Prior to the announcement of the Discovery candidate missions, I had been exploring options to explore th outer planets within the new fiscally constrained environment. Previous posts had looked at options for Discovery missions. The inclusion of the Titan TiME lake probe makes it more likely that at least some Discovery outer planet missions may be possible and scientifically competitive.
Today, I want to consider options for a much reduced cost Europa orbiter. I'll start by stating that I don't know how much costs can be reduced. ESA's proposed Jupiter Ganymede Orbiter would approximately fit within the new proposed New Frontiers budgets planned for the next competition. (Direct cost comparisons using public information are difficult between ESA and NASA missions, because they use different accounting rules and the exchange rates may not reflect actual purchasing power.) JGO, however, doesn't face the extreme radiation that an equivalent Europa mission would face. The costs of radiation hardened electronics and additional shielding certainly would add substantial costs if the same mission were flown to Europa. NASA's Jupiter Europa Orbiter study group estimated that the cost of a minimal mission would be approximately FY07 $2.1B (although the report didn't specify the exact set of options behind that estimate). However, the same group also estimated that the cost of the full JEO mission would be FY07 $2.7B or $3.8B in real year costs. The Decadal Survey estimated the real year JEO costs would be $4.7B, putting the FY07 $2.1B minimum cost estimate in doubt.
This post, therefore, may be an exercise if futility -- there may be no way to orbit Europa for less than a major Flagship cost in an era where Flagship missions don't appear affordable. From the meetings I've listened to, however, the Europa science community would like to see how low the cost of the minimally justifiable mission could be driven. I'll explore two related areas where costs might be reduced: limiting the science goals and shortening the time in Europa orbit.
The JEO study team carefully laid out priorities for studying Europa and for the instruments supporting each study. The phasing of the mission timeline also corresponded to those priorities. One approach to defining a minimally acceptable mission would be to pare back the priorities to a minimum core, which also reduces instruments and potentially time needed in orbit. Because the radiation damage is cumulative over time, the shorter the orbital mission, the lower the cost to design a radiation hardened spacecraft and instruments.
The JEO study team identified four overall goals (for simplicity, I won't reproduce the list of sub goals) and core supporting instruments in priority order:
- "Ocean -- Characterize the extent of the ocean and its relation to the deeper interior" -- Laser altimeter and radio science for gravity measurements
- "Ice -- Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange" -- Ice penetrating radar
- "Chemistry -- Determine global surface compositions and chemistry, especially as related to habitability" -- Visible-IR Imaging Spectrometer, UV Spectrometer, Ion and Neutral Mass Spectrometer
- "Geology -- Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration" -- Thermal Instrument, Narrow Angle Camera, Wide Angle Camera and Medium Angle Camera"
In addition to these instruments were a magnetometer and a plasma instrument to characterize the induced Europan magnetosphere as a method to explore the ocean and the coupling of the surface with Jupiter's magnetosphere.
The relationship between instruments and goals is more complicated than that I presented here (which comes from the summary charts in the front of the report). The exhaustive detail charts in the body of the report shows that several instruments could contribute multiple goals. The wide-angle camera, for example, would contribute to the ice, chemistry, and geology studies. The JEO report authors concluded that the minimum instrument set should be:
- Radio Science
- Laser Altimeter
- Near-IR spectrometer (less capable than the Visible-IR Imaging Spectrometer planned for JEO)
- Ice penetrating radar
- Wide and medium cameras
- Magnetometers
- Plasma instrument
A new proposal for a pared down mission might further reduce this list and probably would recommend less complex instruments than envisioned for JEO. If the new proposal were to address just the ocean and ice goals, the cameras and spectrometer might be dropped. Those instruments also require the high data rates, and dropping them would reduce power and communications requirements.
The second place to look for mission savings would be in the length of time spent in orbit around Europa. The JEO study members identified several distinct mission campaigns, with earlier campaigns addressing higher priority goals:
Europa Campaign 1, Global Framework (200 km orbit, ~28 days): First order characterization of the ocean and ice shell through studies of tidal deformation (laser instrument), gravity (radio science), and magnetic field (magnetometers and plasma instrument). Global stereo and color maps (wide angle camera). Identification of shallow water and deep ocean search (ice penetrating radar). Measurements of surface composition (Visible-IR Imaging Spectrometer operating in a profiling mode where the 1-D location directly beneath the spacecraft would be measured in contrast to the mapping mode where 2-D images are produced)
Europa Campaign 2, Regional Processes (100 km orbit, ~43 days): The studies from campaign 1 continue, and higher resolution studies at regional scales are added.
During these first two campaigns some targeted studies using the full instrument suite were planned. Two campaigns, however, were planned for higher resolution studies, Europa Campaign 3: Targeted Processes (1-2 months), and Campaign 4: Focused Studies (~5 months). These additional campaigns would also provide time for more orbits that would narrow the spacing between ground tracks for the laser altimeter and ice penetrating radar.
The JEO authors listed 3.5 months as the minimally acceptable mission, but perhaps in the new budget realities just the 28 days of Campaign 1 or an additional month or two for Campaign 2 would constitute the minimum mission.
You can download the JEO report at http://opfm.jpl.nasa.gov/library/
Editorial Thoughts: Defining a new lower cost Europa mission will be a multifaceted approach where every possible cut in goals may reduce costs in multiple areas. (For simplicity, and because I'm not a spacecraft engineer, I didn't discuss the power or data requirements that would also be intertwined with all these elements.) The final proposal has to do more than meet a budget figure, however, it must also provide compelling science equal to that which other missions for the same budget might provide. If a mission were defined that could fit within the New Frontiers budget but carried only a laser altimeter and radio science, would this be a compelling use of >$1B of NASA's budget when the same amount might return a sample from a comet or put a lander on Venus? I don't know the answer, but I expect that this question will be in the minds of the team that relooks at an Europa orbiter.
One Decadal Survey White Paper (A budget phasing approach to Europa Jupiter System Mission Science) by David E. Smith of the Goddard Spaceflight Center recommended splitting the JEO goals across three smaller missions. The total cost for implementing the JEO science was expected to be about the same as for JEO, but the costs could be spread over a number of missions. My impression is that the goal of the paper was to point out that there were alternative approaches to exploring Europa rather than to present a rigorous analysis of particular mission concepts that would provide solid feasibility and cost estimates that would form the basis for a mission proposal. Rather the paper pointed to the direction of reducing mission goals as the way to reduce costs for individual mission. In the current budget constrained environment, my feeling is that the Europa community will get at most one scaled back mission, and not a series.
My sense in having read many reports (but having no expertise, so take this with a large grain of salt) is that the minimum mission that would be scientifically compelling for the cost could focus on only the ocean and ice priorities but with some attention to the chemistry and geology goals. I speculate that a minimum mission might consist of:
- 2 months in orbit, 200 km
- Ice penetrating radar
- Wide-angle color camera
- Profiling near IR Spectrometer
- Magnetometer
- Laser altimeter
- Radio science
Even within this limited mission scope, there is considerable room for examining options. An European mission study (IAC-04-Q.2.a.02 SYSTEM CONCEPTS AND ENABLING TECHNOLOGIES FOR AN ESA LOW-COST MISSION TO JUPITER/EUROPA) estimated the mass of this instrument set at 18 kg while the equivalent JEO instruments would have been 75 kg. Eighteen kilograms seems like it might be on the low side, but there may well be room to reduce costs by lowering instrument capabilities.
The European study also showed that there may be many ways to skin the Europa orbiter cat. It recommended a truly minimalistic solar powered Europa orbiter that used a second relay craft in orbit around Jupiter to send the data back to Earth. I expect that we'll see some creativity as the science community tries to find a way to enable an Europa orbiter.
0 comments:
Post a Comment