Statement on the solar power alternatives to nuclear for interplanetary space probes


Ross McCluney, Ph.D., Principal Research Scientist, Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922, 14 August 1997.


(Note: The views expressed by Dr. McCluney are his personal views and not necessarily those of the University of Central Florida or the Florida Solar Energy Center. Dr. McCluney's affiliation with the Florida Solar Energy Center and the University of Central Florida is for identification purposes only.)


I’m an optical physicist and author of a textbook on radiant energy transfer [Introduction to Radiometry and Photometry, Artech House, Boston, 1994]. I was a research scientist at NASA’s Goddard Space Flight Center from 1973 to 1976. For a couple of months I’ve been studying the possible use of solar energy as an alternative to nuclear energy power for interplanetary space probes such as the Cassini mission scheduled to launch from Cape Canaveral this October.


It is clear to me that NASA has a large variety of options available to it—and could mix them all together to make solar power work—for converting the Cassini probe to solar power, and for nearly all future interplanetary missions. Here are the available strategies:


1. Use new high-efficiency solar cells, described by many authors, which boast efficiencies as great as 20%. One doesn’t have to base this figure on the claim of one aggressive solar photovoltaic manufacturer in Europe. This is a factor of two increase over the efficiencies of older solar cells, and could reduce the area and mass requirements for solar power for future space probes by that same factor. Many research laboratories are achieving greatly improved solar cell performance, some claiming efficiencies as high as 24%. Of course it may take a year or two to get them in a durable and tested form for use in space, but what’s the rush?


2. Use new, strong, lightweight solar concentrators, taking advantage of a factor of 4 advantage (at the theoretically ideal limit) which a new class of optical devices called nonimaging concentrators have to offer over traditional imaging optical systems. For more information about these concentrators, you can read the textbook High Collection Nonimaging Optics by W.T. Welford and Roland Winston [Academic Press, 1989], or the numerous articles in the Procedings of SPIE the International Optical Engineering Society and Applied Optics, a technical journal of the Optical Society of America, which have appeared since the publication of the Welford/Winston book. Winston holds the world’s record for the concentration of solar radiation on the earth, achieving a concentration ratio of 56,000 to 1 in 1988. [P. Gleckman, Applied Optics, Vol. 27, p.4385.] Of course it will take a little time and a major increase in the tiny budget NASA is already using to explore the use of solar concentrators in space, but what’s the rush?


3. Use new low-power electronics that need far less power to perform the electronic and computer portions of the mission. For more information, see the July 1997 issue of Military and Aerospace Electronics, an article titled "Low power experiment set for space." If the power requirements are cut in half, for example, so is the size of the solar collection system to meet these requirements. Of course NASA will say that these low-power electronic systems are still in the research stage and some time will be needed to assure their readiness for space, but what’s the rush?


4. Split the mission into more, smaller, better, faster, cheaper ones. Cassini is a relic of the Cold War, and of the era of large, long-drawn-out, expensive space missions, a direct violation of NASA Administrator Golden’s much touted new dictum for "smaller, better, cheaper" missions. Yes it would probably take a lot of time to split the Cassini probe into several smaller ones, basically redesigning much of the vehicle that delivers the payloads to their destinations, but what’s the rush?


5. Put ALL these together for Cassini and other future interplanetary probes.


With these strategies to choose from, and the chance to use them all together, there is no reason to use risky nuclear generators for interplanetary probes. To take that risk in view of the above alternatives is unconscionable, especially in view of the fact that, according to Michio Kaku, the Cassini probe contains 400,000 curies of plutonium 238, a factor of 30,000 times the 13 curies authorities claimed were released from the Three Mile Island accident which created billions of dollars in lawsuits.




Responses to a Supporter of RTG’s for Cassini


George William Herbert, on his web site, offers a reasoned explanation of why the solar power option to RTG’s for the Cassini spacecraft to Saturn won’t work. Here is my response. – Ross McCluney


G. Herbert says:

Could Cassini use Solar Power instead of RTGs?


Many have asked if RTGs are necessary for the Cassini mission. Solar power, after all, is used for most space probes and satellites. Why can't it be used for Cassini?


Many of the opponents of Cassini have circulated reports that new European Space Agency solar cells could allow Cassini to operate on Solar Power at Saturn.


The Inverse-Square Law and Sunlight at Saturn


How strong light is at various distances from a light source is governed by a physical law called the Inverse Square law. To find out what the amount of light power is relative to another distance, you square the ratio of the two distances and then take the inverse of it, or the ratio 1 over that squared number (1/r^2). [Drawing showing increasing areas for solar flux at increasing distances from the source, an illustration of the well-known "inverse-square law" from high school physics.]


If the amount of light falling on one square foot one foot away from a lightbulb is 1 unit of light, then the amount of light falling on that one square foot area if it were two feet away is 0.25 unit (1/(2^2) = 1/4 ), 3 feet away is about 0.11 unit ( 1/(3^2) = 1/9 ), etc.


[Distance Diagram from the SEDS Nine Planets page at]


Saturn is on average 9.54 times as far from the Sun as Earth is. The Inverse-Square law tells us that the intensity of sunlight falling on Saturn is thus 1/(9.54^2) or about 1/91 or 0.011 of the light falling on Earth. The standard value for the energy in sunlight is 1350 watts per square meter at Earth's distance from the sun. As far away as Saturn, you see about 15 (14.83) watts per square meter.


How far can 15 watts per square meter get you?


Nasa is officially somewhat nervous about whether Solar Cells would work at that distance (see section "Non-Nuclear Alternatives to RTGs").


Generally, solar cells work more or less as well under low light as they do under more intense light.


McCluney response:

The biggest problem, according to Sheila Bailey of NASA/LeRC's Photovoltaics Branch is the low temperature they would operate at at that distance. But of course, if a concentrator is used, it will increase the flux on the cells and heat them up, to acceptable temperature levels where their performances will be less degraded.


G. Herbert:

Though we have never flown one out as far as Saturn, it is not unreasonable to assume that they can be made to work at that distance from the sun, if properly tested. You definitely need more of them, about 90 times more since you have about 1% of the light.



True in principle, but it ignores the use of strong, lightweight concentrators, which could reduce the mass requirements by factors of 4 or more, even if the mass of the concentrators is factored in. It also ignores further reductions by using low-power electronics, and reduced mission sizes.


G. Herbert:

If you don't change the power requirements for Cassini, it needs about 600 square meters of solar cell array at Saturn. One (apparently NASA diagram, with someone else's sketch on it for a "more efficient" circular array) design is shown below:.... [Drawing of a large solar array connected to the Cassini spacecraft, like the one shown in the NASA EIS for the Cassini mission.]



Yes, if you don't use concentrators you have to collect all that sunlight directly on the cells and there has to be a large area (and mass) of them. On the other hand, you can intercept that same 600 square meters with a strong lightweight concentrator (or an array of smaller ones) and focus the radiation down to a more reasonable area of actual PV cells, say 100 square meters. If you reduce electronic power requirements by 2 you are now down to 50 sq. m. If you break the mission in half you are down to only 25 sq. m.


G. Herbert: [Referring to]

There are several problems with any of these array concepts:


• Weight

• Volume

• Spacecraft redesign for

m Balance

m New Rotation / Attitude Control

m Operations restrictions: Solar panels always must point at sun

• Some instruments are blocked no matter where the panels go



Of course. And since all of these have presumably been "solved" for Cassini, with a huge expenditure of time, energy, and money, over many years, who wants to stop it now and go back to the drawing board and almost start over? I think this is one of the real reasons for the resistance to canceling the Cassini launch. And it is understandable. A lot of very smart planetary scientists and astronomers have invested a huge amount of time and effort, and considerable reputations, in the Cassini mission, for a lot of years, and they counted on NASA to provide them with a stable and durable power supply. They got this too, barring a terrible mistake or disaster. In a way these scientists were betrayed, because NASA put its eggs into the nuclear/military basket, exposing those scientists to a possible cancellation for reasons seemingly beyond their control. I think they should be spending their time and energy now complaining to NASA rather than trying to out-rationalize the anti-Cassini protesters.


I've said before that if all the scientists with payloads on the Cassini probe would sign a statement against ANY further use of nuclear materials in space, beyond a few short-lived microcuries for diagnostic and scientific purposes, such as was used on the Mars Pathfinder, then I would recommend a halt to the protest against the Cassini mission. The real danger, in my view, is not so much Cassini by itself, but in what's to follow. Without a solid commitment for no more nukes in space, however, we have to draw the line right here and right now.


G. Herbert:

All of these are big problems. The most critical problem is just weight. This is, roughly, the same sized solar arrays as are needed by the International Space Station Alpha which is under construction and about to start being launched. An optimistic estimate of how much these solar arrays would weigh is about 2500 kilograms (5,500 lbs). Right now, the whole Cassini spacecraft weighs 5630 kilograms (over 12,000 lbs) of which 3130 kilograms (about 6,900 lbs) is fuel so it can stop at Saturn and not keep going past it. That means the spacecraft now is massing 2500 kilograms by itself (5,500 lbs), without the fuel. Looking at the vehicle design, you replace about 165 kilos (350 lbs) of RTGs with 15 times that mass of solar panels, in other words nearly doubling the total dry (without fuel) weight of the spacecraft. The resulting spacecraft with all of Cassini's instruments would weigh a total of around 5000 kilograms (11,000 lbs) without any fuel, would require over 6000 kilograms (13,000 lbs) of fuel to stop at Saturn, and would thus mass over 11,000 kilograms (24,000 lbs) fully loaded, over twelve tons.



It’s interesting that the writer chose a comparison with the space station. This is a good example of going with big, old, expensive projects. According to the NASA people I spoke to, the solar cells for that station were purchased some time ago and are sitting around waiting to be sent up. In the meantime they are becoming obsolete, with less than the best available efficiency. If NASA or the contractor had held off on that purchase until the launch was more imminent, I believe that they could have purchased much better cells, lowering their mass, thereby freeing up mass for valuable experimental space on the station, or lessening the fuel requirements, or for some other benefit. This is a good example of taking too narrow a view. The author seeks to show that a simple replacement of the Cassini RTG’s with standard solar arrays creates an enormous number of design problems. Of course. What he fails to do is step back a bit and look at the bigger picture, examining all the options.

G. Herbert:

There is no rocket launcher on earth which could launch twelve tons on a transfer orbit towards Saturn (or the Venus-Venus-Earth-Saturn flyby series planned for Cassini). Cassini cannot be redesigned to use solar cells, no matter how much some of the opponents would like to.



Well, who said we had to send 12 tons on a transfer orbit toward Saturn? I've indicated several ways to reduce the mass to a more reasonable level. Of course, for Cassini, these would require a very substantial delay in the launch. My response is, so what's the big rush? Saturn has been around for a very long time and will presumably stay there a while. Why do we have to go there in 1997? Why violate Dan Goldin's dictum of "smaller, better, faster, cheaper?" Better to redesign the whole exploration concept around this principle, taking the hits in short-term losses, but getting a much better, and more sustainable, vision for NASA as a truly civilian agency, one looking after the needs of all humans, not just the few space scientists who hitched their careers to a failing concept.


G. Herbert:

What about those ESA Solar Cell reports?


Karl Grossman writes in Despite the risks, push for nuclear technology in space steps up,


The use of plutonium on the $3.4 billion Cassini mission and the horrific danger it represents is unnecessary. The European Space Agency (ESA) announced in 1994 what it called a "technological milestone" the development of high-efficieny solar cells to generate the modest amount of electricity needed for deep space probes like Cassini. An ESA physicist told the newspaper Florida Today in 1995 that if given the money to do the work ESA "within five years could have solar cells ready to power a space mission to Saturn."


This statement is factually correct (those statements were all made by the persons involved) but gives a highly misleading impression overall, and leaves out several important developments. The ESA physicist was corrected by his own ESA engineers after analysis of the whole power and mass problem: it won't work. The ESA new solar cells improved the conversion efficiency (light power to electrical power) of cells from around 20% to around 25%, a significant but not immense jump. Their new cells were not much lighter per watt of power produced, however, which means that the mass of a spacecraft wouldn't be much improved by the new cells. While future R&D might improve both the efficiency (30% or more is hoped for) and weight of these new solar cells, there is nothing on earth, in ESA labs or drawing boards or anywhere else, good enough to replace the RTGs on Cassini on a pound-for-pound basis.



I don’t have a problem with this. We’ve learned to be a bit doubtful of claims of PV manufacturers for new cells which haven't been produced in volume yet, or in this case fully space rated yet, but we don't have to listen to one manufacturer to see that the trend is for more efficient cells, and 20% efficiency coupled with the other strategies I've outlined can bring the Cassini power system mass down considerably.


Of course the RTG's are better pound for pound if you ignore radiation safety problems. They are a really marvelous engineering solution. Baseload power for years and years at very little degradation, constant, and fairly impervious to the odd micrometeorite or two, and with no moving parts to wear out and break down, and no need to steer the concentrating collector array toward the much smaller solar disk as seen from Saturn. The use of concentrating optical systems and high efficiency solar cells for long-term interplanetary missions won’t be easy. However, the nuclear risks of large RTG’s just aren't acceptable, never have been, and NASA is deluding itself, or being deluded by DOE and/or the Air Force and various industrial interests into thinking otherwise. The problem lies in the unwillingness of the scientists, the companies, the Air Force, and NASA brass to cut their losses and rethink the role of nuclear in space, cancelling Cassini now while they think this out.


G. Herbert:

Predicting 5 years worth of R&D is not possible; no theoretical improvements of the magnitude needed are understood, so there is no reason to assume that they can be made. Perhaps they could be, but you cannot plan missions around nonexistent technology.



Right, so you don't do those missions until the research produces demonstrably acceptable alternatives to nuclear. There's another point that must be considered. The budget for solar power systems in space is miniscule by almost any measure. Thus we have a "self-fulfilling-prophecy" kind of thing, where the desired technology is not available because the research budgets were never sufficient to make it available. Actually I think the desired technology either IS available or is close to being available, if the basic scope and plan of the mission were altered to meet with the new paradigm, the one followed by the wonderfully successful Mars Pathfinder Mission.


G. Herbert:

Similarly, in her Cassini Fact Sheet, Fredrica Russell states:


Can't the Cassini mission be performed safely-without plutonium? Sure, says Dr. Kaku and others. The plutonium, placed in three radioisotope thermal gererators (RTG's), is only to be used as a power source to produce electricity for instruments on the probe, not for propulsion. They say that if NASA had the will, the needed electricity could be obtained through solar energy gathered by photovoltaic panels on Cassini and, when the sun is too distant, from long-lived fuel cells.


Unfortunately, powering Cassini for years using fuel cells would require more fuel for the fuel cells than Cassini can possibly carry to Saturn. The best hydrogen/oxygen fuel cells, like those on the Shuttle, produce about 1000 watt-hours per pound of fuel. Cassini's mission is intended to last 4 years at Saturn, about 1500 days or 35,000 hours. With a power usage of about 650 watts, that works out to a total power usage of about 23 million watt-hours. That would require about 23 thousand pounds of hydrogen and oxygen for the fuel cells, more than 10,000 kilograms, which would make a fuel-cell powered Cassini larger than a solar-cell powered Cassini (see above), and even more impossible to send on an orbit off to Saturn.



I don't think fuel cells are the answer for very long duration, deep space missions, because they are based on the use of consumables, the mass of which has to be increased as the mission duration increases. If the probes are split into many smaller, better, cheaper, faster ones, then possibly fuel cells could be used in conjunction with solar arrays to provide an adequate mix. Has anyone in NASA looked at various combinations of hybrid power systems? Solar for 2 to 6 astronomical units (AU's) from the sun, fuel cells plus solar for 6 to 8, and just don't go any farther than about 9 AU's from the sun. Call that inaccessible with current technology and just don't go there. Now that's a space policy I think the people of the earth can live with, literally.


G. Herbert:

Could another Saturn probe be designed to use Solar Cells?


Theoretically, yes, it is possible to design space probes for Saturn missions that are mostly solar cell, about Cassini's size, and have much less scientific return per pound because of the less mass-efficient solar cells. This would not be a redesign of Cassini, this would have to be an all-new design, throwing out all the $1.5 billion spent on Cassini so far. There are strong disadvantages to doing so, however. Much less of the probe will be used for science instruments, cameras, etc. Even worse, there is a minimum size which such probes can be built to, because computers and radios that are good enough for the job can't be miniaturized beyond a certain point. That size, plus the solar cells needed, still needs some of the biggest and most expensive rockets (like the Titan 4B) to get to Saturn. Titan 4B's cost around $350 million dollars each, so the solar Saturn missions will end up costing $700 million or more all told, with perhaps a tenth of the scientific instruments on each one that Cassini has. It costs much, much more to do the same work at Saturn using Solar Cells.



This but another example of the narrow thinking that got NASA into this mess in the first place. Start with unreasonable assumptions (doing the Cassini mission or a version of it as currently planned) and you get unreasonable conclusions. We need some really new, really creative thinking here, not the same tired old arguments trying to justify a wrong decision made years ago and kept alive by more wrong decisions each year since.



A rebuttal from George William Herbert, August 15th, 1997

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