# Space sunshade

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A space sunshade or sunshield is an orbiting system located between the sun and a spacecraft or planet so as to reduce solar insolation and cool the target object by diverting or reducing the Sun's radiation. Light can be diverted by alternative methods. The concept of the construction of a sunshade as a method of climate engineering dates as early as 1923 with the physicist Hermann Oberth. Space mirrors in orbit around the Earth with a diameter of 100 to 300 km, were designed by Oberth to heat or cool individual regions of the Earth’s surface in a controlled manner by respectively focusing sunlight toward the region, or deflecting the radiation into space and away from the region of interest. Another conceptual space sunshade proposed in 1989 by James T. Early involved positioning a large occulting disc, or technology of equivalent purpose, between the Earth and Sun.

## Quotes
### Ways to Spaceflight (1970) Tr. Wege zur Raumschiffahrt (1929)

    by Hermann Oberth, Ways to Spaceflight is a public domain Creative Commons translation of Oberth's Wege zur Raumschiffahrt (1929) for NASA, by the Agence Tunisienne de Public-Relations, Tunis, Tunesia (1970). Excerpts are from Ch. 20 Stations in Interplanetary Space pp. 477-506.
* The purpose of these observer stations... 2) A circular wire net... could be spread out about its centre by pivoting. In the gaps between the single wires... movable reflectors made out of light metal sheeting could be fixed so that they could be given any position to the plane of the wire net from the station by means of electric currents. The whole reflector would gravitate about the earth in a plane perpendicular to the plane of the orbit, and the net would have an inclination of 45° toward's the sun's rays... By suitably adjusting the single facets, all the solar energy reflected by the sun could be concentrated on a single point on the earth or also spread out over wide stretches of land as needed or, if there is no use for it, allowed to radiate into interplanetary space.
* In the question of the material of this reflector, it is clear that 1) no oxygen must be present, and 2) it must heat up but little itself. It will remain colder if we leave the back side rough or even paint it black.
* As material, I would suggest sodium which, under the respective conditions, has a specific weight of 1, considerable tensile strength, and a silvery lustre. It could be taken along in large pieces by the single rockets and, since it still has the usual temperature up above, can there be rolled out to sheeting or pressed out as wire or strap from the rocket. Joining of the single pieces as well as polishing can be done by men in diver's suits.
* If the reflecting plate is 0.005 mm thick and the wires, etc., have the same mass as the plate, the whole weighs 10 g per square metre or 100 kg per hectare.
* With regular traffic to the observer station, the ascent of one rocket, which, beside all else, can carry... 2,000 kg of sodium, costs 8,000 to 60,000 Mark all told. Thus, one hectare of reflector costs at the most 3,500 Mark altogether, If we figure that 1 hectare of reflector surface could make 3 hectares of polar land arable, we see that a time may come when this reflector and the whole invention becomes a paying proposition.
* I could have restricted myself in this book to only the most sober physical calculations. But in order to create the necessary respect for my idea (otherwise a realization of this idea is unthinkable), I felt impelled to draw a few pictures of the future... and I have set up some fantastic claims. Naturally, here also, I have said nothing that might not be possible by present scientific standards, and I will now show that I am also on completely scientific ground with this idea of a reflector.
* A rocket with the necessary equipment is sent aloft and there given a lateral propulsion which puts it into an elliptical orbit around the earth. I will call this rotation about the earth "revolution". Major axis perpendicular to the ecliptic, perigee in the south 1,000 km above the earth's surface, apogee in the north 5,000 km above the earth's surface.
* Now the axis of the rocket is turned perpendicular to the future plane of the wire net and, by means of side nozzles, the rocket is made to rotate this axis 4-5 times per hour. This motion I will call "rotation". If now wires are let out which are attached to the rocket at one end... they will in short time take a position perpendicular to the rocket axis due to centrifugal force and the lack of air; and that the more promptly, the longer they are and the greater the relative speed... Of course, until they have erected themselves, that takes place at the cost of the rotative speed of the rocket, so the side nozzles must sometimes be used.
* Finally, as the diameter of the net increases, the rotative speed should decrease in order not to put unnecessary strain on the material. Now the workmen—naturally weighing nothing here—can move along these wires, it they do not prefer to use rearward thrust machines to move about, and draw the cross-wires, etc. The rigidity of the net is based on the absence of a force that could bend it or, more precisely, on the minuteness of these forces as compared to the centrifugal force due to the rotation about the centre.
* Adjusting the reflecting surfaces is done electrically...
* The miniature facets are adjusted by hand... It is sufficient to simply let the sun shine on the miniature reflector and then turn the facets so that the reflected light strikes those parts of the globe corresponding to the region to be irradiated. In so doing, even the bending of the net due to radiation pressure and precession forces... could be taken into account. Either the grill bars could be pliable and bent beforehand at the discretion of the reflector pilot... or, at the adjusting mechanism of the reflector surfaces, directional gyroscopes could be situated according to which the reflecting surfaces adjust themselves, at which the gyroscope would naturally not serve as support but the wire net.
* According to MAXWELL, the light-pressure at the distance of the earth with the rays striking a completely black surface perpendicularly amounts to 0.4 mg/m$^2$, with a completely reflecting surface twice that. With a sodium surface standing at... 45° to the sun... the reflector together with reinforcements, observer's cabin, etc., weighs 10 g/m$^2$. So the radiation pressure gives it an acceleration of less than 0.1 cm/sec$^2$. (The exact value can be found experimentally when rocket ascents are made; beside the pressure posited by MAXWELL's theory, all sorts of other factors are involved. Here I... show... in principle only.) The reflector does not rise higher than two earth's radii above the centre of the earth. In so doing, the acceleration due to gravity remains over 240 cm/sec$^2$. But even 10 earth's radii high, it would still be about 10 cm/sec$^2$, which is [a] hundred times greater than the acceleration due to light-pressure.
* I am introducing three new disignations relating to direction. The direction to the sun I call sagittal (s-direction), the direction from the centre of the reflector to the centre of the earth vertical (x-direction) the direction perpendicular to the s-x-plane transverse (t-direction). We will begin our considerations with the following assumptions: The orbital plane is to be perpendicular to s, at the same time forming the t-x-plane; the reflector surfaces are to be perpendicular to the s-x-plane. They are to be inclined at 45° to the other two fundamental planes, so that the reflected light falls on the earth vertically. When the reflector revolves about the earth, the s-direction is maintained in space while t and x rotate once with reference to a fixed system of coordinates.
* Whether we must seek to constantly reflect the light on the earth vertically or are, in fact, able to is another question. I will assume that we can in order to study the single elements which determine the path of the reflector. (Imaginary numbers are also used in calculations, although it is known that they do not exist.)
* In certain purposes, types of guidance can be more advisable in which the periods of precession and rotation do not coincide. They have the advantage of making it possible to illuminate certain regions more strongly than with the methods of guidance just described. These methods are extremely manifold. When studying them one has the feeling similar to that when examining the question of how best to begin a game of chess. This is an extremely productive field for mathematicians who would like to work on something new.
* All these possibilities of guidance I would like to combine as the group of guiding methods with mechanical precession of rotation. The radiation pressure, however, is also a means by which to influence the rotation speed and the rotation axis.
* Peculiar to all the guiding methods so far is the fact that the rotation appears as a function of the revolution. Still another method of guidance is possible in which the net plane, rotation plane, and trajectory plane coincide and are perpendicular to the s-direction. Steering is done solely by means of the light pressure, which is especially strong here.
* In the south... the main task of large reflectors... of making polar regions arable, is not feasible. If the glaciers of Antarctica were melted, the level of the ocean would rise uncomfortably (6-8 m). Hopefully, by then man will be sensible enough at least to leave a cold zone for the protection of nature.
* So for the southern hemisphere and the tropics there would only remain the illumination of large cities at night and perhaps supplying solar plants with more light as well as the influencing of the weather.
* In the north, on the other hand, outside of Greenland, there are no such masses of land-ice (however much ice there may be, no danger arises from melting ice that floats in the water), and the glaciers of Greenland will remain because if their high location and because there will be more snowfall on Greenland if the polar sea melts.
* Since the reflector is to work mainly over the northern hemisphere; according the KEPLER's second law, that occurs if the reflector follows an ellipse whose perigee lies in the south. The perigee is determined by the fact that the reflector is not supposed to enter the atmosphere even with unforeseen disturbances in the trajectory: meteors, inadvertence of the pilot, the effect of extraneous fields of gravity not calculated beforehand, etc. An altitude of 1,000 km should suffice. (With large reflectors, the perigee is also given by the fact that the net must not tear under the influence of the difference in gravitation.)
* The apogee is determined by the fact that the light reflected to earth must have the necessary concentration in order to fulfill the purpose of the reflector. The light patch of a reflector 6,000 km high, for example, cannot... be smaller than 56 km, no matter how well the reflector works.
* In order to concentrate the radiation energy more strongly (in case of war) the reflector would have to be brought closer to earth by decelerating its revolving speed at the perigee so that the reflector gravitates in a circle near the earth; the deceleration would occur by means of the light-pressure, if one has time (it would take 2 or 3 months), or by means of rearward thrust, if one does not have much time.—With low guidance, precession-free guidance would be in place.
* It could easily appear as though the reflector would have to be extremely thin and light for the light-pressure to have... effect. This is not absolutely true. With appropriate guidance, the reflecting surface could be 10-20 times as thick, yet this precession of the plane of revolution could always be effected in the course of one year.
* The trajectory disturbances caused by the sun, moon, and planets... tend to rotate the plane of revolution about the north-south axis, while the axis itself is preserved. In their total effect, they produce a precession moment which... is opposite to the light-pressure moment, but smaller.
* [W]ith suitable guidance of reflectors, which, in relation to the reflecting surface, are 100 times as heavy as the one described, this precession moment could be greater than the light-pressure moment. These trajectory disturbances are extremely diverse and, in part, can hardly be investigated mathematically; so it might appear as... insurmountable difficulties to the reflector guide. Actually, he need pay no attention... to smaller trajectory disturbances; he must simply re-adjust the reflector in the south each time with the use of the light-pressure.
* Determining the position is... very simple; and, if everything else comes off... any 6th-semester student of astronomy could be instructed... in 2-3 months to be entrusted with the reflector without concern.
* I want to discuss several objections to the reflector idea. For example: The slightest pressure would shatter the reflector like glass. I already spoke about the temperatures of different bodies when exposed to solar radiation... It will not be difficult to paint and guide the reflector so that it always has a temperature at which the sodium is firm yet always elastic. ...[I]t need not pass through the earth's shadow at all, for the light-pressure causes an annual precession which always keeps it above the edge of the earth's shadow. ...[T]he forces which act on the reflector are so small that, although we still determine them mathematically, we can really no longer visualize them at all.
* Another objection: The energy reflected by the reflector will not be sufficient... The strategic effects can be achieved with the reflector under all circumstances. Even the thicket clouds reflect at the most 3/4 of the striking rays. 1/4 is absorbed and, with closest concentration of the rays, the heat... is sufficient to produce a tornado in a few minutes that can destroy enemy forces. For the same reason, the objection is invalid that the cloud cap that... must form above the rising air stream makes further action of the reflector impossible. But this cloud cap... [i]n a calm... forms at... 3-10 km vertically above the affected area. With precession-free guidance of the reflector... as in the case of war, the light falls in on a slant. When no calm prevails... wind blows the cap farther and farther away, so that the attacked region becomes open to further influence. (Except when the reflector is situated exactly in the wind direction behind the region. But, with the rapid north-south movement of the reflector in precession-free guidance, that can be the case only for a few minutes.)
* The cultural tasks are... possible to fulfill. For example, if a sea route to the ports of Siberia is to be kept ice-free, a route must only be chosen that runs approximately in the direction of the winter wind from the Gulf Stream... light is thrown on a relatively narrow and short strip running from east to west... to the extent that the sky clouds over at this place. ...[T]he direction of the wind and ...the earth's rotation coincide. Hereby the earth always rotates as we need it ...By the time the light patch has passed along the whole stretch, the fog at the beginning will either have settled or been blown away... Then one can begin at the beginning again, Since the clouds hold the heat above the shipping lane... a reflector 100 km in diameter is... sufficient.
* Another objection is that the sodium plate would quickly lose its luster due to cosmic dust or the short-wave rays of the sun. ...[W]ith their apparent weightlessness, it will be very easy to bring the reflector facets to the station occasionally and pass them through the rollers once more after first turning their rough side to the sun,thus making them soft by heating. In this way, a km$^2$, of the reflector, could be repolished without expenditure worth mentioning; I hope... that the reflecting capacity of such a reflector facet will not require renewal for at least 30 years.
* I was told that sodium plate 0.05 mm thick would allow light to pass through and not reflect. I have done an experiment in this regard... No basic problem would arise... even if had been made 1/2 mm thick, for the radiation pressure is so great that... a precession of the plane of revolution about the north-south axis in one year would be possible.
* Enough of this. They are only dreams of the future. Bold ones? Perhaps, but we have already experienced... bolder ideas. Who would have believed in 1894 that, a few years later, one would see through a person by means of Roentgen rays? PHILANDER's statement (Medical Fairy Tales), "Man will be made transparent like a jelly-fish", was bolder than this dream of the future; that required finding something completely new, while here we are dealing with laws of nature already known.—Accomplishing these things will certainly require the conversion of enormous energies. But were not hundred times greater sums of money expended during the World War? In one year, the nations of Europe spend more on smoking and drinking than the whole sodium reflector would cost. War and narcotics are quite unnecessary things, yet more money is spent on them than on something useful. Should not mankind, in an exceptional case, also save something for constructive work?
* Ref: Philander, Medizinische Märchen

### "Space-based Solar Shield to Offset Greenhouse Effect" (1989)

    Journal of The British Interplanetary Society Vol. 42, pp. 567-569, by James T. Early, Lawrence Livermore National Laboratory.
* A thin glass shield built from lunar materials and located near the first Lagrange point of the Earth-Sun system could offset the greenhouse effects caused by CO$_2$ buildup in the Earth's atmosphere.
* A suggested method for the control of planetary temperatures is the use of space-based shields to modify the incident flux.
* Terraforming shields for planets such as Venus or Mars would... be large, complex structures requiring vast amounts of lunar or asteroidal material... space manufacturing and long-range transportation... One... stepping stone to understanding and mastering the technologies and processes... would be the construction of a shield to offset the greenhouse effect on... Earth. Such... would not require interplanetary capabilities.
* The time required for the removal of... [greenhouse] gases from the atmosphere by natural processes is... uncertain: current estimates are several centuries. The uncertainties... [have] led to calls for... restrictions on the generation of greenhouse gases. ...The existence of a possible technical solution could... have a major short-term impact in influencing short term consumption restrictions, even if the solution could not be implemented until the next century.
* A conceptually simple method for offsetting the greenhouse radiation trapping effects would be to decrease the solar heating by the use of a space-based solar shield.
* Approximately 2% of the solar radiation reaching Earth must be blocked to offset the predicted greenhouse trapping in the next century.
* The screen postulated would be 2000 km in diameter and located 1.5 x 10$^6$ km from Earth, near the first Lagrange point between the Earth and the Sun.
* A shield 10μ thick would weigh approximately 10$^{11}$ kg [100 million tons] and may cost from one to ten trillion dollars. It would be fabricated from lunar materials launched by a mass driver.
* The space shield must be placed in orbit where it remains positioned between the Earth and the Sun. This point will be near the classical first Lagrange (L1) point.
* Firstly the angular velocity of the shield around the Sun and of the Earth must be the same so that the shield remains in line between the Earth and the Sun.
* Secondly, there must be an acceleration balance on the shield between the centripetal acceleration from the orbit around the Sun, the gravitational accelerations of the Earth and the Sun and the [photon] acceleration on the shield. A photon thrust of zero would locate the shield exactly at the L1 point.
* The shield orbit will be semi-stable as any small radial perturbation towards or away from the Sun will cause the shield to be pulled out of position. However, perturbations perpendicular to the Earth-Sun axis will be stable.
* Station-keeping at the L-1 point requires constant adjustment... The accelerations required to hold this orbit are very small and well within the capabilities of the shield.
* When a photon is absorbed or emitted, its momentum is transferred to the shield. If the photon is reflected, the momentum transferred is twice the photon momentum.
* Shield acceleration may be controlled by optical design.
* If the shield is opaque, then the Sun side should have a low reflectivity (high absorptivity)... The photon thrust from radiated infrared energy can be used to offset the thrust from absorption.
* The infrared emmissivity should be minimized on the Sun side and maximized on the Earth side.
* An ideal opaque shield would scatter the Earth bound solar energy into diffuse infrared energy.
* The shield may also be transparent and simply scatter the visible photons away from the Earth. ...A glass shield may act as a prism to deflect the sunlight away from the planet in accordance with Snell's Law.
* The shield may have a pattern of shallow parallel grooves on one side and be flat on the other.
** Note: see Fresnel lens
* A transparent shield would reflect some of the light at both the entering and exiting surfaces. The amount reflected could be minimized by applying complex layered coatings to the glass but the relatively small benefit incurred may not justify the cost.
* The effective blockage by the shield is... a function of the shield diameter. For a shield diameter less than 1200 km, the whole projection of the shield area on to the Sun's surface would lie within the Sun's disc when viewed from any point on the Earth's surface. The solar blockage would, therefore, be uniform over the Earth's surface.
* For a shield diameter of 2000 km, the projected shield disc falls partially off the Sun's disc only for Earth locations on the outer 6% rim of the projected Earth disc, i.e. of the standard circle map showing one side of the Earth. Except for arctic regions, these will be the regions near sunrise and sunset. Thus, the shading averaged over the entire day will be almost constant across the Earth with the arctic regions receiving slightly less shading. This condition should avoid some of the potential political problems associated with having some sections of the Earth shielded more than others.
* Lunar soil can be formed into glass for either a transparent or opaque shield.
* Most glasses will be sufficiently transparent to 10μ thicknesses. For an opaque shield, an appropriate coating material would have to be found for the glass substrate. Iron may be obtained from lunar soil with only moderate effort but is probably too dense to use as the base material for an opaque shield. Any material used must be capable of maintaining its properties for centuries in the presence of radiation from solar wind, solar UV radiation and cosmic rays.
* The shield material would be processed into glass ingots then drawn into thin sheets. Glass fabrication techniques must be investigated to determine if 10μ is a viable thickness. Commercial plastic wrap is 13μ thick and aluminum foil is typically 13 to 25μ.
* Designs for advanced solar sails have been proposed using 2μ sail materials.
* The glass may be launched to the shield location by a mass driver. A number of studies have indicated that mass drivers are feasible and economical for launching unmanned payloads from the lunar surface. If the glass sheet is sufficiently flexible it may be formed into sheet on the lunar surface and launched in rolls.

### Hermann Oberth: the father of space flight, 1894-1989 (1994)

    by Boris V. Rauschenbach
* Oberth... began the book Men in Outer Space: New Projects for Rockets and Space Travel... written in the style accessible to the general reader. ...Published in 1955 in German, it was translated into English, French, Italian, Dutch, and even in Croatian! ...Oberth's new book was indeed "cosmic," and this gave it a cardinal distinction from his classic book of the 1920s. ...A supplemental chapter ...is devoted to "space mirrors," a theme which occupied Oberth all his life. A short description ...is already present in the 1923 book. In 1929, when he published his fundamental work, Ways to Space Travel, he included a much more comprehensive description... in the chapter... "Space Stations." ...[A]lmost the entire chapter is devoted to space mirrors. In the 1954 book... a new varient... is presented. Eventually in Bucharest in 1978, an entire book (in German) was devoted to this theme. ...[T]he primary purpose for the space mirror would today be called an ecological one ...At the time ...there was ...no robotics technology, and he assumed all the work after ...erection ...would ...be carried out manually by astronauts.
* [A] giant net (similat to a trawl) would be erected in outer space, constructed in a hexagonal mesh pattern. This net would be stretched out and a tension sufficient to rotate the entire net would be maintained by centripetal force. The rotation would be begun by special rockets and... continue because it was in a vacuum. The diameter of each hexagon would be about 10 kilometers, and the entire mirror would have a circular disk shape with a diameter of about 100 to 200 kilometers. Within each hexagon, a round mirror approximately 10 kilometers in diameter, would be installed. ...[Each] single mirror would be capable of being [independently] tilted... initiated by... electric actuators.
* The giant space mirror... would be positioned... between 1000 and 5000 kilometers from the earth. Its orbital path and orientation... would... make it possible to direct the sunlight towards specific areas of the earth... different points on the earth simultaneously...
* Oberth... mentions... using the sun's pressure to alter the orbital course of the mirror.
* Such a mirror, he asserted, could not only light up the cities at night... but could also have a decisive influence on weather and climate.
* [T]he ice in the usable areas of the north polar seas could be melted... to navigate the northern coasts of Europe and Siberia year round. The warming of the Caspian Sea could... produce rain in arid regions of Central Asia. Directing... rays... where spring and autumn frosts are expected would allow... increase[d] fertility... Oberth claimed... it would open up... predicting the weather, but also... determining the weather.
* Oberth proposed to shade planets located close to the sun... with gigantic cosmic shields... Vice versa, the planets... further from the sun... would be warmed... with... gigantic cosmic mirrors.
* [H]is belief... a reduction in the cost of building large structures in outer space could be achieved by delivering the necessary materials from the moon... [H]e presupposed the existence of the necessary industrial plants on the moon, but the end result would be a thousand-fold reduction in the cost of building large structures in space.

### Global Warming and Ice Ages (August 15, 1997)

    :I. Prospects For Physics-Based Modulation Of Global Change by E. Teller, L. Wood, Roderick Hyde, paper prepared for submittal to the 22nd International Seminar on Planetary Emergencies Erice (Sicily), Italy August 20-23, 1997. UCRL-JC-128715 PrePrint. Lawrence Livermore National Laboratory.
* It has been suggested that large-scale climate changes, mostly due to atmospheric injection of "greenhouse gases" connected with fossil-fired energy production, should be forestalled by internationally-agreed reductions in, e.g., electricity generation. The potential economic impacts of such limitations are obviously large: ≥$10$^{11}$/year [$100 billion/yr]. We propose that for far smaller—<1%—costs, the mean thermal effects of "greenhouse gases" may be obviated in... several distinct ways, some... novel. These suggestions are all based on scatterers that prevent a small fraction of solar radiation from reaching all or part of the Earth. We propose research directed to... near-term realization of one or more of these inexpensive approaches to cancel the effects of the "greenhouse gas" injection.
** Abstract, p. 1.
* While the magnitude of the climatic impact of "greenhouse gases" is currently uncertain, the prospect of severe failure of the climate, for instance at the onset of the next Ice Age, is undeniable. The proposals in this paper may lead to quite practical methods to reduce or eliminate all climate failures.
** Abstract, p. 1.
* [I]ncreases in average world-wide temperature of the magnitude currently predicted can be canceled by preventing about 1% of incoming solar radiation—insolation—from reaching the Earth. This could be done by scattering into space from the vicinity of the Earth an appropriately small fraction of total insolation. If performed near-optimally, we believe that the total cost of such an enhanced scattering operation would probably be at most $1 billion per year, an expenditure that is two orders of magnitude smaller... than... proposed limitations on fossil-fired energy production. Some of these insolation-modulating scattering systems may be re-configured to effectively increase insolation by an amount—perhaps 3%—sufficient to prevent another Ice Age.
** Introduction, pp. 2-3.
* [T]he problem of possible changes in climate may be better solved by cooperative application of modern technologies rather than by international measures focused on prohibitions.
** Introduction, p. 3.
* In general, three basic types of scatterers exist, for scattering any type of electromagnetic radiation, including sunlight. The simplest type is based on any material in which the electric fields of light cause a displacement of electric charges; thus, any material at all can be used. The magnitude of the displacement of charges by an electric field of unit strength is measured by the dielectric constant ε, where ε=1 means there is no displacement. The scattering is proportional to (ε-1)$^2$, that is, highly polarizable materials generally will be more useful. This class of scatterer requires the near-optimal deployment of an estimated several million tons of scattering material in order to prevent an estimated (global- and time-)average temperature increase of 3±1.5°C associated with a doubling of atmospheric CO$_2$ during the coming century; the corresponding cost is ~$0.5 billion/year.
** Scattering Fundamentals, pp. 3-4.
* More effective scatterers can be realized by employing that subset of materials which exhibit high electrical conductivity. In this special case, electrons may be separated from their original locations by any distance, and it is the magnitude of the optical-frequency current carried by these electrons that characterize the effectiveness of such scattering materials—which are generally metals. Employed near-optimally, tens of thousands of tons of high-conductivity metal—roughly 1% of the required mass of dielectric materials—are required to scatter 1% of the Earth's total insolation; the corresponding costs are $0.07-0.14 billion/year.
** Scattering Fundamentals, p. 4.
* In principle, the most effective of all possible scatterers are atoms or molecules that scatter light in resonance. Such extremely strong scattering can be obtained for light of a frequency adapted to a specific atom or molecule. The simplest example would be scattering of a narrow band of red light by lithium atoms or of yellow light by sodium atoms. Unfortunately, such exceptionally strong scatterings occur only in the immediate neighborhood of an atomic transition-frequency, and the atom will selectively interact with light of frequencies which deviates from the resonant one by about one part in ten million (for visible light). This difficulty can be overcome by broadening the resonance (accompanied by a proportionate weakening of the scattering-strength) or by using scatterers that have many separate resonances – or, most effectively, by a combination of these two approaches. Of the order of 1 million tons of such resonant-type scattering material are estimated to suffice to remove 1% of the total insolation of the Earth; the corresponding cost may be $0.3-0.75 billion/year.
** Scattering Fundamentals, p. 4.
* The intrinsic scattering strengths of dielectrics, electrical conductors, and resonant scatterers are in the approximate ratio of 1 to 10$^4$ to 10$^6$, respectively, for visible light; in practical implementations useful for insolation modulation, however, these ratios may be much different.
** Scattering Fundamentals, p. 4.
* There are three obvious choices for deployment-sites for scatterers on scales of interest for insolation modulation. One is the terrestrial stratosphere, the second is in a low-Earth orbit (i.e., an orbit whose radius may be as much as twice the radius of the Earth), and the third is a position along the line between the centers of the Earth and the Sun (approximately one hundred times the Earth's radius distant from the Earth).
** How Should Scatterers Be Deployed? p. 5.
* [T]he stratosphere is a chemically uncongenial location due to the high flux of ultraviolet radiation from the Sun and the presence of oxygen, particularly in the more reactive form of ozone.
** How Should Scatterers Be Deployed? p. 6.
* An interesting though not necessarily a practical case comprises the third alternative. Terrestrial insolation has an angular definition of one part in 120. Thus, if the scattering system is deployed ~102 times the Earth's diameter [~1.3 million km] distant from the Earth, small-angle (≤1°) scattering will suffice for an appropriate deflection of the Earth-directed sunlight (either toward the Earth, if warming is desired, or away from it, if cooling is sought). This small angle permits the use of relatively very modest quantities of either conductors or dielectrics to comprise the scattering system—approximately 10$^2$-fold [100-fold] smaller than... when deployed near the Earth. The management of the radial and angular momenta of the sunlight scattered poses basic, albeit quite tractable, issues with respect to position maintenance.
** How Should Scatterers Be Deployed? p. 7-8. Note: 102*Earth's diameter = 102*12,742 km ≈ 1.3 million km, near Lagrange point (L1) at ~1.5 million km.
* There are obviously numerous ways in which the potentialities and difficulties mentioned above can give rise to a workable scattering system, one of a scale adequate to modulate the total insolation of the Earth by 1%. In the following, we shall provide some details regarding specific possibilities, ones selected to illustrate basic features of each of the major classes of scattering systems.
** Some Specific Proposals, pp. 8-9.
* Two insolation modulation systems which we have considered—quasi-resonant scatterers for intra-atmospheric applications and the small-angle-scattering system for deep space use—are apparently novel. These involve total system masses of the order of 10$^3$-10$^6$ tons—which is 2-5 orders of magnitude less mass than that of the most interesting previous proposals. We conclude that the insolation modulation approach to prevention of climate failure is certainly technically feasible-in-principle, and that the total costs-to-own its best examples may be de minimis.
** Conclusions, p. 16.
* We believe that research along several lines to study the deployment and operation in sub-scale—perhaps 10$^{-3}$ of a full-scale, 1% insolation-equivalent system—of appropriate scatterers of sunlight is justified immediately by considerations of basic technical feasibility and possible cost-to-benefit. ...[E]ven very preliminary estimates of performance and practicality suffice to make us optimistic about ultimate workability and utility.
** Conclusions, p. 16.
* Success can be expected to be more significant than merely counteracting the global climate modifications arising from large-scale injection of greenhouse gases. Straightforward modifications of what we have discussed including the scattering not of incoming sunlight but of the long-wavelength infrared radiation emitted by the Earth could be effective in preventing onset of both little and full-sized Ice Ages. These may occur with little warning, seemingly at any time, and could severely impact human affairs on notably brief time-scales. Indeed, the Earth's climate system may manifest finite-amplitude instability along several axes, with small perturbations occasionally resulting in large shifts, some swiftly executed.
** Conclusions, pp. 16-17.
* Greenhouse warming of the Earth due to human activities is a possibility, moreover one for which mitigative/remedial actions of the types proposed here can be at once deliberate and effective. In contrast, Ice Age-severity cooling... that have occurred quasi-periodically many times during the last 1.2 million years, is a practical certainty. Moreover, a several-decade duration cold snap of Ice Age Maximum temperature-drop is known to have occurred in the Northern Hemisphere with essentially no warning during the last interglacial period, under precursor climatic conditions only slightly warmer than the present-day one.
** Conclusions, p. 17.
* Today, our scientific knowledge and our technological capability already are likely sufficient to provide solutions to these problems; both knowledge and capability in time-to-come will certainly be greater. Whether exercising of present capability can be done in an internationally acceptable way is an undeniably difficult issue, but one seemingly far simpler than securing international consensus on near-term, large-scale reductions in fossil fuel-based energy production—especially in a world exhibiting very large geographical and cultural differences in per capita energy use, past, present and future.
** Conclusions, pp. 17-18.
* [P]rior to any actual deployment of any scattering system aimed at full-scale 1% insolation modulation, completely transparent and fully international research in sub-scale could result in public opinion conducive to a reasonable technology-based approach to prevention of large-scale climatic failures of all types. International cooperation in the research phase, based on complete openness, is necessary and may be sufficient to secure the understanding and support without which any of these approaches will fail.
** Conclusions, p. 18.

### Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Lagrange point (L1) (Sep 18, 2006)

    Article by Roger Angel
* The concept considered here is to block 1.8% of the solar flux with a space sunshade orbited near the inner Lagrange point (L1), in-line between the Earth and sun.
* Following the work of J. Early [Early, JT (1989) J Br Interplanet Soc 42:567–569], transparent material would be used to deflect the sunlight, rather than to absorb it, to minimize the shift in balance out from L1 caused by radiation pressure.
* Three advances aimed at practical implementation are presented. First is... a very thin refractive screen with low reflectivity, leading to a total sunshade mass of ~20 million tons. Second is... reducing transportation cost to $50/kg by using electromagnetic acceleration to escape Earth’s gravity, followed by ion propulsion. Third is... the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure. These meter-sized "flyers" would be assembled completely before launch, avoiding any need for construction or unfolding in space.
* They would weigh a gram each, be launched in stacks of 800,000, and remain for a projected lifetime of 50 years within a 100,000-km-long cloud.
* [I]t could be developed and deployed in ~25 years at a cost of a few trillion dollars, <0.5% of world gross domestic product (GDP) over that time.
* [A] cloud of spacecraft holding their orbits by active station-keeping could have a lifetime of many decades.
* Stabilizing forces could be obtained by modulating solar radiation pressure, with no need for expendable propellants. The same controls could be used, if desired, to stop the cooling at any time by displacing the orbit slightly.
* [T]he composition of the atmosphere and ocean would not be altered further, beyond their loading with greenhouse gases, and because only a single parameter is modified, the flux of solar radiation, the results should be predictable.
* [T]he sunshade would be manufactured completely and launched from Earth, and it would take the form of many small autonomous spacecraft ("flyers").
* Early recognized that the orbit of a lightweight sunshade would be disturbed by radiation pressure.
** Ref: J. T. Early, "Space-based solar shield to offset greenhouse effect" Journal of the British Interplanetary Society (1989) 42:567–569

### Was breaking the taboo on research on climate engineering via albedo modification a moral hazard, or a moral imperative? (Nov 17, 2016)

    Open Access Article by Mark G. Lawrence, Paul J. Crutzen
* A wide range of techniques has been proposed for increasing the planetary albedo, ranging from painting surfaces white to placing mirrors in orbit between the Earth and the sun.
* This sense of taboo was based on a range of arguments against research on albedo modification that have been raised by the broader scientific community, including: 1. the so-called moral hazard issue, that is, the possibility that research on climate engineering could be perceived as an implicit legitimization, and thus reduce the motivation for mitigating anthropogenic emissions; 2. the concern that reducing temperatures by albedo modification could distract from other impacts of a fossil-fuel-based economy and the resulting CO2 emissions, such as ocean acidification; 3. the “slippery slope” concern that research into understanding the potential effectiveness could cascade toward the development and deployment of the techniques under investigation; and 4. contention about the perceived “techno-fix” approach to address environmental challenges, that is, the notion that technology-caused problems can simply be fixed with more and better technology.
* [G]iven the balance of results of model studies over the last decade... and the challenging directions that this implies both for future research and also for sociopolitical aspects, especially public perception and the development of good governance principles, we have to conclude that the overall verdict is still out. The responsibility still resides with the scientific community to conduct research and engage in the broader dialogue in a responsible way, so that whatever the outcome, historians will hopefully look back and conclude that it was indeed of value—and in that sense a moral imperative—to begin carefully investigating this topic... Perhaps this will already be clear by the time of the next special section like this one, which, following those in 1996 and 2006, should be due in 2026.

## See also

* Climate action
* Climate Action Network
* Climate change denial
* Global warming
* Green New Deal
* How to Avoid a Climate Disaster by Bill Gates
* Scientific consensus on climate change
* Space exploration
* Sun

## External links

* YouTube videos
** Can We Block the Sun to Stop Climate Change @Real Engineering channel
** A huge sun shade in space? @BBC Studios channel