White Paper: Io

Jason Perry has a nice summary of the Io white papers at his website: Io Decadal Survey White Paper. There are actually two white papers, one which discusses science goals and one which discusses possible missions. Neither had anything particularly new, and Jason does a good job of summarizing them. The most interesting part of the discussion was the endorsement of the Io Volcano Observer Discovery proposal (~$450) and in the next breadth discussing New Frontiers (~$650M)missions to conduct essentially the same goals. My guess -- no knowledge -- is that the community is nervous about the mission fitting within the Discovery budget. The last budget estimate I saw for the Io Volcano Observer put it slightly over the Discovery budget limit. The key issue, I would guess, is likely the technological risks of the radiation hardening fitting in the budget with acceptable margins.

An Io observer mission requires a plutonium power supply. The next Discovery mission can include such a plutonium power supply (to test the ASRG supply) while the next New Frontiers mission will not (to husband dwindling plutonium supplies). If an Io observer cannot fit into a Discovery budget (or can, but isn't selected), then the next opportunity is the late 20 teens New Frontiers opportunity. That would put Io in competition with the proposed Argo mission to Jupiter, Saturn, Neptune, Triton, and one or more Kuiper Belt objects (as well as other good missions to non-outer planet destinations.)

The key issue for any dedicated Io mission, however, is the planned flybys of Io by the Jupiter Europa Orbiter (JEO). A dedicated mission would provide much better Io science through optimized instruments and many more flybys, but will the community accept a flagship mission to Jupiter and another Jovian dedicated mission (given the Juno Jupiter polar orbiter and JEO)? My guess is not. If JEO proves too expensive to fly, then I think that a dedicated Io mission has a good shot.


Resources

Jason's summary

Io Science Goals White Paper

Io Missions White Paper

Io Missions - Part 3: Io Volcano Observer

Previous posts discussed general goals and constraints on a mission to Io and the science goals for such a mission. This entry discusses a specific mission concept, the Io Volcano Observer (IVO). Also see Jason Perry's write up on IVO.

NASA is facing a shortage of plutonium to use as a power source for future missions to the outer solar system (and other destinations with limited sunlight). A new design called the Advanced Stirling Radioisotope Generator (ASRG) would dramatically reduce the amount of plutonium required for future missions. The techology is new, however, and NASA may want to fly it first on an inexpensive Discovery-class ($450M) mission instead of on a more expensive New Frontiers ($650M) or Flagship (~$3B) mission.

Approximately a year ago, NASA began funding several studies of Discovery class missions that would provide a first use of ASRG's. One of those studies, led by Alfred McEwen of the University of Arizona, is for a Io Volcano Observer (IVO). Dr. McEwen was kind enough to provide me a copy of a presentation on the mission concept that he presented to the Berkeley Io Workshop on December 12. (The images come from his presentation.)

IVO would orbiter Jupiter in a highly inclined (>45 degrees) orbit with periapsis at Io's orbit. This orbit minimizes the craft's exposure to the intense radiation fields of Jupiter. On the other hand, it keeps the craft well away from Jupiter's other large moons. While significant obserations of Jupiter itself would be made, the focus of the mission is squarely on Io. Fans of other Jovian moons would be left waiting for other missions. (However, it is possible that IVO might be able to image some of the small moons that orbit inside Io's orbit. In addition, some long term observations of Europas extended atmosphere might also be possible.)


Red orbits are IVO's; white orbits are the major Galilean moons. IVO will not come close enough to Europa, Ganymede, or Callisto for detailed imaging, although studies of Europa's tenuous atmosphere may be possible.


McEwen points out that even if the Jupiter-Europa Orbiter (JEO) is selected as the next Flagship mission that IVO would still provide important complimentary science. JEO would provide 4 science flybys of Io before moving to targets further out in the Jovian system. IVO, however, would provide "unique polar viewing and in-situ sampling geometry of Io, torus, etc; spectral bandpasses designed for Io science, orbit designed to answer key Io questions, and the more flybys the better as Io always changes." IVO would also reach Io a number of years before JEO.

Most of the rest of this blog entry focuses on the nuts and bolts of the proposal, but I'll add my thoughts here. I am extremely impressed with the capabilities of this proposed mission, especially given the target budget of $450M (which it isn't quite hitting yet but is within striking distance). However, coming within striking distance may be enabled by the assumption (provided as a baseline for the concept study) that NASA would provide the ASRG's outside of the mission budget cap. Put another way, if the mission had to pay for its own power source (ASRG or solar cell), it might not be able to fit within a Discovery mission budget. NASA has yet to decide if it will do so for the next Discovery competition. I hope that McEwen or others will also propose a similar mission (using solar cells) for the New Frontiers competition in progress. The New Frontiers program would appear to have sufficient budget to fly the mission and include some additional instruments beyond the baseline planned for IVO.

[A note on mission classes: The price caps for each Discovery and New Frontiers mission are set with each selection competition. For Discovery missions, a working figure for mission costs is $350-450M for the spacecraft, instruments, and launch vehicle (plus other costs). For the next New Frontiers mission, the cap is $650M not including the launch vehicle. While exact comparisons are difficult, as a rule of thumb, Discovery missions appear to spend approximately half as much on spacecraft and instruments as New Frontiers missions.]

McEwen's e-mails to me suggest that he will not propose the mission as a New Frontiers candidate because the high data bandwidth needed to study the Io requires the power levels of the ASRG's. He also says that there are other issues with solar power. He didn't elaborate, but radiation damage to the solar cells and the difficulty of point the instruments at Io while keeping large solar panels toward the sun may be among them.

I would very much like to see a mission like this fly, whether within the Dicovery or New Frontiers program. (Although if Europa/Jupiter is picked as the next target for a Flagship mission, it would perform much of the science proposed for this mission.)

Here's a summary of key facts about the proposed mission:

"Primary science objectives:

1. Understand active volcanic processes on Io
2. Understand tidal heating of Io
3. Understand the loss of matrial from Io and effects on the magnetosphere, plasma torus and neutral clouds"

Launch in January 2015 on a Venus-Earth-Earth gravity assist trajectory with arrival at Jupiter in 2021.

Baseline mission has an Io flyby at orbit insertion, and six additional flybys over approximately a year and a half. Additional flybys in an extended mission are possible (>4 mentioned in the presentation).

Flyby distances range from 100-1000 km.

It appears that approximately 20Gbits of data would be sent to Earth per month. (Compare this with 0.2Gbits of data returned by the Galileo mission for Io.) Early in the mission, when orbit periods are longer, it appears that this data would be a mixture of Io and Jupiter observations. Later in the missions when the orbits are approximaly one month long, Io observations would dominate the available bandwidth.

Mission ends with impact on Io for planetary protection. (Keeps the spacecraft from accidently delivering Earth organisms to Europa.)

Total costs are currently conservatively estimated at $471M (including launch vehicle) versus a cap for the exercise of $450M. (No indication of how risky the mission implementation is within this budget compared to other Discovery program mission proposals.)

The baseline instrument complement focuses on the essential science (and weighs just 32 kg):

Narrow angle radiation-hardened camera with both a black and white framing mode and filters for pushbroom imaging in multiple colors that can provide 1 km/pixel resolution at 100,000 km or 10 m/pixel at 1,000 km. Science goals: monitor eruptions, measure peak lava temperatures, limited topography, Io surface composition (if budget allows for sufficient spectral filters), observe Jupiter's cloud deck (Tech specs: 10 urad/pixel compared to 5 urad/pixel for the New Horizon's LORRI camera; 2000 x 2000 pixel array with up to 1000 lines dedicated to multispectral imaging in pusbroom mode; spectral range ~200-1000 nm; up to 15 spectral filters; ~15 kg)

Thermal mapper to "map and monitor temperatures, heat flow pattern related to internal structure and tidal heating mechanisms." If budgets permit, the instrument could be enhanced to study Io surface composition using thermal emission spectroscopy and map Jovian hot spots. (Tech specs for this instrument appear to stil be in definition. Possibilities listed in the presentation are 640x480 detectors; ~2-20 microns; 1 km/pixel from 8,000 km; up to 10 bandpasses; ~12 kg).

Ion and neutral mass spectrometer to study spatial distribution of neutrals contributing to Io plasma torus , gas composition of Io plumes, composition of Io's (very thin!) atmosphere. (Tech specs: 1-300 amu; measurements every 5 seconds; 4 kg).

Magnetometer to study Jupiter's magnetosphere and "place tighter constraints on Io's internally generated magnetosphere (hard)" (Tech specs: 2 at 1 kg each)

This payload focuses on the essential Io science. Additional desired instruments in apparent order of priority are: more spectral bandpasses on camera and thermal imager; second, medium gain antenna to enhance gravity science; second neutral mass spectrometer with a different view; wide angle camera for better Io imaging close up, especially stereo imaging of topography; near-infrared spectrometer for minerology studies [my note: would also be useful for Jupiter observations]; UV spectrometer (s) for torus studies and Io atmosphere/plume composition.; energetic partical detector for "science and future exploration."

Io Missions - Part Two: Science Goals

As discussed in previous posts, NASA is soliciting mission proposals for its next New Frontiers ($650M) planetary mission. Among the missions under consideration is an Io Observer. Government solicitations of proposals follow specific processes to ensure fairness to the proposers and to ensure that the government receives proposals relevant to its goals. NASA has published a 122 page preliminary Announcement of Opportunity (AO) for the New Frontiers solicitation. Among many other topics it lists the science goals for each possible mission. Since a great deal of thought goes into these AOs, this document is the best summary of the planetary science community's goals for each mission. For your convenience, I've copied the section on goals for an Io observer here (if you want to read them in the original AO, they start on page 8) as a succinct statement of science priorities for future Io missions.

Io Observer
Tidal heating, a process that can greatly expand the habitable zones in the solar system and elsewhere, is best studied at Io because it provides the most extreme example of this process in the solar system. Io provides the best place in the solar system, beyond Earth, to study volcanism, a process of fundamental importance on many planetary bodies. Io also provides some of the most dramatic, freshest, and easily-studied examples of fundamental geological processes such as mountain-building and mass wasting. The volcanic activity on Io drives interlocking processes on a variety of time scales. While resurfacing/recycling the surface, the activity also provides volatile contributions to the Jovian sulfur and sodium nebulae via a time-varying atmosphere and exosphere. By providing approximately 1 ton per second of material deep within the magnetosphere of Jupiter, Io is a primary driver for most magnetospheric activity. With transport of material to Europa and the rest of the system, reenergization processes, and the Alfvénic interaction between Io and the upper atmosphere of Jupiter itself, scientists know that multiple, nonlinear feedback processes are present on many spatial and temporal scales.

An Io Observer mission should address some of the following science objectives, which are not listed in order of priority:

• Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating;
• Determine Io’s interior structure, e.g., does it have a magma ocean;
• Determine whether Io has a magnetic field;
• 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;
• 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 Io’s surface chemistry, volatile and silicate, 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; and
• 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.

It is likely that there are more objectives here than can be included in a single New Frontiers mission; proposals must state the science goals for the proposed investigation
and provide a rationale for the choice of science objectives. Any mission architecture that achieves the majority of the science objectives stated above for Frontiers cost cap will be considered responsive to this AO.

Io Missions: Part One

In case you haven't noticed, there's a poll just to the right of this blog entry with readers' votes on their preferred destination for the next New Frontiers mission. The current voting has Venus as the favorite destination, with Io as a distance second but well ahead of the third favorite, a lunar sample return. A previous (and future) blog entries discussed options for a Venus mission. This and the next two entries looks at options for a return to Io.

Topics will be (1) Background on options for a return to Io, (2) science goals for an Io mission, (3) the proposed Io Volcano Observer, and (4) ideas for extended missions. This entry addresses the first topic, and entries later this week will address the other topics.

On a side note, I'm writing this while traveling to the American Geophysics Union (AGU) fall conference. While this conference is usually jam packed with results from missions in progress (and it will be again this time), this meeting will also have two sessions devoted to future Venus missions as well as poster presentations on several other proposed missions. It will take me awhile to work through the riches, but I will report on all the sessions over the next few weeks.

Background on options for a return to Io

A return to study Io has been a goal of a subset of the planetary science community (and based on our poll, the public that follows planetary exploration) ever since the end of the Galileo mission. While the Galileo spacecraft made several close flybys of Io, relatively little scientific data was returned in part because of the malfunctioning main antenna and in part because the intense radiation near Io caused the spacecraft to repeatedly go into safe mode just prior to closest approach. I don't want to downplay the value of the data returned. It was scientifically invaluable. However, only small portions of the moon were imaged and temporal coverage of this very dynamic moon was limited. One presentation I saw stated that just 0.2Gbytes of data were returned. That represented a very tiny window of data for what is a very dynamic moon.

The planetary science community has made a return to Io a priority. It is one of a handful of missions listed a priority targets for the next New Frontiers mission ($650M). (For a list of all the targets, see the previously mentioned poll.) Why is Io a priority?

o Because of extreme tidal heating, Io is the most active volcanic body in the solar system. The style of volcanism is believed to resemble that of the terrestrial planets in their extreme youth.

o Io is the best place to study tidal heating, a phenomenon that maintains a liquid ocean beneath the crust of Europa and possibly Enceladus.

o Io is a geologist's dream with interesting surface chemistry, grand mountains....

o Io is intimately connected to the Jovian magnetosphere and has even been called the heartbeat of the magnetosphere.

Currently, there are three possible routes for a return to Io. The most likely (in that there's a 50-50 chance the mission will be selected to fly) is the next Flagship mission (~$3B). (Titan is the other candidate target in this selection, which will occur in the next few months.) If Jupiter is the target of the next outer planets Flagship mission, it will include three close flybys of Io for science, including one pass that would go through (presumably the outer fringe) of a volcanic plume. (On Jupiter orbiter insertion, a flyby would also be done to use an Io gravity assist to reduce speed, but scientific observations apparently would not be done, at least with the remote sensing instruments.) Following the Io flybys, continued long range observations would be made for the remainder of the mission. If this mission flies, I doubt that any of the other possible missions that would target Io would be selected.

Let’s assume here, however, that Titan is chosen for the flagship mission. In that case, a New Frontiers mission to Jupiter would still be in the running. (The decision on the New Frontier's target will come in a couple of years.) Return missions have been studied for at least the past decade. I have a paper copy of a proposed Jupiter orbiter/multiple Io flyby Discovery program mission from about ten years ago. I believe that Io missions have been proposed for the Discovery program at least once and if not multiple times. None have ever become finalists.

If the mission(s) were proposed, two factors probably kept them out of the running. First, a $300-450M (budget caps have increased over time) budget is very tight for an outer planets mission. Both the New Horizons Pluto flyby craft and the Juno Jovian orbiter required the approximately doubled budget cap of the New Frontiers mission. In addition, radioactive power sources have not been allowed for Discovery missions. Solar cells can be used at Jupiter (the Juno Jupiter mission will use them, for example.) However, the intense radiation fields in the inner Jovian system where Io lies will gradually degrade performance of the solar cells, limiting the number of close passes. I've not seen a definitive answer as to how much, although at least a couple of studies have suggested that even the Europa orbiter mission (which would face much higher high radiation levels) could be done with solar cells.

Whatever its power source, an Io mission would orbit Jupiter and conduct multiple flybys of Io. The radiation field at Io is so strong that an Io orbiter's lifetime would be measured in hours to days – assuming the craft survived the radiation to even enter orbit. The total number of flybys would ultimately be limited by the cumulative radiation damage to the craft's electronics or (if used) solar cells. The craft can be designed to tolerate high levels of radiation exposure. An Io mission with the same radiation hardening as the proposed Europa flagship mission could do 50 – 100 flybys of Io within its radiation tolerance. (Note that this number is an extrapolation from a couple of presentations with different assumptions about radiation hardening; if any readers have better information, please pass it along.)

An Io mission study group that provided input to the 2003 Decadal Study proposed an Io mission that would encounter Io multiple times (up to 50) at the same place in its orbit. This way, lighting conditions would remain identical with each encounter. Different lighting conditions can make it difficult to determine whether apparent surface changes at Io are the result of true changes in the surface material or changes in lighting.

The Decadal Survey mission concept assumed a very capable craft with a radiation tolerance half that of the then proposed Europa mission. I don't know what degree of radiation hardening can be purchased within the $650M New Frontiers budget, but it is likely less. Radiation is a major design driver for the Juno Jupiter orbiter, which uses a “hole” in the radiation belt near Jupiter to minimize radiation exposure. I suspect that providing radiation hardening to the level of a Flagship mission is probably outside the budget of a New Frontiers mission.

On the other hand, we know that the New Frontiers budget cap is large enough to implement a mission to the outer solar system. Both the New Horizon Pluto and the Juno Jupiter missions are New Frontier missions. It is a reasonable guess that a robust Io mission could also be implemented within the budget cap.

In in the third part of this series, I'll report on a proposal to implement an Io mission within the much tighter Discovery mission budget.