Editing Space sunshade (section)

==Proposed designs ==

=== Cloud of small spacecraft ===
One proposed sunshade would be composed of 16&nbsp;trillion small disks at the Sun-Earth L1 [[Lagrangian point]], 1.5 million kilometers from Earth and between it and the Sun. Each disk is proposed to have a 0.6-meter diameter and a thickness of about 5&nbsp;micrometers. The mass of each disk would be about a gram, adding up to a total of almost 20&nbsp;million tonnes.<ref name="eurekalert">{{cite web |title=Space sunshade might be feasible in global warming emergency |work=EurekAlert |date=2006-11-03 |url=https://www.eurekalert.org/pub_releases/2006-11/uoa-ssm110306.php |accessdate=2010-11-11 |archive-date=23 October 2020 |archive-url=https://web.archive.org/web/20201023000247/https://www.eurekalert.org/pub_releases/2006-11/uoa-ssm110306.php |url-status=live }}</ref> Such a group of small sunshades that blocks 2% of the sunlight, deflecting it off into space, would be enough to halt global warming.<ref>{{cite web |title=Global Sunshade |work=BBC News |date=2007-02-19 |url=http://news.bbc.co.uk/1/shared/spl/hi/picture_gallery/07/programmes_global_sunshade/html/1.stm |accessdate=2010-11-11 |archive-date=1 March 2007 |archive-url=https://web.archive.org/web/20070301025733/http://news.bbc.co.uk/1/shared/spl/hi/picture_gallery/07/programmes_global_sunshade/html/1.stm |url-status=live }}</ref> If 100 tonnes of disks were launched to [[low Earth orbit]] every day, it would take 550 years to launch all of them.

The individual autonomous flyers building up the cloud of sunshades are proposed not to reflect the sunlight but rather to be transparent lenses, deflecting the light slightly so it does not hit Earth.  This minimizes the effect of [[solar radiation pressure]] on the units, requiring less effort to hold them in place at the L1 point. An optical prototype has been constructed by [[Roger Angel]] with funding from [[NASA Institute for Advanced Concepts|NIAC]].<ref>{{cite web
  | last = Tnenbaum
  | first = David
  | title = Pies in the Sky: A Solution to Global Warming
  | work = Astrobiology Magazine
  | date = 2007-04-23
  | url = http://www.astrobio.net/news-exclusive/pies-in-the-sky-a-solution-to-global-warming/
  | accessdate = 2010-11-14
  | archive-date = 2 February 2016
  | archive-url = https://web.archive.org/web/20160202043152/http://www.astrobio.net/news-exclusive/pies-in-the-sky-a-solution-to-global-warming/
  | url-status = live
  }}</ref>

The remaining solar pressure and the fact that the L1 point is one of [[unstable equilibrium]], easily disturbed by the wobble of the Earth due to gravitational effects from the Moon, requires the small autonomous flyers to be capable of maneuvering themselves to hold position.  A suggested solution is to place mirrors capable of rotation on the surface of the flyers.  By using the solar radiation pressure on the mirrors as [[solar sail]]s and tilting them in the right direction, the flyer will be capable of altering its speed and direction to keep in position.<ref name="L1_cooling">{{cite journal
  | last = Angel
  | first = Roger
  | title = Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Lagrange point (L1)
  | journal = Proceedings of the National Academy of Sciences of the United States of America
 | publisher =PNAS
  | date =2006-09-18
  | volume = 103
 | issue = 46
 | pages = 17184–9
 | doi =10.1073/pnas.0608163103
  | pmid = 17085589
 | pmc = 1859907
 | bibcode = 2006PNAS..10317184A
 | doi-access = free
 }}</ref>

Such a group of sunshades would need to occupy an area of about 3.8 million square kilometers if placed at the L1 point<ref name="L1_cooling" /> (see other lower disc size estimates below).
 
It would still take years to launch enough of the disks into [[orbit]] to have any effect.  This means a long [[lead time]]. Roger Angel of the University of Arizona<ref name="eurekalert" />  presented the idea for a sunshade at the [[United States National Academy of Sciences|U.S. National Academy of Sciences]] in April 2006 and won a [[NASA]] Institute for Advanced Concepts grant for further research in July 2006. Creating this sunshade in space was estimated to cost in excess of US$130&nbsp;billion over 20 years with an estimated lifetime of 50-100 years.<ref>{{cite web
  | last =Konecny
  | first =Pavel
  | title =We need SpaceX BFR not just get us to MARS but to save EARTH from Global Warming
  | publisher =Medium
  | date =2018-12-06
  | url =https://medium.com/@konecny.pavel/we-need-spacex-bfr-not-just-get-us-to-mars-but-to-save-earth-from-global-warming-4b2c49fb2b27
  | doi =
  | accessdate =2019-03-11
  | archive-date =22 November 2021
  | archive-url =https://web.archive.org/web/20211122074425/https://medium.com/age-of-awareness/we-need-spacex-bfr-not-just-get-us-to-mars-but-to-save-earth-from-global-warming-4b2c49fb2b27
  | url-status =live
  }}</ref> Thus leading Professor Angel to conclude that "the sunshade is no substitute for developing [[renewable energy]], the only permanent solution. A similar massive level of technological innovation and financial investment could ensure that. But if the planet gets into an abrupt [[climate crisis]] that can only be fixed by cooling, it would be good to be ready with some shading solutions that have been worked out."<ref name="L1_cooling" /><ref name="Earth Observatory 2006-11-03">{{cite press release
 |title=Space Sunshade Might Be Feasible In Global Warming Emergency
 |url=http://earthobservatory.nasa.gov/Newsroom/view.php?id=31280
 |archive-url=https://web.archive.org/web/20100316223357/http://earthobservatory.nasa.gov/Newsroom/view.php?id=31280
 |url-status=dead
 |archive-date=2010-03-16
 |publisher=University of Arizona
 |date = 2006-11-06
 |accessdate = 2009-04-29
}}</ref>

Researchers from the University of Stuttgart, Institute of Space Systems described a roadmap for the development, construction and transport of an international planetary sun shield (IPSS) at the [[Lagrange point|Lagrange point 1]] in 2021, which would also be a [[photovoltaic]] plant. Here, too, as with Hermann Oberth, production on the Moon, the use of an electromagnetic Moon slingshot (lunar coilgun) and the transport of the components from the Moon to the [[Lagrange point|Lagrange point 1]] between the Earth and the Sun are discussed by means of electric spaceships (alternatively with sun sails) assumed. The authors refer to the many international activities and the chance to put the sunlight shield into operation by 2060.<ref>{{Cite book |last1=Maheswaran |first1=Tharshan |url=https://iafastro.directory/iac/archive/browse/IAC-21/D4/1/64164/ |title=roadmap for an international planetary sunshade (IPSS) |last2=Fix |first2=Sebastian Fix |date=2021 |publisher=international astronautical federation IAC-21-D4.1.6 |language=en}}</ref>

==== Lightweight solutions and "Space bubbles" ====
A more recent design has been proposed by Olivia Borgue and Andreas M. Hein in 2022, proposing a distributed sunshade with a mass on the order of 100,000 tons, composed of ultra-thin polymeric films and SiO2 nanotubes.<ref name=":1" /> The author estimated that launching such mass would require 399 yearly launches of a vehicle such as [[SpaceX Starship]] for 10 years.<ref name=":1" />

A 2022 concept by [[MIT Senseable City Lab]] proposes using thin-film structures ("space bubbles") manufactured in outer space to solve the problem of launching the required mass to space.<ref>{{Cite web |title=Space bubbles |url=https://senseable.mit.edu/space-bubbles/ |access-date=24 May 2023 |website=MIT Senseable City Lab}}</ref> MIT scientists led by [[Carlo Ratti]] believe deflecting 1.8 percent of solar radiation can fully reverse climate change. The full raft of inflatable bubbles would be roughly the size of Brazil and include a control system to regulate its distance from the Sun and optimise its effects.<ref name=":2">{{Cite web |date=2022-07-07 |title=Space Bubbles Could Be the Wild Idea We Need to Deflect Solar Radiation |url=https://www.popularmechanics.com/space/a40486004/space-bubbles-climate-change/ |access-date=2023-05-23 |website=Popular Mechanics |language=en-US}}</ref> The shell of the thin-film bubbles would be made of [[silicon]], tested in outer space-like conditions at a pressure of .0028 atm and at -50 degrees Celsius.<ref name=":2" /> They plan to investigate low vapor-pressure materials to rapidly inflate the bubbles, such as a silicon-based melt or a graphene-reinforced ionic liquid.<ref name=":2" />

In July 2022, a pair of researchers from [[MIT Senseable City Lab]], Olivia Borgue and Andreas M. Hein, have instead proposed integrating [[Nanotube|nanotubes]] made out of [[silicon dioxide]] into ultra-thin polymeric films (described as "space bubbles" in the media <ref name="Newcomb2022">{{Cite web |author=Tim Newcomb |date=7 July 2022 |title=Space Bubbles Could Be the Wild Idea We Need to Deflect Solar Radiation |url=https://www.popularmechanics.com/space/a40486004/space-bubbles-climate-change/ |archive-url=https://web.archive.org/web/20230401131841/https://www.popularmechanics.com/space/a40486004/space-bubbles-climate-change/ |archive-date=1 April 2023 |access-date=23 May 2023 |website=Popular Mechanics |language=en-US}}</ref>), whose semi-transparent nature would allow them to resist the pressure of [[solar wind]] at L1 point better than any alternative with the same weight. The use of these "bubbles" would limit the mass of a distributed sunshade roughly the size of [[Brazil]] to about 100,000 tons, much lower than the earlier proposals. However, it would still require between 399 and 899 yearly launches of a vehicle such as [[SpaceX Starship]] for a period of around 10 years, even though the production of the bubbles themselves would have to be done in space. The flights would not begin until research into production and maintenance of these bubbles is completed, which the authors estimate would require a minimum of 10–15 years. After that, the space shield may be large enough by 2050 to prevent crossing of the {{convert|2|C-change|F-change}} threshold.<ref name="Borgue2022">{{cite journal |last1=Borgue |first1=Olivia |last2=Hein |first2=Andreas M. |date=10 December 2022 |title=Transparent occulters: A nearly zero-radiation pressure sunshade to support climate change mitigation |journal=Acta Astronautica |volume=203 |issue=in press |pages=308–318 |doi=10.1016/j.actaastro.2022.12.006 |s2cid=254479656 |doi-access=free}}</ref><ref name="Newcomb2022" /><ref>{{Cite web |title=Space bubbles |url=https://senseable.mit.edu/space-bubbles/ |access-date=24 May 2023 |website=MIT Senseable City Lab}}</ref>

In 2023, three astronomers revisited the space dust concept, instead advocating for a lunar colony which would continuously mine the Moon in order to eject [[lunar dust]] into space on a trajectory where it would interfere with sunlight streaming towards the Earth. Ejections would have to be near-continuous, as since the dust would scatter in a matter of days, and about 10 million tons would have to be dug out and launched annually.<ref>{{cite journal |last1=Bromley |first1=Benjamin C. |last2=Khan |first2=Sameer H. |last3=Kenyon |first3=Scott J. |date=February 8, 2023 |title=Dust as a solar shield |journal=PLOS Climate |volume=2 |issue=2 |pages=e0000133 |doi=10.1371/journal.pclm.0000133 |doi-access=free}}</ref> The authors admit that they lack a background in either climate or rocket science, and the proposal may not be logistically feasible.<ref>{{Cite web |date=8 February 2023 |title=Space dust as Earth's sun shield |url=https://phys.org/news/2023-02-space-earth-sun-shield.html |access-date=2 July 2023 |website=[[Phys.org]] |language=en-US}}</ref>

===One Fresnel lens===
Several authors have proposed dispersing light before it reaches the Earth by putting a very large lens in space, perhaps at the [[Lagrangian point#L1|L1]] point between the Earth and the Sun. This plan was proposed in 1989 by J.&nbsp;T. Early.<ref name="Early">{{Citation
|author=J. T. Early
|title=Space-Based Solar Shield To Offset Greenhouse Effect
|year=1989
|periodical=Journal of the British Interplanetary Society
|volume=42
|pages=567–569|bibcode = 1989JBIS...42..567E }}. This proposal is also discussed in footnote 23 of {{Citation
|author1=Edward Teller
|author2=Roderick Hyde
|author3=Lowell Wood
|name-list-style=amp
|title=Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change
|year=1997
|publisher=Lawrence Livermore National Laboratory
|url=https://e-reports-ext.llnl.gov/pdf/231636.pdf
|accessdate=2010-10-30
|archive-date=27 January 2016
|archive-url=https://web.archive.org/web/20160127185550/https://e-reports-ext.llnl.gov/pdf/231636.pdf
|url-status=live
}}.</ref> His design involved making a large glass (2,000 km) occulter from lunar material and placing at the L1 point.  Issues included the large amount of material needed to make the disc and also the energy to launch it to its orbit.<ref name=":0" />

In 2004, physicist and science fiction author [[Gregory Benford]] calculated that a [[concave lens|concave]] rotating [[Fresnel lens]] 1000 kilometres across, yet only a few millimeters thick, floating in space at the {{L1}} point, would reduce the solar energy reaching the Earth by approximately 0.5% to 1%.<ref name="Benford1">See [http://www.kuro5hin.org/story/2005/4/7/41932/19363 Russell Dovey, "Supervillainy: Astroengineering Global Warming] {{Webarchive|url=https://archive.today/20120804114154/http://www.kuro5hin.org/story/2005/4/7/41932/19363 |date=4 August 2012 }} and [http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=370 Bill Christensen, "Reduce Global Warming by Blocking Sunlight"] {{webarchive|url=https://web.archive.org/web/20090417153949/http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=370 |date=2009-04-17 }}.</ref>

The cost of such a lens has been disputed. At a science fiction convention in 2004, Benford estimated that it would cost about [[United States dollar|US$]]10 [[1000000000 (number)|billion]] up front, and another $10&nbsp;billion in supportive cost during its lifespan.<ref name="Benford1"/>

===One diffraction grating===
A similar approach involves placing a very large [[diffraction grating]] (thin wire mesh) in space, perhaps at the [[Lagrangian point#L1|L1]] point between the Earth and the Sun. A proposal for a 3,000 ton diffraction mesh was made in 1997 by [[Edward Teller]], [[Lowell Wood]], and [[Roderick Hyde]],<ref name="Teller1997">{{Citation
|author1=Edward Teller
|author2=Roderick Hyde
|author3=Lowell Wood
|name-list-style=amp
|title=Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change
|year=1997
|publisher=Lawrence Livermore National Laboratory
|url=https://e-reports-ext.llnl.gov/pdf/231636.pdf
|accessdate=2010-10-30
|archive-date=27 January 2016
|archive-url=https://web.archive.org/web/20160127185550/https://e-reports-ext.llnl.gov/pdf/231636.pdf
|url-status=live
}}. See pages 10–14 in particular.</ref> although in 2002 these same authors argued for blocking solar radiation in the stratosphere rather than in orbit given then-current space launch technologies.<ref name="Teller2002">{{Citation
 |author1         = Edward Teller, Roderick Hyde
 |author2         = Lowell Wood
 |name-list-style = amp
 |title           = Active Climate Stabilization: Practical Physics-Based Approaches to Prevention of Climate Change
 |year            = 2002
 |publisher       = Lawrence Livermore National Laboratory
 |url             = https://e-reports-ext.llnl.gov/pdf/244671.pdf
 |accessdate      = 2010-10-30
 |archive-date    = 13 May 2009
 |archive-url     = https://web.archive.org/web/20090513044038/https://e-reports-ext.llnl.gov/pdf/244671.pdf
 |url-status      = live
}}</ref>

'''Other Lower Disc Size Estimates'''

{{Too technical|date=January 2025|section}}

Recent work by Feinberg (2022)<ref>{{Cite journal |last=Feinberg |first=Alec |date=2022 |title=Solar Geoengineering Modeling and Applications for Mitigating Global Warming: Assessing Key Parameters and the Urban Heat Island Influence |journal=Frontiers in Climate |volume=4 |doi=10.3389/fclim.2022.870071 |doi-access=free |bibcode=2022FrCli...4.0071F |issn=2624-9553}}</ref> illustrate that lower disc area sizes (factor of approximately 3.5 reduction) are feasible when the background climate response is considered. For example, the background Earth climate would yield less re-radiation and feedback. In addition, disc area sizes can be further reduced by 50 times using an Annual Solar Geoengineering approach as indicated by Feinberg (2024).<ref>{{Cite journal |last=Feinberg |first=Alec |date=February 2024 |title=Annual Solar Geoengineering: Mitigating Yearly Global Warming Increases |journal=Climate |language=en |volume=12 |issue=2 |pages=26 |doi=10.3390/cli12020026 |doi-access=free |bibcode=2024Clim...12...26F |issn=2225-1154}}</ref>