Responses to NASA claims that solar canít work for the Cassini space probe

by Dr. Ross McCluney
Principal Research Scientist,
Florida Solar Energy Center,
1679 Clearlake Rd.,
Cocoa, FL 32922-5703.

ESA's Rosetta Mission
ESA's Rosetta mission slated to go beyond Jupiter using solar power

NASAís statement at web page on the use of non-nuclear alternatives to power the Cassini mission to planet Saturn follows, with my questions and responses in italics.

Non-Nuclear Alternatives to RTGs

JPL studies have concluded that neither fuel cells nor spacecraft batteries demonstrate the operational life needed for planetary missions, whose duration can exceed 10 years from launch. In addition, the large mass of batteries that would be needed to power a mission such as Cassini greatly exceeds current launch vehicle lift capabilities.


What about combined source options, such as small batteries and fuel cells for short-term high-electrical-demand operations, and solar for baseload and recharging of batteries? What about recent advances in reducing the voltages and power consumption for the same task with new electronic and computer circuits? What about reducing mission size so that current launch vehicles can be used? If these strategies mean a delay, thatís unfortunate for a number of scientists, and probably a number of contractors, unless the latter get paid for their work done so far, and then get new contracts for splitting Cassini into smaller, non-nuclear powered spacecraft. A delay to solarize a fleet of smaller Cassinis would be good news for the group of people on Planet Earth that is not willing to accept the risks of a nuclear accident with the current spacecraft.

JPL's rigorous analysis has also taken into account the advances in solar power technologies that have occurred over the last decade. The conclusion reached by the researchers at JPL is that solar technology is still not capable of providing sufficient and reliable electrical power for the Cassini mission. The mass of solar arrays required would make the spacecraft too heavy for available launch vehicles.


If its so rigorous, why provide only three sentences on it. Show your calculations, or at least the strategies taken in reaching this conclusion. How was the analysis done, and what were the assumptions used in preparing alternative scenarios? Did the analyses include the possible use of inflatable concentrators, stiffened by chemical processes after inflation? What about small, lightweight, flexible Fresnel lenses that can be compacted for launch and released for use with small, lightweight solar cells? What about the use of non-imaging concentrators, together with light pipes to deliver the solar radiation to small solar cells inside a protective housing, making the system less susceptible to micrometeorite or space junk impact?

Even if a sufficiently powerful launch vehicle were available for an all-solar Cassini, other limitations exist with current and near-term solar technologies, including:

  1. The behavior of solar cells at vast distances from the Sun is not well understood and would add significant risk to the success of a solar-powered mission to Saturn. Saturn is located approximately 1.42 billion kilometers (882 million miles) from the Sun, nearly twice as far from the Sun as Jupiter, the next closest planet.


    The space environment near Saturn is not that unfamiliar to NASA, so the behavior of solar cells there should not be a great unknown. It sounds like you are saying that there might be some mysterious force or some mysterious chemical around Saturn which would affect solar cell operations but would not affect any other part of the existing Cassini design. This is highly unlikely. Perhaps you are referring to the low temperature which cells without concentration would operate at near Saturn, and a reduction in operating efficiency due to the cold. If this is the case, then why not consider using solar concentrators to increase the solar flux on the cells considerably, thereby also increasing their operating temperatures and restoring operating efficiency?

  2. The size of solar arrays that would be needed, about one-quarter the area of a football field, would not only be difficult to deploy reliably, but would significantly increase the orbiter's moments of inertia, making turns and other timely maneuvers extraordinarily difficult to perform This would severely inhibit Cassini's ability to achieve its science objectives.


    If you are thinking of conventional solar cell technology, without the use of strong, lightweight concentrators, and no gimballing of the collectors with respect to the spacecraft, this might be a reasonable objection. If, however, you include the use of light weight concentrators, the same (or slightly more) area of flux can be collected, but with less mass. The concentrators would direct concentrated sunlight onto much smaller solar cells. These concentrators could be large, with only a few being used, with a correspondingly fewer solar cell arrays. Or they could be small, but numerous, with correspondingly smaller but more numerous solar cells at concentrator exit apertures. The use of a large number of small concentrating solar cell modules should increase the ease with which they could be deployed, and decrease the adverse consequences of micrometeorite impact damage to just a few of these modules. By gimballing the solar power system and displacing it from the Cassini payload, you could have the solar system always pointing toward the sun and the payload could be directed in almost any direction, independently. Of course this is would be difficult, but so is going to Saturn with such a huge payload to begin with. Why not scale back the mission to make these problems and difficulties much easier to solve?

  3. The large arrays would seriously interfere with the fields of view of many of the science experiments and navigation sensors, further limiting the Cassini mission's ability to achieve the science objectives.


    This is another good reason for separating the solar power system from the Cassini spacecraft, linking them only by flexible optical or electrical cables and support struts in a gimbaled arrangement to permit a high degree of independent motion. Surely your intelligent and very highly capable scientists and engineers can rise to this challenge, thinking in new ways, expanding the possible options, to find a way to make a smaller Saturn mission work with solar energy.

  4. Large arrays could generate serious electromagnetic and electrostatic interference, which would adversely impact the operation of the science experiments and the spacecraft's communications equipment and computers.


    Not if you used optical concentrators, which are free of electromagnetic interference, and optical light pipes to deliver solar flux to an electrically shielded and otherwise protected array of photovoltaic cells. Iíve also heard of innovative new strategies for converting the heat from concentrated solar radiation into electricity in space, efficiently. Whatís the status of these and other innovative solar options being explored by NASA scientists, engineers, and contractors? It looks like you dismiss these options too quickly, because you decided to use the nuclear option a long time ago and donít want to have to rethink that decision.

    These arguments sound more like someone trying to grab at anything, to justify a decision already made, than the exasperation of a creative genius who tried this and that and the other, and who consulted colleagues in other fields, and who was really determined to find a way to make solar work for interplanetary probes, but in the end had to admit failure based on currently existing technology, and even technology expected to be ready in a few years time.

    I have difficulty believing that this problem cannot be solved, that even a slightly reduced size Cassini mission using no nuclear power sources cannot be made to work. Surely, with alternative energy conversion systems to choose from, with low-power electronics soon to be available for use in space, with innovative new non-imaging concentrators coupled to flexible light pipes, with smaller, better, faster, cheaper system planning, and with the remarkable talent that made the Mars Pathfinder mission such a success, surely you can make a solar-powered Saturn probe work, and work well.

From "Final Supplemental Environmental Impact Statement for the Cassini Mission," Office of Space Science, NASA, Washington, DC 20546, June 1997.

On pages 2-6 through 2-9 NASA responds to comments on the previous, 1995, Environmental Impact Statement (EIS) about the use of new solar cells in an explanation of "why solar arrays, even arrays using the new ESA cells, are not feasible for the Cassini mission." It is stated that "The simplest and most immediate explanation for this is that the arrays, in order to meet Cassiniís electrical power requirements, would have to be so large that the spacecraft as a whole would be too massive to launch."


If you start with the assumption that you must launch the already completed Cassini spacecraft, and power it as it was designed to be powered by RTG power sources, then you are placing design constraints on possible solar solutions to the problem. If, however, you are willing to change some mission objectives, change the power demand profile, and postpone some of the experiments to later missions, then it should be possible to reduce the total payload, freeing up valuable mass for solar cells, and still be able to use existing launch vehicles. Just using more efficient solar cells, however, is probably not going to make it possible to power the current Cassini mission without any major changes in the payload.

The statement points out that the new, highly efficient laboratory-produced ESA solar cells were tested only for sunlight and temperature conditions approximating those near Jupiterís orbit, not at the much larger distance Saturn is from the sun. The statement admits that the new solar cells donít show as much efficiency loss at low temperatures as currently available cells, but it says that these new cells "could be less efficient at Saturn,...."

The statement shows in its Fig. 2-3 the size of arrays of the new ESA GaAs cells in relation to the Cassini spacecraft, and says that the collector area required with the new, more efficient cells is greater than 500 sq. meters (5,380 sq.ft.) A drop of only 98 sq. m. from the area needed using the older cells, according to the 1995 EIS. The statement further claims that "Attaching two such huge solar arrays to the Cassini spacecraft would severely impact the design, mass and operation of the spacecraft. One significant factor would be the array itself, which is a mechanical structure that ties the many solar cells together. This structure would have to be deployable, which means that it would have to be stowed for launch so that it could fit inside the Titan IV payload fairing, and then unfold once the spacecraft was on its way to Saturn. This in turn would require mechanical components to fold and unfold the arrays and support the long array arms when extended."

"Such components and support structures would increase the size and mass of the spacecraft considerably. The long and unwieldy solar arrays would also severely complicate spacecraft maneuvering and turning for scientific observations and data transmission back to Earth. Therefore, special devices would have to be added to enable the spacecraft to turn, again adding significantly to the mass. Finally, to properly regulate electrical power on board the spacecraft, special regulators and batteries would be required. This, too, would increase the overall mass."

"As with other solar power options studied for the Cassini spacecraft, the extremely large mass of even the lightest solar configuration is beyond the lift capability of the Titan IV (SMRU)/Centaur launch vehicle. Even if a heavy-lift booster and a suitable upper stage could be developed and certified for such a massive solar-powered spacecraft, the adjustments necessary to accommodate solar power would have substantial negative effects on the mission. First, they would make spacecraft maneuvering so slow and difficult that the mission would run out of time for scientific data collection, causing some crucial observations to be lost. Second, the addition of so many moving parts susceptible to mechanical failure would add considerably to the overall risk to mission success. As a final note, the researchers who developed the ESA solar cells evaluated the JPL solar study and concluded that ĎLow (insolation) intensity and low temperature (LILT) solar cells (including those developed by ESA) are not a viable power source alternative for the presently defined Cassini mission of NASAí...."


Ask a silly question and you get a silly answer. The "silly question" here is something like, "Why not just power the existing Cassini spacecraft with new, more efficient solar cells?" The question is silly because the current spacecraft was designed to work with RTGís, not solar cells. Of course there are going to be all sorts of problems making an already optimized spacecraft and launch vehicle work when all you try to do is patch in a radically different power source. The only way to have a viable argument against the use of solar for Cassini is to spend the time and effort of some of your wonderfully bright NASA scientists and engineers, to rethink the whole mission in the light of using solar as the primary power source. Letting your designers "get out of the box" of having to design a solar system for the current Cassini spacecraft can be wonderfully freeing in permitting innovative new design solutions.

The real problem is that NASA has invested so much money, time, and effort into a large, RTG powered mission, and that it is more than reluctant to consider the alternatives seriously. In the end it is, as are many big problems, as much a matter or philosophy and politics as a problem of technology. Forcing the writers of the two EISís to try and come up with solar power for an unaltered Cassini spacecraft, thereby preventing the creative mind from actively seeking real solutions to the problem, merely insures failure of the exercise, making it an exercise in futility from the beginning. If a more comprehensive study of solar power alternatives to the current Cassini mission had been performed by NASA, including the possibility of dropping one or more of the science missions or payloads, and the possibility of delaying the launch a few years, and the possiblity of using strong, lightweight solar concentrators, and the possibility for reducing payload power requirements through innovative new electronic systems, then NASAís arguments against solar power for a Saturn probe might be more convincing.

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