Editing Solar sail (section)

===Interstellar flight===
In the 1970s, [[Robert Forward]] proposed two [[beam-powered propulsion]] schemes using either lasers or [[maser]]s to push giant sails to a significant fraction of the [[speed of light]].<ref name="Forward1984RoundtripInterstellar">{{Cite journal| author=Forward, R.L. | title=Roundtrip Interstellar Travel Using Laser-Pushed Lightsails | journal=J Spacecraft | volume=21 | issue=2 | pages=187–195 | year=1984 | doi=10.2514/3.8632 |bibcode = 1984JSpRo..21..187F }}</ref>

In the science fiction novel ''[[Rocheworld]]'', Forward described a light sail propelled by super lasers. As the starship neared its destination, the outer portion of the sail would detach. The outer sail would then refocus and reflect the lasers back onto a smaller, inner sail. This would provide braking thrust to stop the ship in the destination star system.

Both methods pose monumental engineering challenges. The lasers would have to operate for years continuously at [[gigawatt]] strength. Forward's solution to this requires enormous solar panel arrays to be built at or near the planet Mercury. A planet-sized mirror or [[Fresnel lens]] would need to be located at several dozen [[astronomical unit]]s from the Sun to keep the lasers focused on the sail. The giant braking sail would have to act as a precision mirror to focus the braking beam onto the inner "deceleration" sail.

A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves directed at the sail, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "[[Starwisp]]" interstellar probe design<ref name=starwisp>Forward, Robert L., "Starwisp: An Ultralight Interstellar Probe,” ''J. Spacecraft and Rockets, Vol. 22'', May–June 1985, pp. 345-350.</ref><ref name=starwisp2>Landis, Geoffrey A., "Microwave Pushed Interstellar Sail: Starwisp Revisited," paper AIAA-2000-3337, 36th Joint Propulsion Conference, Huntsville AL, July 17–19, 2000.</ref> would use microwaves, rather than visible light, to push it. Masers spread out more rapidly than optical lasers owing to their longer wavelength, and so would not have as great an effective range.

Masers could also be used to power a painted solar sail, a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation.<ref>{{cite web|url=http://www.space.com/businesstechnology/technology/technovel_sail_050211.html |title=Earth To Mars in a Month With Painted Solar Sail |publisher=SPACE.com |date=2005-02-11 |access-date=2011-01-18}}</ref> The momentum generated by this [[evaporation]] could significantly increase the [[thrust]] generated by solar sails, as a form of lightweight [[ablative laser propulsion]].

To further focus the energy on a distant solar sail, Forward proposed a lens designed as a large [[zone plate]]. This would be placed at a location between the laser or maser and the spacecraft.<ref name="Forward1984RoundtripInterstellar" />

Another more physically realistic approach would be to use the light from the Sun to accelerate the spacecraft.<ref>"Solar Sail Starships:Clipper Ships of the Galaxy," chapter 6, [[Eugene F. Mallove]] and [[Gregory L. Matloff]], ''The Starflight Handbook: A Pioneer's Guide to Interstellar Travel'', pp. 89-106, John Wiley & Sons, 1989. {{ISBN|978-0471619123}}</ref> The ship would first drop into an orbit making a close pass to the Sun, to maximize the solar energy input on the sail, then it would begin to accelerate away from the system using the light from the Sun. Acceleration will drop approximately as the inverse square of the distance from the Sun, and beyond some distance, the ship would no longer receive enough light to accelerate it significantly, but would maintain the final velocity attained. When nearing the target star, the ship could turn its sails toward it and begin to use the outward pressure of the destination star to decelerate. Rockets could augment the solar thrust.

Similar solar sailing launch and capture were suggested for [[directed panspermia]] to expand life in other solar systems. Velocities of 0.05% the speed of light could be obtained by solar sails carrying 10&nbsp;kg payloads, using thin solar sail vehicles with effective areal densities of 0.1&nbsp;g/m<sup>2</sup> with thin sails of 0.1&nbsp;[[micrometre|μm]] thickness and sizes on the order of one square kilometer. Alternatively, swarms of 1&nbsp;mm capsules could be launched on solar sails with radii of 42&nbsp;cm, each carrying 10,000 capsules of a hundred million [[extremophile]] microorganisms to seed [[life]] in diverse target environments.<ref>{{Cite journal|last1=Meot-Ner (Mautner) |first1=Michael N. |last2=Matloff |first2=Gregory L. |title=Directed panspermia: A technical and ethical evaluation of seeding nearby solar systems |journal=Journal of the British Interplanetary Society |year=1979 |volume=32 |pages=419–423 |url=https://ui.adsabs.harvard.edu/abs/1979JBIS...32..419M/abstract |bibcode=1979JBIS...32..419M }}</ref><ref>{{Cite journal|last = Mautner |first = Michael N. | title = Directed panspermia. 2. Technological advances toward seeding other solar systems, and the foundations of panbiotic ethics | journal = Journal of the British Interplanetary Society | year = 1995 | volume = 48 | pages = 435–440 }}</ref>

Theoretical studies suggest relativistic speeds if the solar sail harnesses a supernova.<ref>{{cite news |last1=Loeb |first1=Abraham |title=Surfing a Supernova |url=https://blogs.scientificamerican.com/observations/surfing-a-supernova/ |access-date=14 February 2020 |work=[[Scientific American]] Blogs |date=3 February 2019}}</ref>