Editing Solar sail (section)

===Materials===

The most common material in current designs is a thin layer of aluminum coating on a polymer (plastic) sheet, such as aluminized 2&nbsp;μm [[Kapton]] film. The polymer provides mechanical support as well as flexibility, while the thin metal layer provides the reflectivity.  Such material resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminum reflecting film is on the Sun side. The sails of ''[[Cosmos 1]]'' were made of [[metallized polyethylene terephthalate|aluminized PET film]] ([[PET film (biaxially oriented)|Mylar]]).

[[Eric Drexler]] developed a concept for a sail in which the polymer was removed.<ref name=Drexler>{{cite web|url=http://dspace.mit.edu/bitstream/handle/1721.1/16234/06483741.pdf?sequence=1|archive-url=https://web.archive.org/web/20110604235745/http://dspace.mit.edu/bitstream/handle/1721.1/16234/06483741.pdf?sequence=1|url-status=dead|archive-date=2011-06-04|author=Drexler, K. E.|year=1977|title=Design of a High Performance Solar Sail System, MS Thesis |publisher=Dept. of Aeronautics and Astronautics, Massachusetts Institute of Techniology, Boston}}</ref> He proposed very high thrust-to-mass solar sails, and made prototypes of the sail material. His sail would use panels of thin aluminium film (30 to 100 [[nanometre]]s thick) supported by a [[tension (physics)|tensile]] structure. The sail would rotate and would have to be continually under thrust. He made and handled samples of the film in the laboratory, but the material was too delicate to survive folding, launch, and deployment. The design planned to rely on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer high area per unit mass and hence accelerations up to "fifty times higher" than designs based on deploy-able plastic films.<ref name=Drexler />
The material developed for the Drexler solar sail was a thin aluminium film with a baseline thickness of 0.1&nbsp;μm, to be fabricated by vapor deposition in a space-based system. Drexler used a similar process to prepare films on the ground. As anticipated, these films demonstrated adequate strength and robustness for handling in the laboratory and for use in space, but not for folding, launch, and deployment.

Research by [[Geoffrey Landis]] in 1998–1999, funded by the [[NASA Institute for Advanced Concepts]], showed that various materials such as [[alumina]] for laser lightsails and [[carbon fiber]] for microwave pushed lightsails were superior sail materials to the previously standard aluminium or Kapton films.<ref>{{cite web|url=http://www.niac.usra.edu/files/studies/final_report/4Landis.pdf|title=Advanced Solar- and Laser-pushed Lightsail Concepts|author=Geoffrey A. Landis, Ohio Aerospace Institute|year=1999}}</ref>

In 2000, Energy Science Laboratories developed a new [[carbon fiber]] material that might be useful for solar sails.<ref name="carbonsail_000302.html">{{cite web |url=http://www.space.com/businesstechnology/technology/carbonsail_000302.html |work=SPACE.com |title=Breakthrough In Solar Sail Technology |archive-url=https://web.archive.org/web/20110101191444/http://www.space.com/businesstechnology/technology/carbonsail_000302.html |archive-date=January 1, 2011 |url-status=dead <!--alive, but it's now a different text-->}}</ref><ref>{{cite web|url=http://sbir.nasa.gov/SBIR/abstracts/99/sbir/phase1/SBIR-99-1-25.02-2034.html|title=Carbon Solar Sail|website=sbir.nasa.gov|access-date=2015-12-25|archive-date=2011-10-22|archive-url=https://web.archive.org/web/20111022203607/http://sbir.nasa.gov/SBIR/abstracts/99/sbir/phase1/SBIR-99-1-25.02-2034.html|url-status=dead}}</ref> The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same mass. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films. The material could self-deploy and should withstand higher temperatures.

There has been some theoretical speculation about using [[molecular manufacturing]] techniques to create advanced, strong, hyper-light sail material, based on [[Carbon nanotube|nanotube]] mesh weaves, where the weave "spaces" are less than half the wavelength of light impinging on the sail. While such materials have so far only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could mass less than 0.1&nbsp;g/m<sup>2</sup>,<ref>{{cite web|url=http://www.physorg.com/news5890.html |title=Researchers produce strong, transparent carbon nanotube sheets |publisher=Physorg.com |date=2005-08-18 |access-date=2011-01-18}}</ref> making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick [[Mylar]] sail material mass 7&nbsp;g/m<sup>2</sup>, aluminized Kapton films have a mass as much as 12&nbsp;g/m<sup>2</sup>,<ref name=jpl /> and Energy Science Laboratories' new carbon fiber material masses 3&nbsp;g/m<sup>2</sup>.<ref name="carbonsail_000302.html"/>

The least dense metal is [[lithium]], about 5 times less dense than aluminium.  Fresh, unoxidized surfaces are reflective.  At a thickness of 20&nbsp;nm, lithium has an area density of 0.011&nbsp;g/m<sup>2</sup>.  A high-performance sail could be made of lithium alone at 20&nbsp;nm (no emission layer).  It would have to be fabricated in space and not used to approach the Sun. In the limit, a sail craft might be constructed with a total areal density of around 0.02&nbsp;g/m<sup>2</sup>, giving it a lightness number of 67 and a<sub>c</sub> of about 400&nbsp;mm/s<sup>2</sup>. [[Magnesium]] and [[beryllium]] are also potential materials for high-performance sails.  These 3 metals can be alloyed with each other and with aluminium.<ref name="Wright" />