space sunshade
A space sunshade or sunshield is a parasol that diverts or otherwise reduces some of the Sun's radiation, preventing it from hitting a spacecraft or planet and thereby reducing its insolation, which results in reduced heating.  Light can be diverted by different methods. The concept of the construction of sunshade as a method of climate engineering dates back to the years 1923, 1929, 1957 and 1978 by the physicist Hermann Oberth. Space mirrors in orbit around the Earth with a diameter of 100 to 300 km, as designed by Hermann Oberth, are intended to focus sunlight on individual regions of the Earth’s surface or deflect it into space so that the solar radiation is weakened in a specifically controlled manner for individual regions on the Earth’s surface. 
First proposed in 1989, another space sunshade concept involves putting a large occulting disc, or technology of equivalent purpose, between the Earth and Sun.
A sunshade is of particular interest as a climate engineering method for mitigating global warming through solar radiation management.  Heightened interest in such projects reflects the concern that internationally negotiated reductions in carbon emissions may be insufficient to stem climate change. Sunshades could also be used to produce space solar power, acting as solar power satellites. Proposed shade designs include a single-piece shade and a shade made by a great number of small objects. Most such proposals contemplate a blocking element at the Sun-Earth L1 Lagrangian point.
Modern proposals are based on some form of distributed sunshade composed of lightweight transparent elements or inflatable "space bubbles" manufactured in space to reduce the cost of launching massive objects to space.

Designs for planetary sunshade
Cloud of small spacecraft
One proposed sunshade would be composed of 16 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 micrometers. The mass of each disk would be about a gram, adding up to a total of almost 20 million tonnes. Such a group of small sunshades that blocks 2% of the sunlight, deflecting it off into space, would be enough to halt global warming. 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 NIAC.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 sails and tilting them in the right direction, the flyer will be capable of altering its speed and direction to keep in position.Such a group of sunshades would need to occupy an area of about 3.8 million square kilometers if placed at the L1 point.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  presented the idea for a sunshade at the 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 billion over 20 years with an estimated lifetime of 50-100 years. 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."

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. The author estimated that launching such mass would require 399 yearly launches of a vehicle such as SpaceX Starship for 10 years.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. 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. 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. 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.

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 L1 point between the Earth and the Sun. This plan was proposed in 1989 by J. T. Early. 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.In 2004, physicist and science fiction author Gregory Benford calculated that a 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%.The cost of such a lens has been disputed. At a science fiction convention in 2004, Benford estimated that it would cost about US$10 billion up front, and another $10 billion in supportive cost during its lifespan.

One diffraction grating
A similar approach involves placing a very large diffraction grating (thin wire mesh) in space, perhaps at the 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, although in 2002 these same authors argued for blocking solar radiation in the stratosphere rather than in orbit given then-current space launch technologies.

Spacecraft sunshades
The James Webb Space Telescope (JWST) infrared telescope has a layered sunshade to keep the telescope cold.
For spacecraft approaching the Sun, the sunshade is usually called a heatshield. Notable spacecraft [designs] with heatshields include:

Messenger, launched 2004, orbited Mercury until 2015, had a ceramic cloth sunshade
Parker Solar Probe (was Solar Probe Plus), launched 2018 (carbon, carbon-foam, carbon sandwich heatshield)
Solar Orbiter, launched Feb 2020
BepiColombo, to orbit Mercury, with Optical Solar Reflectors (acting as a sunshade) on the Planetary Orbiter component.

See also
Starshade – Proposed occulterPages displaying short descriptions of redirect targets
Space mirror (geoengineering) – Artificial satellites designed to change the amount of solar radiation that impacts EarthPages displaying short descriptions of redirect targets
Space-based solar power – Concept of collecting solar power in outer space and distributing it to Earth
Sunshield (JWST) – Main cooling system for the infrared observatoryPages displaying short descriptions of redirect targets
Spacecraft thermal control – Process of keeping all parts of a spacecraft within acceptable temperature ranges
Solar sail – Space propulsion method using Sun radiation

References
External links

Marchis, Franck; Sánchez, Joan-Pau; McInnes, Colin R. (2015). "Optimal Sunshade Configurations for Space-Based Geoengineering near the Sun-Earth L1 Point". PLOS ONE. 10 (8): e0136648. Bibcode:2015PLoSO..1036648S. doi:10.1371/journal.pone.0136648. ISSN 1932-6203. PMC 4550401. PMID 26309047.