Solar Cassini debate:

McCluney Response to George William Herbert, 28 August 1997, shown in bold and italics.

Response to Dr Ross McCluney's Statement on the solar power alternatives to nuclear for interplanetary space probes

George William Herbert, August 15, 1997

Largely responding to my page Could Cassini use Solar Power instead of RTGs?, Dr Ross McCluney wrote the following page. For reference, Dr McCluney's original statement can be found at: Statement on the solar power alternatives to nuclear for interplanetary space probes This page includes his entire commentary on my site and my responses to that commentary. I welcome further discussion at this site and the Stop Cassini site Dr McCluney's statement was posted at.

My response and commentary is unindented and seperated with lines thusly
McCluney's statement is indented thusly

McCluney: 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:

George Herbert response:

While positive steps towards more efficient future space exploration missions, Dr McCluney's proposals do not offer a quick easy solution to fix Cassini and do not address all of the issues involved in spacecraft design.

McCluney writes:

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.

George Herbert response:

Concentrated solar arrays probably will work, and probably are somewhat lighter than conventional solar arrays. We don't know that for sure, though, they have never been flown in space and actually tested on real spacecraft before. The first flight of a test CSA is the Deep Space One mission which is upcoming in 1998.

Sounds interesting. Can you say more about this mission, what kind of concentrator will be tested, and what it hopes to accomplish?

In any case, there are some basic physical laws involved in the design of solar arrays, be they concentrated or not, that limit what advantages concentrated arrays give you. A CSA does not reduce the total area of collector you need. It just splits the collector into two parts:

You still need the 600 m^2 collector on Cassini to bring the same amount of power into the array if you use a concentrating array, which still brings the problems listed below...

Of course. The key is whether you can keep the mass per unit collection area much lower with concentrators than with solar cells alone. As thin film solar cells, with their greatly reduced mass, and ability to operate on flexible substrates, come into use, the competition with concentrators will intensify. The result will be much lower ratios of mass to power output, using either concentrators or thin film cells, or both. I just don't think NASA has invested enough time, money, effort, and intellectual brain power into the solar option for these complaints to be very persuasive.

G. Herbert: Referring to

There are several problems with any of these array concepts:

McCluney: 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.

George Herbert response:

Here we see a gap in worldview. The assumption Dr McCluney makes here is that the scientists involved see Cassini as a risk, and are so tied up in it that they can't see past their own good. No, I don't make this assumption. I assume that they turned the problem of getting them the power they need over to NASA, and that NASA chose a politically untenable option. If they have any complaints about a threatened cancellation, they should address these to NASA, not to the Cassini protestors.

In my opinion and experience, those involved in Cassini and spaceflight in general feel that the risks from RTGs are very low, not high. As stated in Dangers during launch, Dangers during flyby, and The Danger from Plutonium there is general consensus, backed by what we feel are solid engineering and risk assessments, that the risks from Cassini and space use of RTGs in general are minor and quite tolerable. Most of the really involved people will be at the Cape when Cassini lifts off to watch and would be in prime danger if there were a liftoff accident that caused plutonium contamination. The fact that just about anyone who can get the time and pass to be there will be should indicate that we feel that it's not a risk.

This has never been in contention. Just because mission scientists accept NASA claims of safety does not mean that the mission is safe. They are not experts in probablistic risk assessment, nor in the proper application of that discipline to the Cassini project, if that is even possible. The risks here, as in so many nuclear accident cases, are more from the accident scenarios that have not been considered, and have not been put into the risk assessments, than from the ones that have. I mention the multiple event scenarios described by Dr. Kaku, and other possible human failures we haven't thought of yet. This situation is not unlike the old debate about cost/benefit analysis applied to environmental damaging actions. Often it is difficult if not impossible to put dollar values on all the costs of environmental damage, since the damage is diffuse, spread over space and time, and not easy to attribute to the action which caused the damage in the first place. If you can't quantify it, it doesn't get considered in these numerical risk assessment studies. This is why I don't trust NASA's disaster probability figures, especially when those figures seem to have a way of shifting around a lot over time.

I believe that the public has a right to be what you might call a little bit irrational, or better, non-trusting of government probability figures. As a scientist, I have a real problem with the declining scientific literacy amongst the general population, especially at a time when they are putting their lives more and more into a technology they do not understand. So I would be sympathetic to someone discounting the relatively uninformed opinions of many of the anti-nuke protesters, and I find them often exasperating myself, often doing more harm to the cause than good, making it plausible for their opponents to call them wild-eyed fanatics or worse. On the other hand, not matter how scientifically illiterate the public is, it has a right to make its own decisions on matters of safety, regardless how irrational their concerns may be. This is admittedly a difficult position to take, since it brings up all the other areas where the public is at risk, where they don't have demonstrations and protests about those risks, mainly because the risk is so ever-present in their lives, and where they seem to be perfectly happy letting the government take charge. Examples include bridge, train, and airplane safety. I could mention the automobile, but they are so dangerous that none of us should be driving them. This kind of makes my point. The public is irrational, but they get to vote, if they want to, and whatever votes they cast, count, at least in most of the U.S. system. It is the responsibility of the scientist and engineer, however, to do the very best job he or she can to determine risks and benefits, using the highest level of thinking possible, and considering every conceivable error scenario, using realistic numbers to denote probabilities, and then to report the results of this honestly to the public. The scientists amongst us in opposition to nukes in space don't believe a good enough job has been done by the government scientists and engineers, so we advise the public not to trust those numbers. Of course a certain amount of emotionality about the horrible consequences of nuclear accidents comes in, and perhaps makes some protesters unreasonably fearful. My counter is that nuclear accidents can be horrible, including the mild little ones which poison you but the disease is not detectable and doesn't present itself until a few years later, making this an area of great fear and uncertainty. I think this is a rational response to the threats posed by the Cassini launch and the other nuclear launches to follow.

If we felt otherwise, it would be a moral failure to allow Cassini to fly despite the risks. But we believe, and I argue in my web pages and NASA and others have argued, that it's safe. If we are correct, the opponents of Cassini are proposing to throw around two billion taxpaper dollars out of NASA's small and decreasing budget out, as well as throw out ten years of work and planning by thousands of people, over nothing. Of course you can't get scientists and engineers to agree with doing that. They have to be convinced that it's a risk first, and so far the opponents have failed to convince just about anyone in the spaceflight community that the risk is serious.

"You can't get scientists and engineers" with vested interests in the program "to agree with doing that." I don't think these taxpayer dollars would be thrown out for nothing. One could argue that this was a small price to pay for NASA to learn the error of its ways, to learn of its failure of vision, to learn, again, of the dangers of thinking too narrowly and not abandoning a failing mission before it was too late, like the o-ring incident on the Challenger.

This carries on into future missions as well. While the expense and bother involved in using RTGs and other nuclear payloads is large, there are some missions that are simply not practical to do without it, and there is general interest in the planetary science community and the public in doing them. It would be irresponsible to not consider RTGs for future flights without solid evidence of serious risks. Such evidence is sorely lacking.

I don't think there's that much interest in the general public in this mission, if it carries even a small risk of a major nuclear release. The burden of proof is in showing the mission is safe and this is where I see the evidence lacking. The burden of proof should not lie outside the agency, amongst people who even have trouble getting correct information on which to base judgements of safety, much less make the best decision possible with limited information.

That said, there is no reason to use radioactives in space when not appropriate. There is no reason for military use of reactors or RTGs in space. RTGs should not be used for any missions except those to the outer planets (possibly Jupiter, definitely Saturn, Uranus, Neptune, or Pluto). There is no need to do so and the engineering, as well as cost and social strife issues, just doesn't justify them for other uses.

I'm glad you mentioned future missions, because for me this is the real issue here. As I've already said, I'm not that concerned with the Cassini probe, if I thought it would be the last one ever to carry significant quantities of nuclear materials on board. I really do fear the future scenarios that I see mentioned seriously, of large nuclear power plants on Mars so we can mine it of its riches, and of nuclear power plants in space, to power weapons of a variety of kinds. All of these nuclear materials have to be launched through the earth's atmosphere, and all have a risk of depositing unacceptable quantities of deadly radioactive materials into that atmosphere. If there are some missions that are not practical to do without nuclear materials then we shouldn't do them. The line against the nuclearization of space has to be drawn somewhere, and until I see significant movement by our government, at the highest levels, to reverse the nuclearization of space, I'll draw the line at the Cassini probe, and all which come after it.

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.

George Herbert response:

The origional space station was supposed to have been in operation for several years by this time. Is it suprising that some parts were purchased early and have sat around unused, since the project was delayed so long?


The time scales of spaceflight and mission development require that you freeze a design to allow it to be properly analyzed before it is flown, and then built. The years those analysies and construction take always mean spacecraft are obsolete at the time they are launched. In some cases, they are terribly late and terribly obsolete, but predicting which that will happen to ahead of time is impossible.

I don't agree. I think that NASA planners have a pretty good idea of the rate at which technology develops. I think they can plan future missions to use the technology of the future, not of the present. Here is an area for creative thinking, higher level thinking, and innovative problem solving, historically very strong points for NASA.

Given magic solar cells in 1984 and an inkling of the delays and problems the Shuttle would face and how far we might come developing smaller spacecraft, perhaps Cassini could have been rethought from scratch and redone in that manner. We didn't have magic solar cells then (and still don't), and are still struggling to make smaller space probes work right and deliver good data. The bigger picture now is that we have Cassini all wrapped up and ready to go, the way it is, and NASA and the space industry feel it's safe. Alternate histories aside, to not launch Cassini now is a two or more billion dollar toss into the trashcan, and there is no good reason to do so.

Of course this is the crux of our disagreement. I understand that the steamroller is moving, and unlikely to be stopped or be redirected, but that does not mean that it is right to keep it going. This is NOT the bigger picture. The bigger picture is that NASA has lost its way, lost a sense of mission and direction, and does not seem able to get it back. We need a higher level of thinking at the top in this administration, and amongst the policymakers on the hill, in the White House, and over at NASA headquarters. If NASA cannot see the folly of launching nuclear materials in significant quantities into space, then it truly has lost its way and should undergo a major upheaval, shift of leadership, or more general restructuring. If it sees this folly, then announce it to the public and put a ban on future nuclear missions, and then we can call of the Cassini protest and cross our fingers as it launches and returns for the fly-by.

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.

George Herbert response: Cassini is not a violation of "faster, better, cheaper". Cassini predates Dan Goldin's term as NASA director and, along with quite a number of earlier missions, was designed under different circumstances.

A mere technicality. I concede that it get's to be grandfathered in, excused for not obeying the current dictum of the NASA administrator. That does not mean we have an undying responsibility to continue supporting this dinosaur when even a tiny accident could cause a major loss of confidence in NASA, or even the demise of the agency if any nuclear material gets loose. The mayor of Cape Canaveral has pointed out the following. What if there is a launch accident and the rocket goes into the sea off Brevard County's coast? What if the RTG's remain intact? Until NASA recovers the debris and absolutely guarantees that no nuclear material was released, would you swim in those waters? Would you eat the fish caught along this coast, or anywhere along the Florida coast, given the vagaries of the Florida Current and the spin-off eddies which often produce southward counter-flows? Would anyone else? And if not, then what is the impact to the sizeable seafood industry, the tourist industry, and the boating industry in this area, and all up and down the Atlantic coast, all the way to Maine? The Florida Current flows just off our coast and carries waters to the northern Atlantic, warming Europe. What would the Europeans say if NASA could not guarantee their waters were not plutonium contaminated from a Cassini accident?

Here, now, Cassini or other large space probes would not get approved as new projects, simply because the cost and time involved are percieved as impractical today. However, we have already spent that money and time. Cassini costs us nothing more but the cost of the Titan 4B and operations costs to run it in flight. It would be a tremendous waste to throw that investment away for no good reason. If Cassini really risked millions of lives, that would be a reason worth considering, but space experts do not believe that there is any such extreme risk.

Space experts should not be deciding this issue for you, for me, or for my children who live in the vicinity of the launch complex.

The "civilian space agency" issue is one which opponents bring up trying to tie NASA RTG use into military nuclear space use and which I feel is a red herring, although one that seems sincerely felt. As I point out in Policy Risks related to RTGs in Space, while the US military has not completely abandoned research into nuclear reactors in space and nuclear rockets, there are no currently desired missions for such items and the funding is decreasing steadily over time.

Are you sure? How do you know?

It is my opinion and I believe the consensus opinion of the space policy community that there will be no military nuclear space program in the future, regardless of what happens with NASA's planetary exploration program and RTGs. There is no need, no reason for one, and it would be expensive at least and possibly if poorly executed a health risk for those on Earth.

Sounds good. Why am I not convinced?

Answer: because I know a little bit about the military mind, and the feeling of many military technologists that if it is possible, it should be done, before the "other side" does it first.

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 RTGs 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.

George Herbert response:

Again, the opponents of Cassini claim that RTGs are an unacceptable risk to human health and safety. The NASA and space industry consensus, however, is that the risk is minimal and tolerable. The opponents have yet to effectively argue technical points of risk in a rigorous manner and convince us on the pro-Cassini "side" that there's a real danger. We don't want to die of Cancer any more than anyone else does, but we don't think Cassini is going to kill us, or anyone else, even if the worst possible accident happens. And any accident at all is pretty unlikely.

I'm reminded of all the assurances the tobacco industry gave us that smoking was not addictive and didn't cause cancer, and the ones our government gave us that fallout from atmospheric nuclear testing was not poisoning our milk. Then we had a global test ban.

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.

George Herbert response:

The budget for power systems in space in general is fairly low, and most of what is being spent these days on R&D is on solar. The SP-100 reactor program is dead, the Russian reactor programs are dead, and RTG development is essentially dead as well. Solar development, spurred by both science missions and commercial users, continues at a reasonable pace, though some in the solar power industry have always called for more funding (typical of any R&D industry).

There are no theories I am aware of that will enable order of magnitude improvements in solar power in space. What we see as likely in the future are a few small leaps (concentrated arrays, when proven) and slow evolution as materials get slightly better. But the requirements of outer planet missions are much worse than Solar is likely to be able to do anytime soon. If we had a theory that predicted a huge improvement in capabilities then funding to develop it would make sense, but no such theory exists. We have to work in the design space we know about and can predict, and that is considerably more restrictive than we would like. Just throwing more money at the problems would not solve them. We don't know if solutions are even physically possible.

Then we must scale back our expectations for outer planetary exploration.

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.

George Herbert response:

There are investegations and proposals for solar/battery missions, where large solar arrays charge batteries up and instruments and radios are run for a small percentage of the time (minutes or hours a day, depending on the distance from the Sun and the mission). The capabilities of such missions are severely limited, however, especially the data rate returned. Further analysis is warranted. I'd like to know more about these studies. Where did you hear about them. Where can I get more information? I believe that fuel cells are simply the wrong technology and should not be used for any missions to outer planets. The numbers just don't seem to work out. I have no problem with this. But perhaps smaller ones, used in hybrid, combined cycle systems, can have an advantage worth looking at. If not, I'll concede this point. There are plenty of other ways to make these missions succeed without nuclear power.

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.

George Herbert response:

There is a lot of creative thinking going on in spaceflight; Single-Stage-to-Orbit, fully reusable launch vehicles, better-faster-cheaper, new millenium missions, etc. We have seen the first fruits of a revolution in approach to many aspects of space missions. Pathfinder / Sojurner are the first to fly, and we've all seen the photos from Mars and watched the reliable little rover drove around well past its recommended tuneup mileage. Wonderful new and different things are happening in spaceflight.

None of that, however, changes the consensus NASA and industry analysis that RTGs are safe and viable means of powering outer planet space missions. The arguments are not tired or old: as Dr McCluney himself admits, excluding the radiological issues, RTGs are very efficient and reliable power sources. The radiological risks are the only point of contention. Agreed.

What are those radiological risks? Look at the pages: Dangers during launch, Dangers during flyby, and The Danger from Plutonium

There is some risk, yes. The worst case risk, though, is really quite minor, and I and others feel that it is quite acceptable. Cassini can't kill millions of people as some opponents allege. I just think you are wrong here, based on the calculations of several scientists who have come out against Cassini.

Nuclear paranoia should not be allowed to impose unreasonable safety restrictions. The risks from Radon in homes from natural environmental radiation make a Cassini accident look tame. You are far more likely to die driving to work tomorrow than Cassini is to ever kill you over your whole lifetime. Should the danger be ignored? No. The public has a right to have it explained and justified. But that has been done and is being done still in response to the critics and opponents of Cassini. This page is part of that response. I hope that everyone will look at both sides, follow the links to both opponents of Cassini and NASA and pro-space sites, and make up their own minds.

The big difference is that I may choose to live in a home with low-level radon, to ride in my car on marginally safe streets, over government certified bridges, and to eat government certified safe food, even though there are risks in these activities, but I do not choose to be exposed to Cassini risks, however, small. The government exposes me to these risks and the only way I can avoid them in one case is to sell my house and move away from the launch area and in a second case is to choose to live on another planet, one which NASA does not plan to put nuclear material on. Neither of these choices is feasible or fair.

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Dr. Ross McCluney, Principal Research Scientist
Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922-5703
Voice: 407-638-1414 Fax: 407-638-1439 e-mail:
Web Sites: Florida Solar Energy Center:
Introduction to Radiometry and Photometry:
Short course on Radiometry & Photometry, SPIE San Jose, 26 JAN 98
For more info.:

(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.)

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First placed online September 21st, 1997.
Last modified July 10th, 1999.
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