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 Ashwin Kumar Mar 4, 2013 12:44  |
Thanks for the proposal!
1) Low earth orbit refers to quite a wide range of altitudes. Did you have a narrower range in mind?
2) A dust cloud can absorb longwave radiation emitted by the earth, re-radiate in all directions, and thereby contribute to heating the Earth. Presumably the proposal is referring to particles that reflect sunlight but do not interact significantly with longwave radiation? Is that correct? What types of particles did you have in mind?
3) Do you foresee challenges with generating orbital configurations by launching these particles from the surface? For example, how might one deal with the high temperatures produced in the shock waves that would result in the surrounding air -- caused by the very high particle velocities?
4) Also, I am curious to know how one might ensure that these particles, at the time of launch, can be made to ensure they have the desired orbital configuration. For example, they would initially be required to have very high angular momentum in the plane of the polar orbit at the time of launch at the surface-- can this be achieved?
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 Michael Maccracken May 1, 2013 06:29  |
Regarding the proposal to put small particles in low Earth orbit, this suggestion was considered in the 1992 report "Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Scientific Base" prepared by a National Academies of Science panel [free download is available at http://www.nap.edu/catalog.php?record_id=1605 ]. On page 448 they discuss the possibility of what they call "Space Dust": "The space parasol could be designed as an orbiting dust cloud. To minimize launch costs, very small dust particles are required. However, because of solar radiation pressure, small dust is driven out of orbit or into the earth's atmosphere in very short times; the particles are barely orbital (Mueller and Kessler, 1985). Peale (1966), quoted by Mueller and Kessler (1985), gives 1500 cm2/g, or the equivalent of aluminum oxide particles 3 μm in
size, as the limit below which this sweeping effect gives dust a short lifetime. A reflecting cloud of fine dust in orbit does not appear to be practical, and the launch costs for large dust particles appear to be too high to be practical." Basically, the particles have too little mass to stay in orbit and get blown out of orbit by the solar wind,making this not a very economical approach (the book has some cost estimates, etc.).
The NAS panel also include the possibility of injecting space mirrors, but to reflect 1% of incoming solar energy (so to balance half the equivalent of a CO2 doubling), one would need mirrors that covered 1% of Earth's area, so about 5 times 10**12 square meters, which is equivalent to having 50,000 sheet mirrors, each 10 km by 10 km in size, which would cause a flickering of sunlight. An alternative might be to have self inflating balloons, each say a square meter in cross-section and dimpled with corner mirrors that would reflect incoming light back in the direction from which the light has come. Again, one would need to cover the same area, so this would require 5 trillion of these in orbit at any given time. Again, being light, their orbit would be rather quickly deflected. To the extent the corner reflectors were not perfect at sending radiation back out into space, even more would be needed. Also, there would be a problem in that the bottoms of the balloons would reflect some energy back to Earth, and this would require balloons. So, quite problematic.
I would also note that one cannot just orbit dust or satellites in polar areas, so in would really make much more sense to be trying to reduce polar warming by counterbalancing the global absorption of solar radiation. Caldeira and Wood's proposal would really only make sense for injection of particles or something similar into the atmosphere. If one were to go to the effort of lofting materials into space, one would want the material to be reflecting solar radiation as much of the time as possible, and not having them in the polar dark during the winter.
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 Jeremy Cavanagh May 2, 2013 06:09  | Proposal contributor
Thankyou for the points you made, especially pointing out the issue of sunlight pressure on small particles with the Mueller and Kessler paper. I will have to go through a copy of this to see exactly what the author's analysis consists of before I can make a comment.
Regarding the comment in the last paragraph, there seems to be a slight misunderstanding perhaps in the way I worded it in my proposal. I meant a 'polar orbit' not orbiting about the poles and a polar orbit answers your point about wasting time in the polar dark during Winter because 45 minutes, or so, later the cloud would be over the other pole which would be in Summer. I will go over that part of the proposal to make sure I have been clear in that I am proposing the use of a polar orbit but the idea of a polar orbit runs across Calderia and Wood's different analysis using global and high latitude situations and requires further analysis of its effects.
As to space mirrors, I haven't included them as my assumption (only an assumption) is that this sort of idea presents a very difficult engineering control problem.
Finally, my proposal does have cloud decay built into it, its feasability depends on how long a cloud takes to decay due to sunlight pressure (which goes back to your first point), etc which is why I have assumed in my concept that replenishment is a central feature of operating any such cloud system. Replenishment is a feature of other geoengineering ideas e.g. stratospheric aerosols propose a constant fleet of Boeing 747s. One could speculate with a space based cloud that as its particles slowly decay into the uper atmosphere they then may have a second life of reflecting solar insolation from the upper stratophere replacing the need for aerosols sprayed from 747s. This depends on what happens to small particles entering the upper reaches of the atmosphere and I haven't looked at this yet.
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Jeremy Cavanagh May 13, 2013 03:42 | Proposal contributor
I have gone through Mueller and Kessler's paper you listed in your comment. Thank you for singling out their quote of Peale's statement that particles with a mass to area ratio of 1500 cm^2/g, or as they put it less than 3 microns of Al203, stay in orbit as that goes to the heart of whether my proposal is feasible or not. I would like to use this material to alter and refine my proposal. However, in reading over the paper they also quote Peale in saying that particles less than this critical value will stay in orbit indefinitely but if I have understood the paper correctly this seems to be contradicted by their computation results (Table 2) that only 5% of particles 10 microns in size stay in orbit up to one year. The apparent contradiction may be explained in their Ha/Hp (heights: apogee and perigee) time histories (Figs 12 to 16) that set out the different effects of the sun’s initial orientation when the orbit of a particle is established with their conclusion that this changes things considerably. The importance of this is that they present Ha/Hp histories for both 1 micron and 10 micron particles where these two different sized particles stay in orbit for 360 days or longer with different launch dates. My question from this is, if I have understood their paper correctly, does the type and time of orbit bear significantly on the effect over time of radiation pressure on a particle? Coupled with that question I have some more questions:
Mueller and Kessler are dealing with very high altitude orbits with large and changing eccentricities where not only is radiation pressure on a particle present for a large amount of the orbit but the angle of this pressure on a particle changes considerably across each orbit so perhaps causing destabilization?
Mueller and Kessler assume that the shadow affect can be discounted, I presume that’s because of the eccentric orbits of the particles they are considering, but if it’s a lower orbit with a much smaller eccentricity then a particle will spend up to half the orbit in shadow so perhaps slowing down the effect of radiation pressure compared with other types of orbits
In a lower, circular orbit the change in angle of radiation pressure will be smaller so perhaps having a different effect than in the orbits the authors are dealing with
The National Academies Report from which you cited Mueller and Kessler doesn’t seem to have analysed Mueller and Kessler’s work in coming to its conclusion about a dust cloud in orbit and beyond Peale’s paper I cannot find any other work in the references that looked at this topic
I would have to spend much more time working through Peale’s paper in order to understand it and how Mueller and Kessler drew on it.
It perhaps comes down to why do 5% of particles stay in the orbits Mueller and Kessler consider (yes I realize they say even less than that). I would also speculate as to what effect a mass of closely spaced particles have on each other wrt to radiation pressure where perhaps a small electrostatic charge is introduced to each particle to stop them clumping. As to the economic feasibility of this that will come down to launch costs which are changing and how much mass is needed in orbit and I have provided a medium to long term alternative to ground based launches for replenishment in my proposal. The costs, of course, would need to be compared with other proposals against benefits and drawbacks.
Finally, your calculation of how many mirrors would be needed to shield 1% of the earth’s surface spurred me onto a similar calculation using Caldeira and Woods observation that latitudes above 70 Degrees North make up 2.7% of the earth’s surface If we say approximately 50% of this figure is in darkness then that reduces to 1.35% or approximately 6.9 million km^2. Caldeira and Wood’s analysis requires 27% reduction of solar insolation across this area for a scenario of CO2 at twice pre industrial concentrations. If, for arguments sake we take the 27% as equal to the area for shading then that is a cloud with an area a bit larger than approx 1000 km X 1859 km (larger if taking into account the increase in size due to height of orbit). That this would only be over 70 degrees north twice every 90 minutes or so would either be countered by its shielding effect elsewhere on the sunside of its orbit. If feasible this could be extended (taking into account Mueller and Kessler’s analysis using orbits of high eccentricity) by introducing an eccentricity to the orbit so that it stays over Northern latitudes longer in summer when the sun is overhead. Finally the shape or size may have to be altered. Any such cloud would have to be larger in area anyway as it would be both highly unlikely and undesirable to have a cloud that is 100% opaque. This is largely speculation on my part and it needs analysis that I do not have the resources for at this stage but I can think of two groups who are likely to be able to carry out this sort of analysis.
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Jeremy Cavanagh Jun 22, 2013 06:36 | Proposal contributor
The comment below is a reply back March to Dr Ashwin Kumar's comments/questions also in March. It went to the wrong place (my fault) and Dr Kumar found it just a day or two ago and kindly emailed it to me and I post it here to show his original questions were addressed. Since this reply was written in March my thinking has developed on the techncial aspects of this proposal due in part to questions such those put by dr Kumar above.
"ClimateColab user ashwin has sent you the following message:
Subject: RE: Reply to questions on LEO Dust Cloud
-- original message begin --
Ashwin,
Thanks for your thoughtful questions and I have struggled to answer them. I must apologise for not replying before as I have had an extremely busy week and my answers below are a first run at your questions but they most probably require more thinking and most probably more expertise than I have at this stage to answer them completely. So please excuse my inexactness.
1) Low earth orbit refers to quite a wide range of altitudes. Did you have a narrower range in mind?
My thinking, at this stage, is that initially you might want a cloud or a collection of clouds to decay quickly so you can change the cloud's shape through replenishment to alter its shadowing effect or if the shadowing effect causes problems you can stop fairly quickly. The decay would come about through very slight drag from the very slight presence of the upper reaches of the atmosphere and gravitational changes from the earth's composition under the orbit. The other thing is to not affect the use of LEO (e.g. ISS, earth observation, etc). This requires more work and detail but I will hazard a guess of @ 200km. I don't envisage the cloud(s) being more than 100 m in thickness and actually I would hope far less as it seems to me that would make it easier to put in place and maintain. My idea is that the cloud(s) is constantly maintained or needing replenishment but that it requires far less work than the huge clouds envisaged for Lagrange points or being built to last for a thousand years in equatorial orbit.
Eccentric orbits using bigger or differently clouds may provide more efficient attenuation of insolation by reaching the highest and slowest part of the orbit a number of times during the day during summer between the earth and whichever pole so providing attenuation during a longer period than a purely LEO circular orbit. This would have a longer period than something @ 200 kms circular.
2) A dust cloud can absorb longwave radiation emitted by the earth, re-radiate in all directions, and thereby contribute to heating the Earth. Presumably the proposal is referring to particles that reflect sunlight but do not interact significantly with longwave radiation? Is that correct? What types of particles did you have in mind?
Good question. Starting in reverse, in the first instance the material could be dirt. I'm serious, finely milled dirt! However, other materials such as the sulphate aerosols proposed for stratospheric injection might be much more suitable candidates. I have to understand this much more. Using one starting point, Calderia and Wood (2008) state that the near-ultraviolet and near-infrared bands "contain roughly half of the total insolation in energetic terms" and so propose that a reflection system with spectral selectivity be used. Their discussion is based around using stratospheric based particles. Various people have raised the point that reflectors placed somewhere are going to reflect heat back down to the ground or radiate absorbed energy. The question is whether this is of significance compared to the amount of insolation reflected or stopped by a cloud.
I don't know whether it is possible to have particles in a LEO cloud stay in position wrt to other particles (e.g. by possibly imparting a small electrostatic charge to each particle that counteracts the mass attraction with other particles). There is research being undertaken into this by a group at NASA JPL who are looking at clouds in space for a completely different use. They haven't reported or published yet.
3) Do you foresee challenges with generating orbital configurations by launching these particles from the surface? For example, how might one deal with the high temperatures produced in the shock waves that would result in the surrounding air -- caused by the very high particle velocities?
They are not going to be launched from the earth'ssurface instead the cloud(s) would be built up from specialist satellite platforms launched into polar orbit. On my time line I include after 20 or so years of operation that the nascent asteroid mining ventures that created news during 2012 with announcing their aims and goals might be mature enough to supply the replenishment of any such cloud(s) from outside the earth moon system
4) Also, I am curious to know how one might ensure that these particles, at the time of launch, can be made to ensure they have the desired orbital configuration. For example, they would initially be required to have very high angular momentum in the plane of the polar orbit at the time of launch at the surface-- can this be achieved?
Thats answered above. I can't see how you would get particles to get through the atmosphere at high enough speed into orbit. Instead, big dumb rockets with satellites built for the task, on top of them and this maybe similar in scope to the idea of specially equipped 747s flying around spraying sulphates into the stratosphere as once the cloud(s) is in orbit and the optimum shape decided on it might just be a matter of replenishment in a slightly higher orbit to give it a longer life.
I envisage the cloud(s) looking like a large, thin sheet(s) rather than something round and bulbous like a cloud in the sky, instead the goal would be to design the overall shape(s).
Thanks for your questions as they have made me think and realise that my proposal still has huge gaps. I have not yet had the time nor do I have the expertise (currently) to being to approach the technical issues in any depth beyond that of ideas let alone to start to identify trade offs. My idea is that this approach simplifies greatly the ideas of dust clouds in orbit and can be deployed in the very near term with perhaps the similar resources needed to that of sulphate injection in the upper atmosphere but is more controllable than that idea. I would hope that an outcome of such a deployment is that more sea ice is generated so reflecting more sunlight."
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Jeremy Cavanagh Jun 23, 2013 10:28 | Proposal contributor
This won't go in the proposal as it is after the submission deadline but a dust cloud in polar orbit could also inhibit methane release in the Artic by stopping permafrost from melting and as the cloud(s) could be altered to change the rate of permafrost and other environmental factors that contrain the release of emthane. If dealing with methane release becomes both an immediate priority and regional priority then decreasing the amount of solar insolation maybe the only way to deal with it in a defined time window.
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 2013geoengineeringjudges 2013geoengineeringjudges Jul 8, 2013 12:23 |
Thank you for the proposal and the very interesting discussion that followed. Your excellent efforts to respond to questions and consider the relevant literature as you develop this idea are highly appreciated. As you know, this scheme is appearing to become a more complex and expensive endeavor compared to simply altering the sources of energy here on Earth. Basically, such a scheme would have to be considered as an additional cost for fossil fuel energy sources to bear and and these could be quite large. And this is all before unintended consequences are accounted for--for example, while the particles might be put into a relatively narrow band, solar wind and other factors would broaden it out. These particles would then not just disappear once out of the layer, but would create a haze down to the top of the atmosphere and likely also spread into more eccentric orbits at some times higher than the band.
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