Tuesday, December 16, 2008

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."

1 comment:

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