cost the planetary program a Discovery or New Frontiers mission.

On top of this, NASA's human spaceflight program has been given an ambitious set of tasks that many don't believe it will have funds to fulfill without transferring funds from other programs (which has happened before and resulted in cuts to NASA's science programs).

I have been a manager at a major high technology firm, and I understand the challenges of budget crunches and trying to scramble to keep programs going and recraft roadmaps.  Dr. Green and his colleagues have my admiration for what they are doing and their honest assessments of the situation.

I also applaud the efforts of the planetary science community, Dr. Friedman, and The Planetary Society for advocating for a strong planetary program.  The political process, which will decide the level of funding, responds to strong community commitment.

With the information I have, however, I don't believe that OMB is necesarrily the bogey man preventing the planetary program from pursuing a great program.  NASA's two biggest programs, human spaceflight and JWST, are under funded.  Major new budget cuts to all of NASA are a real possibility.  If OMB approves the Mars funding, can it meet the commitments made to the Europeans in the out years without gutting the rest of the planetary program?  In addition, what America buys through the investment in the joint Mars program is the first mission in three needed to return samples from Mars.  Another ~$6B (Decadal Survey estimate) would be needed to complete the program.  If OMB doesn't believe the rest of the program can be afforded or has the necessary political support within Congress, then is the first payment a wise move?

As a strong supporter of planetary exploration, I favor the investment in the joint program because it also enables the Mars Trace Gas Orbiter and the European ExoMars rover, which would be a great mission in its own right.  However, spending American taxpayer dollars to fly European rover instruments to Mars probably do not make a strong sell within OMB.

The total cost for building the JWST and the three missions needed for the Mars sample return would be about $8B each.  European investments in a joint program might reduce the U.S. cost for the sample return by $2-3B.  OMB has the option to support JWST and build on the $3B already spent and the option to begin investment in a Mars sample return program now and save $2-3B through cooperative investments by the European Space Agency.

Dr. Green points out that the astronomical community has done a great job of making the case for the science of JWST and building political support.  My guess is that the real issue here is that the planetary science community has not made the political sale for the Mars sample return program, and as a result, OMB is reluctant to make the down payment.  

I remember the 1980 budget disaster mentioned by Dr. Friedman in the quote above well.  It led me to publish one of my first articles.  In those days, the seriously proposed planetary missions were Flagship-class.  A few people called for flying small, focused, and relatively inexpensive missions, some of which I profiled in that article.  Eventually these ideas would lead to the highly successful Discovery and New Frontiers mission programs that do great science at less than Flagship prices.  Check out the Messenger Mercury, Dawn Vesta and Ceres, and the Juno Jupiter missions for examples of what can be done.

Ultimately, the President's office and Congress will have to sort out priorities.  Is the James Webb Space Telescope's science higher priority than the joint Mars program with ESA and an eventual Mars sample return?  Is reducing the Federal budget deficit so important that NASA and many, many other programs should be cut?  (Funding for my research comes from some of the programs facing cuts, so I have skin in this game.)  

I believe that NASA's top science priority should be to understand the changes occuring to the Earth's biosphere on which we all depend.  I personally prioritize planetary science over astronomical science, but believe both are important.  Making the JWST's science NASA's top science priority after Earth Science is in no way a stupid call, but not the one I would make.  My major concern with JWST is that a single launch failure or design flaw could make several billion dollars in future investment a waste.  By moving forward with JWST, NASA is betting the bulk of its science program for the coming decade on a single mission.

In 1980, the only missions being proposed seriously were Flagship-scale missions.  Today, NASA has strong Discovery and New Frontiers mission programs that fly missions for much less than Flagship prices.  If the coming decade were to have the OSIRIS-REx asteroid sample return and each of the three Discovery missions that are current finalists (the Titan Mare Explorer, the Comet Hopper, and the Mars GEM geopysical mission) or equivalently good mission in their turn, it would be a good but not great decade.  

At the last Outer Planets Analysis Group (OPAG) meeting that I listened to, it was said that the last Discovery mission selection included three outer planet missions that would fit into the Discovery mission budget cap.  This is a new development and a major achievement for NASA and planetary science community.  The Discovery and New Frontiers programs can advance planetary exploration across most of the solar system.

NASA also is not only game in town.  Europe has two planetary missions in competition for selection, Russia has a Phobos mission ready for launch and lunar and Venus missions in development, Japan has another asteroid sample return mission in development, and India and China are building their programs of planetary exploration.  

As for Zubrin's claim, the quick denial by a NASA manager known for being straightforward leaves me doubtful of his claim.  There have been no other hints of this radical of a move by the administration.  Congress has supported NASA's science programs, and a sudden change like this requires conccurance of the administration and Congress.  Sometimes Chicken Little is right, but extraordinary claims require more evidence than a single uncited source.

I'll close with some words from a second article from Dr. Freedman that resonated with me: "As I have said many times, it makes no difference to Mars when it is explored or by whom, but it makes a huge difference to us, the people who own the program and carry it out. Just as when, nearly 20 years ago, the US abandoned the Superconducting Supercollider effort, it is one more piece of evidence of a great country ceding its greatness and reducing its hopes and investments for the future... For those who like to think short-term only, it also reduces jobs and national capability. Universities and companies around the country have motivated academic achievement and inspirational jobs with Mars and other planetary exploration." 

Saturday, October 22, 2011

Europa - New Options

Click on the slide or here to go to the site with the presentation of the Europa mission study team

At this week's meeting of the Outer Planets Analysis Group (OPAG), NASA unveiled new options for exploring Europa as a possible abode of life (and with a third option still to come).  The results are a work in progress, with completion of the studies to come next year.  However, the results so far present entirely new options to finally moving on with the study of Europa.

The new mission proposals come after the sticker shock from the previous proposal, the Jupiter Europa Orbiter (JEO).  When JEO was defined, NASA's budget outlook seemed robust and multi-billion dollar missions possible.  At NASA management's request, its engineers designed a mission that hit the science "sweet spot."  After many months of exploring the Jupiter system, JEO would have carried an extensive suite of instruments into Europa orbit.  In what would have been an engineering tour de force, the spacecraft would have been designed to survive the hellish radiation fields around Europa for a full 90 days (and likely even longer).

Unfortunately, the independent cost estimate for JEO from the Decadal Survey's review pegged JEO's cost at close to $5 billion.  Even for a robust planetary budget, the Survey's members concluded that the price tag was too high and asked NASA to explore cheaper alternatives.

The two mission concepts presented at the OPAG meeting are NASA's response to that request.  To understand how the team arrived at its proposal, it's useful to compare the JEO approach to the Mars exploration strategy.  In the former, a single mission would be flown that would have addressed all high priority goals for Europa exploration except for those that that required a lander.  In comparison, the Mars program has depended on a series of relatively inexpensive missions that each address a subset of questions.  For less than a billion dollars a mission, NASA flew Mars Pathfinder, Mars Odyssey, the Spirit and Opportunity rovers, the Mars Reconnaissance Orbiter, and the Phoenix lander.  Only the soon-to-launch Mars Science Laboratory Curiosity broke the billion dollar mark (and rather substantially).  Looking forward, the Mars program has (or perhaps given the new budget realities, perhaps its more correct to say, 'had') planned to break up the multi-billion dollar Mars sample return into three separate missions to reduced peak funding costs.

The question for the Europa mission study team was how to divide the JEO mission goals into manageable chunks.  The job of the Europa mission study team was helped by the fact that the next steps in Europa exploration naturally divide into multiple goals:

  • The ocean goal would measure the extent of the ocean beneath Europa's icy shell and determine its connection to the rocky core of Europa.  This is also, because of the nature of the measurements required, is the only goal that requires orbiting Europa.  
  • The other three goals -- measure the vertical structure and depth of the ice shell, measure the composition of trace materials on the surface to understand the composition of the ocean and whether it might harbor life, and image the surface to understand the processes acting on the ice shell -- would ideally be done from orbit but could be done from a multitude (>30) Europa flybys.

Two key factors had driven JEO's costs upward.  First, to meet JEO's goals from Europa orbit, the spacecraft needed to survive for 90 days or more in the harsh radiation field around Europa.  This required extensive shielding and exotic technologies for both the spacecraft electronics and for the instruments.  Second, collecting and returning all the data in just 90 days required high communications rates and as result high power rates.

Comparison of the two proposed spacecraft with the JEO proposed spacecraft

The ocean measurements that had to be done from Europa orbit required relatively small instruments, low to modest data rates, and only 30 days in orbit.  By focusing the orbiter only on those required measurements, the radiation hardening, data rate, and power requirements dropped substantially.

The remaining measurements still required substantial instruments and high data rates.  The team chose to place these instruments on a flyby spacecraft that would orbit Jupiter and encounter Europa 30+ times over the course of the mission.  (The spacecraft also would encounter Ganymede a number of times, but those flybys were planned for gravity assists and were not science drivers for the mission.)  By going to a multi-flyby strategy, the spacecraft spends only a little time each orbit in the highest radiation fields.  There are also days each orbit to return the data, dropping the requirement for peak data rates and power.

Proposed instruments

The study team estimates that the orbiter and flyby mission each would cost around $1.5B, not including the costs of the launch vehicles.  The assumption is that for budgetary reasons the missions would not launch together, and their launches might actually be years or many years apart. Assuming that funding can be found, this would place the science community in the difficult predicament of deciding which to fly first, knowing that the second mission might never fly.  Is ocean science more important or is the combination of icy shell, composition, and high resolution mapping science more important?

Proposed flyby (top) and orbiter (bottom) spacecraft

Representatives from NASA's headquarters stated their delight in having new, much lower cost mission options to present to Congress for funding.  They are so delighted that they have asked to have the same focused approach applied to a possible Europa lander mission.  Presumably, once the studies for the three possible missions are complete, NASA will ask the science community to prioritize them.  Then NASA will begin the job of securing funding to fly the highest priority mission.  NASA's representatives emphasized that there is no money in the current budget plan to fly any of these missions.  The President's office (OMB) and Congress will have to increase NASA's planetary budget for any Europa mission to begin development.

Note: A previous post, A Pragmatic Approach to Investigating Europa's Habitability provides additional background on the new approach.

Editorial Thoughts: It's exciting to have new Europa options on the table in a cost range that I can imagine one of them eventually being funded. However, looked at another way, the studies demonstrate again that exploring Europa is a multi-billion dollar proposition.  Add the cost of the orbiter and flyby with their launch vehicles together, and you are approaching $4B, not too far from the estimated cost of JEO.

Which mission would I chose?  As currently defined, I can't decide.  The eventual goal would be to get a lander on the surface of Europa, which requires picking a location that is safe (high resolution imaging) and has interesting surface chemistry (imaging spectroscopy).  This would argue for the flyby mission as the higher priority.  That mission, however, would image only small portions of the surface at higher resolution than the orbiter mission would.  (The orbiter would produce a 100 m global stereo map in three colors.)  The flyby craft would carry a sophisticated mapping spectrometer (think of a camera that takes images in hundreds of colors) that would be too large and data hungry for the orbiter.  If a simple spectrometer (for those of you familiar with these instruments, a profiling instrument) could be added or if the imager could image in more colors to get at spectral information, I would favor the orbiter.  I certainly have no special insights into these issues, and included this thought experiment only to suggest the tough choices the planetary community will face if funding becomes available.