Europa Clipper Update

Planetary geologist Philip Horzempa returns with a new post giving an update on the proposed Europa Clipper mission.  

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President Obama’s recently released FY2014 budget proposal, unfortunately, contains no funding for a mission to Europa.  In fact, the budget document states that NASA not only is not funding such a mission, but that it cannot fund it.  Several sources of budget constraints appear to be stalling any new start.  In addition to the sequester, NASA’s Science Mission Directorate has the ongoing money drain of the Jams Webb Space Telescope (JWST).  That funding burden will not lessen until about 2017 – 2018.  One could imagine that NASA may see a funding wedge appear at about that time, with a new start for a Europa mission possible in FY 2015 or 2016.  The early years of a space project require minimal funding, allowing a program to begin Phase A and Phase B (design and definition) a few years before the fiscal “heavy-lifting” of Phases C and D (detailed design, construction and testing) .In the meantime, the Europa team has continued to refine the design of what they refer to as the Europa Clipper (see Van’s post of September 24, 2012).  Whenever they are given the “go-ahead” from the White House, they will have a mission ready to proceed to implementation. 

The Europa science community believe they have developed a cost-effective, yet scientifically compelling, mission to the ice-covered Galilean satellite.  After considering an orbiter, the consensus is that a multi-flyby spacecraft would return more science for the same cost ceiling.  The Europa Clipper embodies the modified FBC (faster, better, cheaper) approach.  It is seeking to capture as much of the Jupiter Europa Orbiter (JEO) flagship science as possible using a smart, elegant, lower-cost design.  This past January, the Europa team presented the results of their latest “scrub” of the Clipper mission.  This Europa Clipper design refinement can be seen here.

The plan is to launch in 2021, followed by one Venus and two Earth gravity assists.  Six years after launch, with the gravity assist of a Ganymede flyby, the Clipper will enter orbit around Jupiter.  Over the next 2.5 years, it will perform 32 flybys during its prime mission, with closest approach altitudes of 25 - 100 kilometers (actually 34 total flybys will occur, but only 32 are optimal for science).  In order to reduce planning costs, the timeline of each flyby will be essentially identical. (Figure 1) However, the trajectory of each flyby will bring it over a different sector of Europa.  This will provide global medium-resolution coverage from the Topographic Imager. 

Figure 1.  Flyby timeline.  Click on image for a larger version.

It was felt that the Europa Clipper mission should also provide data that would feed-forward to a future soft lander.  This concept of reconnaissance has seen a rebirth at NASA, with ongoing orbital missions at the Moon and Mars.  The addition of a Reconnaissance Camera was deemed to be essential for providing images for landing site surveys ( lander-scale characterization of the surface is needed).  The Recon camera (a push-broom design) will produce 20 x n km images at resolutions of as fine as 0.5 meters.  The limitation on the number of such high-resolution images comes from the large amount of data in each photo.  In turn, the swath length will be determined by the amount of down-link time available.  The Recon camera will utilize an innovative flip-mirror to enable stereo imaging of a scene in a single pass.  It will be able to obtain views 15 degrees from nadir (Figure 2)  It is believed that about 15 candidate landing sites will need to be surveyed in order to be able to down-select to 2, a primary and a backup.  That selection will be done by some future team of Europa Lander scientists and engineers.

Figure 2. High resolution camera flip mirror to allow stereo imaging.

A separate, smaller, and gimbaled gravity science antenna will allow the collection of gravity data during flybys. (Figure 3).  Because the cameras and other remote sensing instruments are mounted to the spacecraft body, the main antenna cannot be pointed to Earth during flybys to allow tracking for gravity measurements.  The separate antenna will be kept pointed at Earth during flybys to permit the important gravity measurements that will reveal much of the internal structure of Europa.

Figure 3. Gravity science antenna.

During this latest iteration, the Europa team was allowed to raise the cost cap from $1.7 billion to a total of $2.0 billion.  (This is still less than half the estimated cost of the previously proposed Jupiter Europa Orbiter.)  This increase allowed the addition of a Magnetometer and Langmuir Probes to the payload suite.  Rounding out the instrument complement are an Ice-Penetrating Radar, a Thermal Imager, a Neutral-Mass Spectrometer and a Short-Wave Infra-Red Spectrometer.  Figure 4 shows some of the payload complement and where they will sit on the spacecraft. 

Figure 4.  Europa Clipper instruments.

The highly-capable instrument suite is one reason that the Europa Clipper would cost more than missions such as JUICE or the proposed Io Observer.  The scope and resilience of the Clipper mission means that it must survive an intense radiation exposure over its 2.5-year mission.  This data-intensive mission must also use a reliable, high-energy power source. 

The Europa Clipper spacecraft benefits from the heritage of the Galileo and Juno Jupiter Orbiters in its approach to radiation protection.The Clipper will utilize 150 kg. of dedicated radiation shielding which is one-half of that planned for the earlier JEO (Jupiter Europa Orbiter)  proposal.  The Clipper will use a scheme of nested radiation protection for its electronics (Figure 5).

Figure 5. Nested radiation protection for the spacecraft's electronics.

For example, the Spacecraft structure and propulsion system will provide a measure of radiation protection, essentially for free.  With intelligent placement, the project will utilize much less expensive 100 and 300 kilo-rad hard parts.  Individual payload electronics have their own shielding, while the use of a central electronics vault is also part of the protection plan.  As a result of this approach, the Clipper team will not need to fund an expensive development effort to build mega-rad hard avionics.

The Europa Clipper mission will be data-intensive.  In order to downlink this data efficiently and cheaply, the Clipper will use mass-memory-storage. The spacecraft will leisurely downlink the data from each close encounter with Europa during the two weeks between flybys.  This will avoid the more costly, and power-hungry, approach of near-real-time broadcast during flyby.

Over the course of its prime mission, the Clipper will return a Terabit of data, including high-resolution images, radar soundings, magnetic field measurements, compostion spectra, and gravity science.  In order to return all of this date, a robust energy source is required.  There are three energy supply options, two of which are thermal-electric and one solar.

Solar panels would be the lowest cost, highest mass option.  However, they pose the risk of not providing enough power over the lifetime of the mission.  The Europa Clipper's orbit has a low inclination causing it to pass through the most intense radiation environment in the solar system.This would cause aggressive degradation of solar cells, such that their power output would be increasingly compromised as the mission progressed.  The Juno orbiter is able to use solar power because its high-inclination polar trajectory enables it to avoid most of the high radiation zones that are concentrated over Jupiter's equator.  This is true even though it flies much closer to the gas giant than the Clipper ever will.  ESA’s JUICE spacecraft is able to use solar energy mainly because it only flies near Europa twice during its mission. 

The proposed Io Observer would also use solar panels.  It avoids high doses of radiation by orbiting Jupiter in an inclined orbit.  Europa Clipper is unable to utilize such a high-inclination orbit because that would result in flyby velocities too great to allow its Infra-Red and Ice-Penetrating Radar to gather useful data. 

This leaves the two thermal-electric options.  These power systems utilize the heat generated by the decay of Plutonium-238 to drive thermal-electric power conversion units.  One of these, the Advanced Stirling Radioisotope Generator (ASRG) design is actually still in development, although at a high level of maturity.  NASA chose not to pursue a Discovery mission that would have utilized one of these units.  In light of that decision, the agency will still take the two ASRG development units to flight status this year.  They will then be placed in storage, awaiting a mission.  If this power source is chosen, then the Clipper would utilize four ASRG units. 

However, before the ASRG design would be approved for the Europa Clipper,  more work would need to be done.  The radiation hardness of the Generation-1 ASRG units is not sufficient for the Europa mission and there are also lifetime demonstration issues.

The other thermal-electric option is the Multi-Mission Radioisotope Thermal-electric Generator design (MMRTG). This system is the 1^st new radioisotope power system developed in over 20 years.It has advanced to actual flight status, with the first MMRTG flight unit, F-1, now sitting on the surface of Mars, powering the MSL rover.  Its backup, F-2, is in bonded storage at the Rocketdyne plant in Canoga Park.  It has been operated and has shown good performance.  It is now slated to fly onboard the 2020 Mars Rover, i.e., MSL-2. 

The next unit, F-3, is the flight spare for Mars 2020.It is now under construction, with completion set for this month.  If not needed for the Mars 2020 rover, then F-3 would be available for a mission to Europa.In addition to F-3, three more MMRTG units would be needed for the Clipper.  There are plans for infusing new technologies in the next generation of MMRTG.These would produce 150, or even 180 watts, as compared to 120 watts for the 1^st generation. 

There are a number of issues that need to be considered if one of the thermal-electric options is chosen.  ASRG development seems to have begun during the short-lived Prometheus program.  An engineering unit at NASA Glenn has accumulated over 10,000 hours (14 months) of operation so far.  The maturity level for the ASRG units is high, but they are more expensive than an MMRTG and have yet to fly in space.  On the other hand, their power conversion efficiency of 30% means that they are more frugal than MMRTG units (9% efficiency) with the Plutonium supply. 

The MMRTG design has several advantages over the ASRG.  First, as noted, an MMRTG is now in space.  The design has high reliability and low cost.  In addition, the ASRG utilizes kinetic energy as one stage in it power conversion.  It is still to be determined whether the resulting vibrations would make it incompatible with a Europa mission.  If so, then the vibration-free MMRTG would be at an advantage.  In addition, the re-start of Plutonium production in the U.S. may make the use of an MMRTG for Europa more plausible.  One factor that had favored the use of ASRG units for space missions was the shrinking inventory of Pu-238 in this country.  However, if the goal of producing 1.5 – 2.0 kg of Pu-238 per year is met, then that concern will be eased.America now has about 10 kg of older, aging Pu-238.  The new Pu-238 can be blended with the old material producing the desired power density.

Over the next 18 months the Europa project team will be conducting a comprehensive trade study, comparing all viable energy options.  The variables to be considered include cost, risk, robustness, design compatibility, and implementation feasibility.  This effort will go a long way towards choosing the most appropriate system for the Clipper. 

The Clipper team is very interested in the idea of hosting several nanosats that would be deployed in the vicinity of Europa.  This is contingent upon the use of the Space Launch System (SLS) heavy-lifter.  Only that rocket would provide the needed mass margin required if the Clipper is to carry small satellite payload elements.  However, if pursued, the working concept for the Clipper could provide the necessary housekeeping, deployment and radio-relay capabilities.  In addition, thought is being given to utilizing an intermediate orbit insertion module that would allow several nanosats to enter orbit around Europa. 

If these nanosats can be accommodated, then the Europa team would like to cooperate with the growing American small-sat community.  There is a desire to get feedback from engineers and scientists on the best way to use these probes.  There are a variety of options that could use a single smallsat, or a network, with instruments such as magnetometers or cameras.  These probes could be orbiters, “Ranger-style” crash landers, or even hard landers that might operate for a short time after impact.  Resource and cost constraints will be tight, but if these mini-probes  could fit, then the Europa team is interested. 

Still to be decided this year is how, or if, a total of $75 million of new funding is to be spent.  In this year’s budget, Congress specifically earmarked that sum for development of a Europa mission.  There have been rumors that NASA’s operating plan for this year’s budget, due to be delivered to Congress soon, will seek to spend that money on other agency projects.  In response to such concerns, Senators Diane Feinstein and Barbara Boxer joined with  Congressmen Adam Schiff and John Culberson in sending a letter to NASA.  They point out to NASA that funding levels for its science programs “will remain consistent with the structure directed by Congress.”  Essentially, they are reminding the agency that the Constitution gives the power to say how the nation’s money is spent to the Congress.  The Executive branch has limited leeway in how it interprets Congress’ appropriations legislation.

How this will turn out is difficult to gauge.  This is not the first time such a struggle has occurred.  For years, the Congress earmarked funds for development of a Solar Probe mission.  Eventually, NASA got the message and awarded a new start for the Solar Probe Plus spacecraft.  About 10 years ago, when NASA was trying to eliminate funding for the New Horizons Pluto probe, Congress specifically earmarked funding for that mission, enabling it to proceed.More recently, after the Obama Administration canceled the Ares 5 heavy lift rocket in its FY 2011 budget proposal, the Congress (especially the Senate) was not pleased.  They directed NASA to pursue an alternate heavy lifter, the SLS (Space Launch System), which is essentially a scaled-back version of the Ares 5.That launcher is now on track for its first mission in 2017.

If NASA does agree to spend the $75 million (more like $70 million after sequestration) this year for Europa mission preparation, there are several ways that the money could be usefully spent.  Instrument development, launch vehicle requirements and power system options could be funded, as well as studies to define the loads on the Clipper during launch.  Much will also depend on whether Congress again earmarks funds for a Europa mission in the new FY 2014 budget.  If it does, then the tug-of-war with the Administration will continue with the future of Europa exploration hanging in the balance. 

Editorial Note from Van: If you are an American citizen and you would like to see NASA continue work on the Europa Clipper, remember to let your Congressional representatives know.  Visit the Planetary Society's website for instructions on how to do so.  You can also follow the latest information on the budget on Twitter at #fundPlanetary.