Trojan and Centaur Mission Concepts

The last couple of months worth of blog entries have focused on Decadal Survey mission concepts to the outer solar system.  Today, I'll look at missions to some of the smallest members of the outer solar system, the Trojan and Centaur objects.  The two populations of small, primitive bodies differ in their location.  The Trojans share the orbit of Jupiter and either lead or trail the gas giant (with the population that leading Jupiter sometimes called 'Greeks').  Centaurs orbit between Jupiter and Neptune and typically cross the orbit of one or more major planets.  (While the term 'Trojan' now refers to any small object in the Lagrangian points of a major planet, here I use the term only to refer to Jovian Trojans.  Trojan objects also have been discovered for Mars and Neptune.)


The use of the term 'objects' is deliberate.  These worlds may blur the distinction between asteroids and comets (a fuzziness not unique to these families of primitive bodies).  Chiron, a Centaur, periodically displays cometary behavior by outgassing.  Other bodies in these families may be mixtures of rock and ice that defy easy classification as either an icy 'comet' or a rocky 'asteroid' and may have wide compositional variation within their populations.  What makes these worlds interesting target of exploration is that they likely are accessible remnants of interesting places in the early solar system.  Two leading theories for the origin of the Trojans, for example, postulate that they are either left overs from the zone in which Jupiter formed or they are Kuiper Belt Objects captured with the migration of Uranus and Neptune out from the sun early in the solar system's history.  Therefore, Trojans either can teach us about the material from which Jupiter formed or they are easily accessible samples of the far outer solar system.  The Centaurs are all in unstable orbits, meaning they have migrated to their present locations recently (in solar system terms).  Their source has not been determined, but leading theories point to populations of objects beyond Pluto.


The authors of both reports spend considerable time discussing options to reach and orbit these worlds -- they are still distant and 'accessible' is a relative term.  In fact, the Chiron report discusses little else.  Several options were examined for propulsion post launch: all chemical, a combination of chemical and solar electric propulsion, and radioisotope electric propulsion.  The solar electric propulsion would be used increase velocity as the spacecraft transits the inner solar system and into the main asteroid belt.  Chemical propulsion (e.g., a rocket engine) would then be used beyond the main belt.  The radioisotope option would substitute additional radioisotope generators for solar power.  This option has the benefit of being able to operate the engines throughout the mission, much as the Dawn mission is doing (using solar power).


The discussion of the tradeoffs was complicated.  The RPG options provided the best performance for all options, enabling a orbiter for Chiron and potentially enabling orbiting two Trojans.  However, this would also be the most expensive option, requiring a Flagship level of funding (and using larger amounts of the limited supply of plutonium 238).  The other options could fit into a New Frontiers mission budget.  The solar electric propulsion option would enable a Chiron mission, but with a reduced instrument payload.  A chemical-only mission was not possible for Chiron, but would be for some other Centaurs and for a Trojan mission.

Design concept for a Trojan flyby and rendezvous spacecraft from the Trojan mission concept study.

A mission to either type of object would begin with a long cruise (13 years to Chiron and 8-10 years to the Trojans).  Once a spacecraft reached Chiron, it would orbit for at least three years to both map that world and to detect and study the periodic outbursts.  (A Trojan mission would also search for outgassing from its targets.)  A Trojan mission would have a Jupiter flyby, one or more flybys of Trojan objects, and end with a rendezvous and orbit of an additional Trojan object.  If ASRGs were used to power the craft instead of solar panels, it might be possible to land on the final object.  In this latter case, the Trojan mission would be similar to the Ilion Discovery mission concept. 


The strawman instrument lists for the two missions share several candidates:

The Trojan mission list a greater emphasis on surface measurements two surface composition instruments (gamma-ray and neutron spectrometers and LiDAR instrument to measure topography) while the Centaur mission list has a greater emphasis on measuring emitted gases  (ion and neutral mass spectrometer).  (The UV spectrometer common to both lists would also examine outgassing.)


Editorial Thoughts: A mission to Chiron would be scientifically rich -- this body is unique amoung outer solar system primitive bodies with is large outgasing events.  However, implementing the mission feels to me to be at the edge of what would be technically and financially probable for the next decade.  A mission combining flybys of several Trojans followed by a rendezvous with another seems to be scientifically richer than a mission to a single Centaur object.  Either target would open up the exploration of a new class of world with all the possibilities for discovery that portends.  Whether either mission will be prioritized highly by the Decadal Survey may depend on how the Primitive Bodies Panel ranked these missions against missions to orbit, land on, or collect samples from nearer primitive bodies.


Appendix: Science Goals


White papers for the Decadal Survey were submitted for missions to both the Centaur and Trojan objects.  In general, the science goals are similar; here, I quote the goals from the Trojan white paper.  I also published a blog entry summarizing the science rational for a Trojan mission here; the rational for a Chiron mission is similar.


"1. Did the Trojan asteroids originate near Jupiter’s orbit or farther out in the solar system?
2. What do compositions of these primitive bodies tell us about the region(s) of the solar nebula in which they formed?"


These questions would be answered by focusing on a set of specific questions for the body (or preferably, bodies) visited:


"1. How much and what types of ice and organics are present on and within Trojan asteroids?
2. What is the mineralogy of the silicates present on and within Trojans?
3. How do the geological processes that have occurred on the Trojans compare to those that have affected other small bodies?
4. What is the relationship between Trojan asteroids and comets, TNOs, outer planet satellites, and main belt asteroids?
5. Are densities and bulk compositions of Trojans diverse or homogeneous?"


The Decadal Survey mission concept studies can be found here.

NASA Planetary Program Budget Issues

Space News reports that the Mars Science Laboratory requires an additional $82M beyond currently budgeted funds to meet its November to December launch window.  The article reports that NASA considers meeting this launch window is crucial and that funds will be found elsewhere within the Mars program, and if necessary the rest of the planetary program.

The article also briefly discusses potential impacts to the planetary program if NASA's budgets are cut back to FY08 levels, as one political party is seeking to do for most federal discretionary programs.  The head of the programs says that he believes that the program could probably adjust without canceling any programs.  However, if necessary, the upcoming Lunar Atmosphere Dust Environment Explorer or the New Frontiers or Discovery missions in selection could be put on the cutting table.

Editorial Thoughts: I looked at potential impacts of budget cuts in a previous blog.  If the budget is reduced to FY08 levels, then NASA's looses ~$1.5B over a decade compared to the FY10 budget level, or a bit more than the burdened cost of a New Frontiers mission.  If NASA's budget for future missions was frozen for a decade, NASA would lose about the equivalent funding of a New Frontiers program whether the initial level is at FY08 or FY10 budget levels.  If the starting budget was the FY08 level and then frozen, then the result would be the loss of funding equivalent to approximately two New Frontiers missions compared to the FY10 budget level increased for inflation.

If NASA's budget is frozen or cut, however, the impact on the planetary program might not be proportional.  Funding for the manned spaceflight program does not appear sufficient for its mandate, and Congress might direct larger cuts at the science program to preserve funding for the manned program.  As the FY11 and FY12 budgets are debated and hopefully approved over the next year, the size of the program for the next decade should become clearer.  It will be interesting to see if the Decadal Survey's recommendations will have designed in flexibility to respond to a dynamic budget environment.

I will post an analysis of the President's FY12 budget proposal when it is released next month.

Uranus Orbiter Concept Study

In the reader poll of missions you would like to see the Decadal Survey select, the Uranus Orbiter has been the second most popular larger flagship mission -- but a distant second after the Jupiter Europa Orbiter.  (If you haven't voted yet, please do so!)  This mission isn't one that I am considering for my personal list of five missions I would like to see the survey select.  Given readers' interest and the similarities with the Neptune orbiter missions previously described (here and here), I'll summarize this mission here.  Because of the similarities of Uranus and Neptune, the goals for simple orbital missions to the two worlds would be similar.

Uranus and Neptune offer the opportunity to study worlds that are distinctly different than Jupiter and Saturn.  These two worlds are much less dominated by atmospheres of hydrogen and helium than Jupiter and Saturn.  Instead, they contain much larger fractions of heavier elements believed to have been delivered as ices rather than gases.  Studying these worlds provides an opportunity to understand the formation of planets in a different portion of the proto-solar system.  These worlds also have unusual magnetospheres that are strongly offset from the axis of rotation ( ~60 degress in the case of Uranus and ~50 degrees in the case of Neptune).  The moons of Uranus also show signs of tectonic activity and possible cryovolcanic flows in the case of Ariel.  The larger Uranian moons, Titania and Oberon, may also harbor interior oceans.

The group that produced the Uranus orbiter mission originally was charged with examining orbiter missions for both Uranus and Neptune.  However, one ground rule for the study handed down by the Decadal Survey was that missions much launch no earlier than 2020.  By that time, Jupiter gravity assist options are not available for Neptune missions for a number of years.  While Neptune orbiters could still fly, they would require aerocapture (an untested technology) or unacceptably long flight times.  This study group, therefore, dropped Neptune as a focus and concentrated on Uranus orbiter missions.  (The Neptune study group examined earlier launch dates that would avoid these problems for simple orbiters.)  The lack of a Jupiter gravity assist would impact a Uranus orbiter by requiring a solar electric propulsion stage to reach Uranus within acceptable flight times, which are still long at 13 years with the nominal mission launching 2020 and arriving in 2033 for a mission tour of  1.5-2.3 years.

Spacecraft concept with solar electric propulsion stage and orbiter concept design

The baseline mission would include the orbiter and an atmospheric probe that would survive to 1-5 bars (or 1-5 times the atmospheric pressure at sea level on Earth).  The basic mission includes a highly elliptical orbit for observing the atmosphere and exploring the magnetosphere.  Options for an extended mission would provide two close encounters with each major moon (plus four close untargeted encounters with Umbriel) or could lower the periapsis closer to the planet for additional magnetosphere and gravity field measurements.  (It may be that the two extended mission options both could be conducted.)

 

Baseline and satellite tours

Compared to the goal-rich Cassini or Jupiter Europa Orbiter missions, goals for the Uranus orbiter would be modest.  To quote from the concept study (with instruments noted with each goal):

Tier 1:

• "Determine the atmospheric zonal winds, composition, and structure at high spatial resolution, as well as the temporal evolution of atmospheric dynamics." (Wide angle camera and an orbit with apogee above the sunlit hemisphere for long duration imaging)
• "Understand the basic structure of the planet’s magnetosphere as well as the high-order structure and temporal evolution of the planet's interior dynamo."  (Magnetometer and orbit that explores different regions of the magnetic field)

Tier 2:

• "Determine the noble gas abundances (He, Ne, Ar, Kr, and Xe) and isotopic ratios of H, C, N, and O in the planet’s atmosphere and the atmospheric structure at the probe descent location."  (Atmospheric probe with mass spectrometer and pressure-temperature sensors)
• "Determine internal mass distribution."  (Radio tracking with orbital periapsis visible from Earth)
• "Determine horizontal distribution of atmospheric thermal emission, as well as the upper atmospheric thermal structure and changes with time and location at low resolution."  (Mid-infrared thermal detector and UV imaging spectrograph)
• "Remote sensing observations of large satellites."  (Wide- and narrow-angle cameras with visible/near infrared mapping spectrometer)

Tier 3:

• "Measure the magnetic field, plasma, and currents to determine how the tilted/offset/rotating magnetosphere interacts with the solar wind over time."  (Plasma and particle instrument)
• "Remote sensing observations of small satellites and rings." (Same instruments as required for studying large satellites)
• "Determine the vertical profile of zonal winds as a function of depth in the atmosphere, in addition to the location, density, and composition of clouds as a function of depth in the atmosphere."  (Radio tracking of atmospheric probe with ultra-stable oscillator and nephelometer)

The estimated cost for the full mission would place it in at the lower end of the larger flagship range (~$2B FY15 dollars).  The mission could be descoped to the cost of a small flagship missions (~$1.5B) by addressing Tier 1 objectives only.

Editorial Thoughts:  This would be an exciting mission that would perform valuable science.  The lower cost option (Tier 1 science only) cuts out considerable science.  No relative science evaluations of the two mission options were given.  For Neptune, however, the minimal orbiter (similar to the minimal Uranus orbiter) was judged to have significantly less science value than the full orbiter mission.  Probably not coincidently, the cost estimates for the minimal and enhanced orbiter missions for the two planets were similar.  Similar goals, similar instruments, similar long flight times = similar costs, apparently.

More on Neptune Missions

When I have a question or a connection to someone related to a future mission study, I'll send them a preview of my write up or a link to a posted one.  For the Neptune mission concept study, I did the latter with the technical team leader, Tom Spilker of JPL.  He and I met at the AGU conference at his poster on a Saturn ring observer concept (which will be the subject of a future blog entry).  I asked Tom if he had any corrections or additions he might suggest.  In the course of a couple of emails, he expanded on several topics.  With his permission, I have quoted extensively from those two emails and changed them only to connect topics together.

Side note:  In this field, I am an interested amateur.  I notice that a lot of readers are in locations near major centers of development for planetary missions.  If anyone has corrections or additions to what I write, I will publish them.  I presume annonimity unless you specifically give permission to use your name or quote directly.

From Tom:

You might want to mention the advantage of going to Neptune before continuing to a KBO:  you can use Neptune's gravity to change the spacecraft's outbound trajectory significantly (up to 60 degrees bending angle), to steer to a large, scientifically interesting KBO, rather than one that happens to be within 0.9 degrees of the trajectory to Pluto.  This is not to minimize the importance of New Horizons!  The first view of a KBO that is still a part of the main population (for example, hasn't had a close encounter with Neptune) is an important milestone and will teach us much about the Kuiper Belt.  But the ability to choose one that should teach us the most about the primordial Kuiper Belt is also important.

Concerning the trajectory flexibility, if you specify one single KBO and won't settle for any other, then indeed you lose a lot of flexibility from the trajectory in the Neptune system.  If you specify that you must have an optimal or near-optimal radio science flyby at both Neptune and Triton, then you narrow greatly the number of KBOs you could encounter.  The radio science investigations would involve determining the gravity fields (and thus the distribution of mass in Neptune's and Triton's interiors), and possibly atmospheric occultation experiments.  The study found that doing a "best compromise" mission still gets good science returns at all three destinations, you just don't get really hi-quality gravity field data at both Neptune and Triton.

That said, one of the advantages of the Neptune flyby is that the choice of which KBO will be visited can be made before launching, so considerable effort can be put into a trade study examining the many different KBOs we might visit, and, for each one, the trajectory options within the Neptune system and their potential science value.

Concerning the "understatement" of the science value of the flagship-class missions, you could point out that the Giant Planets Panel's interest was primarily in the lower-cost concepts, thus the science objectives the concepts were being judged against were more tuned to the lower-cost missions.  The flagship-class missions did a better job achieving these science objectives and thus scored higher than the low-cost missions, but if you added to the science value assessment the science objectives that only a flagship-class mission could address, the scores of the flagship concepts would increase significantly while those of the lower-cost concepts wouldn't change much.

Until NASA's Deep Space Network (DSN) makes some significant upgrades, the upper limits to practical telemetry rates from the outer solar system are likely to be somewhat greater than those available to Voyager 2, but unfortunately not "much greater".  The increase might be a factor of 2, not a factor of 10.  While the DSN has gained by going to the higher-frequency Ka-band, they have moved to smaller antennas (34 meter diameter, as compared to the 70 meter ones available for use at X-band), somewhat offsetting the gain... There's some uncertainty in the telemetry system data rates from Neptune-like distances, mostly because you can't be sure, this early in the concept's studies, how much electric power can be devoted to the telecommunications system;  in the business we shorten that to "telecom".  But given the likely range, downlinking 256 Gb could take anywhere from 5 months to well over a year.

In the list of instruments, "RSCM" means "Radio Science Celestial Mechanics", a mouthful of words that means measuring the gravity field of a planet (or other object) via very accurate tracking of the spacecraft's trajectory.  This is accomplished using the radio link between the spacecraft and ground stations, specifically by measuring the Doppler shift the spacecraft's motion imposes on the frequency of the radio signal.

I would call Triton a "possible" ice-ocean moon, not a "likely" one.  If Triton is captured, and if it were captured billions of years ago, not millions of years ago, we think there's not enough tidal dissipation there to keep part of it thawed until the present.  Enceladus surprised us, but it's only 4 Saturn radii from Saturn's barycenter where Triton is more than 14 Neptune radii from Neptune's barycenter;  the largest-scale tidal effects go as one over r cubed.

Although the terms, "hydrogen-helium giants" and "methane-rich gas giants" are certainly reasonable, the planetary science community uses different terminology. In that community Jupiter and Saturn are known as the "gas giants" because their compositions are dominated by hydrogen and helium that were delivered to the forming planets as gases.  Out in the frigid outer solar system, where compounds such as water and ammonia occur as "ices" (solids; unless they are in deep atmospheres that are hot at depth, where they can be gases or liquids), Uranus and Neptune appear to have far greater abundances of such compounds, that we think were delivered to those planets as ices, so they are called the "ice giants".

Editorial Note: I was aware of the term 'ice giants,' but wanted to emphasize the compositional difference without taking the space to add the background explanation.  The Uranus orbiter mission concept study (page 5) has a nice explanation of the difference between the two groups of large outer planets: "Uranus and Neptune represent a distinct class of planet. Their composition and interior structure are known much less well than those of the gas giants Jupiter and Saturn. While Jupiter and Saturn are composed mostly of hydrogen (more than 90% by mass) with hydrogen envelopes thought to extend all the way to relatively small rock/ice cores ... Uranus and Neptune possess much smaller hydrogen envelopes (less than 20% by mass)... The bulk composition of these planets [Uranus and Neptune] is dominated by much heavier elements... Since these species are thought to have been incorporated into proto-planets primarily as ices... Uranus and Neptune are often referred to as 'ice giants.'   However, it is thought that there is currently very little ice in these planets, a supercritical fluid being the preferred phase of H2O at depth."

Do Your Own Decadal Survey

In about two months, the Decadal Survey should release its recommended program of new missions for the coming decade.  In the meantime, I've set up a poll (actually three) to get your vote for which missions you think should be selected.

The poll is broken into three parts by mission cost class: Larger flagships (>$2B), smaller flagships (~$1.5B), and New Frontiers (~$1B).  You can vote for one each of the first two classes and for two New Frontiers missions.  All mission costs were taken from the Decadal Survey mission concept reports.  The Survey may decide on a different mix of missions (say, two larger flagships and one New Frontiers), but there's only so much flexibility I can build into a poll.

For inclusiveness, I added a comet sample return mission to the New Frontiers list.  No price tag was given in the mission concept study, so I arbitrarily decided to include it in this class.  I also listed the three New Frontiers missions currently up for selection.  One should be chosen any time now, but the other two still would be fair game to fly later in the decade.  I did not list the Mars sample return orbiter and return vehicles, which would fly in the 2020s and should be the subject of the next Decadal Survey.

By the way, this mix (1 larger flagship, 1 smaller flagship, and 2 New Frontiers) is my best guess at the mix the Decadal Survey will decide on with the addition of 2-3 Discovery missions.

The poll will close on March 10.  I plan to have completed my summary of the missions before then if you want to hold your vote or change it (I think Blogspot allows vote changes) when you hear about more missions.

The following are links to the mission concept studies or to similar mission proposals I've covered in the past:

Approximately New Frontiers
Lunar Geophysical Network
Trojan Tour and Rendezvous
Lunar Polar Volatiles Explorer
Chiron Orbiter Mission
Saturn Atmospheric Probe
Mars Polar Climate mission concepts
Io Observer
Mars Geophysical Network
Venus lander (SAGE)
Near Earth Asteroid Sample Return (OSIRIS-REx)
Lunar Sample Return (MoonRise)


Smaller Flagship Missions
Venus Intrepid Tessera Lander
Titan Lake Probe
Ganymede Orbiter
Enceladus multiflyby
Mercury Lander
Neptune-Triton-KBO or minimum Neptune orbiter
Venus Climate Orbiter/Balloon/Probe
Enceladus Orbiter 

Larger Flagship Missions
Uranus Orbiter with probe
Venus Mobile Explorer 
Mars MAX-C Caching Rover
Jupiter Europa Orbiter
Titan Saturn System Mission 

Neptune-Triton-KBO Flyby Missions

This week, I'll continue to look at possible missions for my personal selection of the five most compelling missions from the list of Decadal Survey mission concepts.  I've already selected the first four candidates and am now looking at options for my fifth selection.  Last week, I wrote about the Mars and lunar geophysical network missions.  This week, I'll look at Neptune missions.

My criteria for inclusion on my list is that a mission must offer the chance to significantly advance our understanding of the solar system.  Humans have flown many missions to the terrestrial planets and a number to the hydrogen-helium giants, Jupiter and Saturn.  The methane-rich gas giants have been visited just one time each by the Voyager 2 spacecraft.  That craft -- state of the art for the 1970s -- carried instruments that by today's standard would be antiquated.  A new mission to Neptune would offer the chance to investigate a key class of worlds with modern instruments and much higher telemetry rates.  As a bonus, the spacecraft would also investigate another likely ice-ocean moon, Triton.  Depending on the mission options chosen, the craft also could continue to flyby a Kuiper Belt Object (KBO) and continue the reconnaissance that New Horizons will begin at Pluto.

Many of the Decadal Survey concept studies looked at specific mission designs.  The two network missions last week were good examples.  Specific spacecraft configurations were chosen and examined in some depth.  The charge given to the Neptune study group was to evaluate a number of mission concepts ranging from simple flybys to Cassini-class orbiters.  The depth of analysis for any particular concept necessary was limited.

The missions break into two cost classes: Small flagship (~$1.5B [all costs in FY15 dollars]) flyby missions and very simple Neptune orbiters and larger flagship (>$2B) orbiters.   The report notes that the flyby missions might be able to be cost optimized to fit within the New Frontiers program (~$1B).   In this case, the mission would come to resemble the proposed Argo mission that I've written about before.
My description will focus on the small flagship mission class because it seems unlikely to me that the budget for the coming decade would fund a larger flagship mission.  Within that group are seven flyby concepts that would all investigate Neptune, Triton, and a KBO:

• Four of the options differ in which world would receive priority and one "best compromise."  (These options seemed to vary primarily on the design of the encounter geometry at Neptune/Triton.  A geometry that optimizes Neptune observations, for example, would limit the quality of the Triton encounter and rule out a range of potential KBO encounters.)
• One option would include multiple free flying magnetometers to study Neptune's unusual magnetosphere from multiple locations.  
• Another concept would add a single free flying magnetometer/transponder to enhance the KBO encounter.  
• A seventh option would include a small atmospheric probe that would sample the upper atmosphere (to 5 times Earth surface pressure).  
• Yet another option would strip the spacecraft of approximately half its instruments to fund a simple orbiter mission that would provide a number of encounters with Triton (but then could not go on to a KBO).

The study assessed the relative science merits of the different mission options.  The simple flyby missions and the free flying magnetometers/transponder were judged to have 1.7-2.0 times the science value of the Voyager 2 flyby.  The flyby mission with the atmospheric probe was estimated to have a relative science value of 2.3.  The simple small flagship class orbiter was given a science value of 1.6.  By comparison, the larger flagship mission concepts ranged from 2.3-3 (although the report emphasized that these values were probably understated).

A mission to Neptune an beyond necessarily involves a long flight.  In one representative flyby mission, launch occurs in 2018, followed by a Jupiter gravity assist in 2021, and the Neptune/Triton encounter in 2029.  The KBO encounter would occur sometime after that with the timing dictated by the target chosen.  The Argo proposal lists possible KBO encounters as late as 2041.

The requirement for a gravity assist limits possible launch dates.  Ideally, the mission would launch by 2018 to enable a Jupiter flyby.  The Argo proposal includes a Saturn flyby with either a preceding Jupiter flyby or a Trojan asteroid flyby.  Forgoing Jupiter for a Trojan, however, adds three years of flight time to reach Neptune.

Editorial thoughts: The arm chair explorer in me really likes the Neptune-Triton-KBO flyby missions.  The science would be excellent, and a chance to return to fascinating worlds after a wait of 40 years would be exploration at its finest.  This mission must either launch by 2018 (with 2019 and 2020 as poorer opportunities) or wait another decade for a favorable planetary line up.  This represents the last reasonable chance in my lifetime (yes, I'm in the boomer generation) to visit these worlds.   The mission might offer the chance for an international partner to provide either the atmospheric probe or free flying magnetometers.

The key issue I see is that this mission must compete with compelling missions to Jupiter-Europa-Ganymede and Titan-Enceladus for selection.  Among these three, which would you select?

Links:

Neptune-Triton-KBO Decadal Survey concept report
Argo mission presentation

Appendix: Here is a list of instruments proposed for the flyby options; those with an asterisk would be dropped for the small flagship class orbiter.

Wide angle imager
Near-IR imaging spectrometer
Magnetometer
RSCM link (the report doesn't spell out the acronym)
Narrow angle imager*
UV imaging spectrometer*
Plasma spectrometer*
Thermal mapper*

Mars and Lunar Geophysical Network Missions

Today I'll begin looking at three mission concepts that I am considering for my the fifth slot in my list of the missions I find most compelling for the next decade.  For this list, I am considering missions that I believe have the greatest chance of fundamentally changing our understanding of our own planet or of the solar system.  You can find the list of the first four missions here.  And again, I'll emphasize that the purpose of this list to stimulate thinking rather than convince anyone that my list is the one correct list.


For the past five decades, we've sent probes to study the surfaces and atmospheres of many worlds.  The interiors of those worlds, however, have remained largely unexplored except through low resolution gravity and magnetosphere studies.  The one exception was the seismographic network left by the Apollo astronauts on the moon in the early 1970s.  While crude by today's standards, these instruments gave us our first detailed look inside another world.  (Just recently, reprocessing of the data from these instruments has revealed that the moon has a liquid core.)  This year, two missions will launch that will further our study of planetary interiors.  The GRAIL mission will create a high resolution gravity map to study the distribution of material within the moon.  The Juno mission will study the interior of Jupiter by mapping its gravity and magnetic fields in high resolution.  


Two of the Decadal Survey mission concepts describe missions that would emplace networks of landers to study the interior of either the moon or Mars.  The goals of these missions are similar.  To quote from the Mars concept report, the top priorities would be to:


"1.  Characterize the internal structure of Mars [the moon] to better understand its early planetary history and internal processes affecting its surface and habitability.

• Characterize crustal structure and thickness.
• Investigate mantle compositional structure and phase transitions.
• Characterize core size, density, state and structure

"2.  Characterize the thermal state of Mars [the moon] to better understand its early planetary history and internal processes affecting the surface and habitability.

• Measure crustal heat flow.
• Characterize thermal profile with depth."

Possible landing sites for a two station Mars network.

Not surprisingly, the list of proposed instruments is similar, with a seismometer getting top billing with precision tracking, heat flow, and electromagnetic sounding instruments rounding out the roster.  The lunar mission would rely on laser retroreflectors for precision tracking while the Mars mission would rely on radio tracking.  The Mars mission also would include a limited number of meteorological instruments to characterize the weather and its effects on the seismic measurements.  

To keep the description of these concepts to a reasonable length, I won't describe each instrument in detail, but I will include some tidbits that I gleaned from reading the reports:

• A seismometer would be sensitive enough to use the tidal effects of the tiny moon Phobos to help determine if the core is liquid
• Precision tracking has as its primary goal detecting faint wobbles in rotation from the distribution of material within each world.  This technique is sensitive enough measure the exchange of material between the Martian ice caps and the atmosphere as ices freeze and sublimate.
• The heat flow measurements would primarily be used to understand the thermal state and evolution of each world.  At Mars, the heat flow could also be used to model the depths at which water would be liquid, with implications for zones of potential habitat.
• Electromagnetic sounding is a common technique for Earth studies that can determine the structure of the interior to hundreds of kilometers beneath the station.  Pockets of underground water would be detectable at Mars.

The key difference between the two missions would be the number of stations placed on each world: two for Mars and four for the moon.  At a 2009 Mars Decadal Survey meeting, Bruce Banerdt of JPL described what would be gained and lost with different numbers of stations.  One station may provide important constraints on our knowledge of the interior, but would rely on modeling for interpretation to an "uncomfortable degree."  Two stations allow the unambiguous detection of seismic events by recording them at two places and allows their sources to be narrowed to regions.  Three stations provide an incremental improvement, but four stations would be needed to fully address seismological goals.  Similarly, four stations would be needed to fully sample all key Martian regions for heat flow and electromagnetic sounding.

Lunar network station concept (astronaut not included :> )

The lunar network would meet the requirement for four stations.  The Martian network would fall short.  An option to increase the Martian network by an additional station was described in a separate Decadal Mission concept study.  This study looked at additional instruments that could be added to the landing pallet for the 2018 MAX-C and ExoMars rovers.  One option would add a seismometer and basic meteorological instruments.  If this enhancement would be made, the seismic measurements might help tune instruments and analysis on a subsequent mission.  If the additional network stations were flown in the 2020 opportunity, the 2018 instrument might be still functioning to provide a third station.

Relative science return of Mars networks with different numbers of stations.

Editorial Thoughts:  The lunar network would be a highly capable mission that would build on the results of the GRAIL mission and extend the data set collected by numerous previous missions to the moon.  The Mars mission as described sounds as if it would be a pathfinder mission, with a full scale network mission still needed at a future opportunity.  The proposed Mars mission appears to be based on a New Frontiers proposal that used the heritage of the then just-flown Mars Phoenix lander.  As such, the study did not look at other lander options that might be cheaper and allow more stations within the same budget.  Europe is planning to demonstrate such a lander as part of the 2016 joint mission with NASA.  Designing descent and landing systems as part of a science mission drives costs and risks up.  I can see why JPL chose to based the concept study on proven technology (the concept studies had to be done quickly and with limited budgets).  By the latter half of this decade, though, that technology will be a decade old and Europe's technology should offer an alternative.  (Note: I don't know if Europe's technology would offer a cost advantage and bring it up only as a possibility.)


The option to enhance the 2018 mission with a minimal geophysical station is intriguing.  Per the concept study, this would add ~$200M to the total mission costs, although cleverness would be needed to keep the total delivered mass within limits.  At this price, this seems like a bargain to me, but a couple of caveats give me pause.  First, the station would be build with single string electronics, that is, no redundancies in case of hardware failure.  That could make operation to overlap a subsequent mission problematic.  For me, though, the bigger concern is that this addition would be the fourth of four priorities for the 2018 mission: land safely, build and operate a capable NASA rover, deliver the ExoMars rover, and implement a pathfinder geophysical station.  If the mission needs to cut costs or weight, I fear that the geophysical instruments could be the first to go.


Network missions make my list of compelling missions to explore because they explore terra incognita: the interiors of worlds with high precision.  What we learn has the potential to change our understanding of how planets formed and evolved.  A lunar mission would provide the richer data set and would compliment the extensive data sets collected by previous missions, especially the GRAIL gravity-mapping mission.  A Martian mission might be more of a pathfinder with just one or two stations, but one that begins the exploration of the interior of another terrestrial planet and that would also compliment an extensive data set.  Either should be a strong candidate to fly in the coming decade.  My choice would be for the Mars mission, but only because of a stronger personal interest in the exploration of that world.

Resources:

The Mars and lunar network mission concept reports can be found here.


The Mars meeting geophysical network presentation (a good overview of the science goals) can be found here.

All images are from these documents.

Compelling Decadal Survey Missions

With the long holidays behind us, I'll restart my selection of the five missions under consideration by the Decadal Survey.  My criteria for selection has been that the missions have, in my opinion, the greatest chance for fundamentally changing our understanding of the solar system or our own home planet.  This list and the analysis presented is intended to stimulate thinking rather than to persuade you that my list is right.  Ray at Vision Restoration has been presenting his own list based on the goals of overall exploration and development of space.

I've been using this series to go through the Survey's mission concept studies to understand some of the issues inherent in planning a roadmap for the coming decade and to explore individual mission concepts.  To date, my list has included:

• 1st priority: Missions leading to a Mars sample return: Mars Trace Gas Orbiter, MAX-C and ExoMars rover, and technology development for the missions that would return the cached samples to Earth
• 2nd priority: A Venus mission (lander, balloon, or orbiter) that would help us understand how Venus diverged so dramatically from Earth even though the planets were apparently more similar early in their history.  NASA's mission might be part of a series of international missions
• 3rd and 4th priorities: Advance the exploration of ice-ocean moons as potential habitats.  The Jupiter Europa Orbiter (JEO) would, in my opinion, provide the greatest return for the dollars spent but hinges on the budget allowing a second large flagship mission (MAX-C would be the first) in a single decade, the technology being ready to allow extended operation in the high radiation at Europa, and sufficient plutonium supplies to power the spacecraft. As my fourth priority, a Discovery or New Frontiers class mission to continue the exploration of Enceladus or Titan.  If JEO doesn't fly, then my priorities would change to put continued exploration of Enceladus and Titan as the third priority and a small flagship class Ganymede orbiter with flybys of Europa and Callisto as my fourth priority.

This weekend, I'll begin looking at three missions that are my candidates for the fifth priority: Mars geophysical network, a Neptune-Triton mission, and a comet sample return.

I'll close this blog with a relisting of the missions under consideration by the Decadal Survey.  Most of the mission concepts come with specific price estimates, but as one report states given the limited time for analysis of each mission, "it would be appropriate to thing of costs at relative levels of ~$1.0B, ~$1.5B, ~$2.0B, etc."  These studies included both principal investigator (spacecraft, instruments, operations, etc) and launch costs.   With that in mind, here is the list of missions broken into three cost bins:

Approximately New Frontiers (<$1.2B estimate in study)

• Lunar Geophysical Network (4 landers)
• Trojan Tour and Rendezvous 
• Lunar Polar Volatiles Explorer 
• Chiron Orbiter Mission 
• Saturn Atmospheric Probe 
• Mars Polar Climate mission concepts
• Io Observer 
• Mars Geophysical Network (2 landers)  

In the FY15 dollars used for these studies, a New Frontiers mission would cost around $1.05B.

Smaller Flagship Missions ($1.3-$1.6B estimate)

• Venus Intrepid Tessera Lander
• Titan Lake Probe (4 options) 
•  Mars Sample Return Orbiter 
• Ganymede Orbiter 
• Enceladus multiflyby 
• Mercury Lander (mid-range option) 
• Neptune-Triton-KBO or minimum Neptune orbiter 
• Venus Climate Mission Enceladus Orbiter  

Larger Flagship Missions (>$1.9B estimate)

• Uranus Orbiter with probe 
• Venus Mobile Explorer 
• Mars MAX-C Caching Rover 
• Mars 2018 Sky Crane Lander 
• Mars Sample Return Lander & Ascent Vehicle 
• Jupiter Europa Orbiter 
• Titan Saturn System Mission  

These mission bins are not necessarily fixed. Several small flagship missions have larger flagship mission alternatives, for example.  In addition, there are proposals for Discovery missionsfor a Titan Lake Lander, a simple multi-flyby mission to Titan and Enceladus, and an Io Observer.  By reducing mission goals and capabilities, significant costs can be saved.  However, it is possible that (1) these missions might no longer be as compelling compared to other Discovery mission proposals and (2) the proposing PI's may be optimistic in how cheaply these missions can be implemented.  (Editorial note:  I personally like these Discovery mission concepts, and hope that neither of these caveats apply.)