Io Observer Concept

In the current tally of your votes for missions you would like to see the Decadal Survey prioritize (if you haven't voted, please do so!), the Io Observer currently has the second highest number of votes for New Frontiers missions.  (The current leader is a Titan lake probe.)  Today, I'll summarize the mission concept prepared for the Decadal Survey for the Io mission.

The concept has many simularities to the Io Volcano Observer Discovery proposal that I wrote about almost two years ago.  This is not surprising since both take their starting point from the same JPL concept study conducted in 2008.  The mission would place an spacecraft into a highly inclined (>45 degree) orbit around Jupiter that would encounter Io at each perijove.  The inclined orbit keeps the spacecraft out of the highest radiation fields except during the actual flyby.  This strategy allows the mission to avoid the high-cost radiation mitigation strategies of the proposed Jupiter Europa mission and instead use the more modest design approaches employed by the upcoming Juno Jupiter orbiter.  (The Io Observer would have less than half the radiation exposure of Juno by the end of their nominal missions.)

At each Io flyby, the spacecraft would map the illuminated hemisphere in eight colors at resolutions of less than 1 km/pixel.  Selected areas would be mapped at 10-100 m/pixel in four colors and small areas at less than 10 m in panchromatic.  A thermal mapper would provide high resolution thermal imaging of the moon during the flybys and whole disk heat flow maps at ~200 km/pixel when Io slips into Jupiter's shadow and the spacecraft is between flybys.  Observations of Jupiter's atmosphere could also be made between flybys.

The six to ten encounters in the nominal mission would employ a variety of encounter strategies at Io.  There would be at least three passes each on the day and night sides of Io.  Two flybys would study magnetic induction by conducting encounters at high but opposite magnetic latitudes.  Two or more flybys would encounter Io at the same latitude to allow searches for changes on the surface.  At least one flyby would occur below 100 km above the surface, and one encounter might take the spacecraft through a volcanic plume.

The instrument list proposed would be modest: a narrow angle camera with filters for multispectral imaging, a thermal mapper operating between 2-20 μm, an ion and neutral mass spectrometer to measure the composition of material ejected from Io, and a magnetometer.  One option studied would add a fast-imaging plasma spectrometer to map the distribution of ions in the space around Io.  Charts tying science goals to instruments note that an ultraviolet spectrometer and a near-IR spectrometer also would  make valuable contributions (with the latter also noted as useful for studies of the Jovian atmosphere).

The Io Observer Decadal Survey concept and the Io Volcanic Observer Discovery concept have two key differences.  The analysis for the Survey concept concluded that solar power was more practical than plutonium ASRGs. The Discovery concept was based on ASRGs.  At the time the Discovery concept was publicly discussed, it would have been powered by ASRGs.  That concept's study, however, was funded to determine the types of missions that would be enabled by ASRG units.  I understand that the PI submitted a proposal to the current Discovery mission selection; I don't know what power source was proposed.  An advantage for proposing an ASRG system for a Discovery proposal would be that NASA would pick up the costs for the ASRGs but not for solar panels.  In addition, ASRG's would allow all instruments to be mounted to the spacecraft body, while solar panels would require the remote sensing instruments to be mounted on a more expensive scan platform.

The second key difference between the two concepts is in the cost estimates.  The Survey concept would fit within a New Frontiers budget while the Discovery concept would cost several hundred million dollars less.  Yet the two concepts appear quite similar.  It could be that the team estimating the Survey concept was too conservative -- under the ground rules established by the Survey, conservative estimates were encouraged and each concept had to carry hefty cost reserves.  The PI for the Discovery concept may be too optimistic, although he is very experienced and has well established credibility.  A similar difference exists between the Survey's Titan lake probe cost estimates in the small Flagship range and a similar Discovery proposal, with the Discovery proposal again led by a very experienced, credible team.  Several of the Survey concept reports speculated that it might be possible to significantly reduce costs below their estimates with a focused design-to-cost approach.

Appendix: Science goals from the report

"Understand the eruption mechanisms for Io’s lavas and plumes and their implications for volcanic processes on Earth, especially early in Earth’s history when its heat flow was similar to Io’s, and elsewhere in the solar system.

"Determine Io’s interior structure, e.g., whether it has a magma ocean, and implications for the coupled orbital-thermal evolution of Io and Europa.

"Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating.

"Investigate the processes that form Io’s mountains and the implications for tectonics under high heat-flow conditions that may have existed early in the history of other planets.

"Understand the composition, structure, and thermal structure of Io’s atmosphere and ionosphere, the dominant mechanisms of mass loss, and the connection to Io’s volcanism.

"Determine whether Io has a magnetic field.

"Understand Io’s surface chemistry, including volatiles and silicates, and derive magma compositions (and ranges thereof), crustal and mantle compositions and implications for the extent of differentiation, and contributions to the atmosphere, magnetosphere, and torus."

"Improve our understanding of Jupiter system science, including meteorology, aerosol structure, tropospheric composition, and auroral phenomena on Jupiter, composition and temporal variability of Europa’s exosphere, Jovian magnetospheric processes, and small inner moons and rings of Jupiter."

The Decadal Survey mission concept studies can be found here.

NASA Planetary Program Update

The coming year and a half will be a hell of a ride.  Can't wait!

Approximately twice a year, the National Academy of Sciences Planetary Science Subcommittee meets and reviews status and issues in NASA's planetary program.  The most recent meeting just completed, and I will summarize highlights.  The meetings typically have an update from NASA's headquarters, special presentations on key issues, and updates from the analysis groups (Outer planets (OPAG), Mars (MEPAG), Venus (VEXAG), lunar (LEAG), and small bodies (SBAG)).  You can view the presentations at www.lpi.usra.edu/pss/jan2011

Much of the meeting consisted of status updates, but four issues seemed to stand out:

1) NASA (and most Federal programs) do not know what their budget will be this year or for coming years.  They are reacting by being conservative both by mandate (e.g., no new initiatives such as the restart of plutonium 238 production) and judgement (e.g, being careful not to over commit research grants).

2) There still is no plan to restart plutonium 238, which is essential to exploring the outer solar system beyond Jupiter for simpler missions and at Jupiter for Flagship missions.

3) Launch vehicle costs are rising (see below).  In an era of expected flat to declining budgets, this translates into fewer missions.

4) Mars Science Laboratory (Curiosity) costs are expected to require more funds.  NASA plans to first take funding from JPL, the implementing organization, then the Mars program, and finally the larger planetary program.

The number of missions being proposed or under development (Venera-D) suggest that Venus may again become a priority destination.  In addition to the  two ESA Cosmic Vision proposals for Venus, I know of proposals for a Titan balloon and a Uranus orbiter.  There are likely additional planetary proposals.  The selected mission would launch in the early 2020s.

The MESSENGER spacecraft is expected to have fuel for a second year of studies in orbit.  NASA is beginning planning to decide whether to fund a second year of operations.

It is possible (probable?) that the Decadal Survey will recommend only one of the two large Flagship missions proposed for the next decade, the Jupiter Europa Orbiter or the MAX-C rover that would cache samples for a Mars sample return mission.  This slide acknowledges that the plans for outer planet exploration may have to be reset if JEO is not recommended.

The Mars program has become a joint NASA-ESA program that is planning a sequence of missions that include ESA's ExoMars rover, NASA's MAX-C rover, and future missions to return samples.  The international cooperation may influence the Decadal Survey's recommendations for priority missions.  If Mars missions are not a recommended priority, then the Mars community is likely to be rethinking its program.

 An example of the cost increases NASA is facing for launching missions.

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.