Solar sails (also known as lightsails, light sails, and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s. The two spacecraft to successfully use the technology for propulsion were IKAROS, launched in 2010, and LightSail-2, launched in 2019. A further demonstrator, Advanced Composite Solar Sail System (ACS3), was launched in 2024, and deployed successfully but is not being actively controlled due to a fault.
A useful analogy to solar sailing may be a sailing boat; the light exerting a force on the large surface is akin to a sail being blown by the wind. High-energy laser beams could be used as an alternative light source to exert much greater force than would be possible using sunlight, a concept known as beam sailing. Solar sail craft offer the possibility of low-cost operations combined with high speeds (relative to chemical rockets) and long operating lifetimes. Since they have few moving parts and use no propellant, they could potentially be used numerous times for the delivery of payloads.
Solar sails use a phenomenon that has a proven, measured effect on astrodynamics. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced thousands of kilometers by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the orientation of a spacecraft, a factor that must be included in spacecraft design.Georgevic, R. M. (1973) "The Solar Radiation Pressure Forces and Torques Model", The Journal of the Astronautical Sciences, Vol. 27, No. 1, Jan–Feb. First known publication describing how solar radiation pressure creates forces and torques that affect spacecraft.
The total force exerted on an solar sail, for example, is about at Earth's distance from the Sun,, and existing technology is limited to much smaller sizes (such as for ACS 3, resulting in approximately ), making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines. However, since it uses no propellant, that force can be exerted indefinitely (as long as the Sun or another star is sufficiently close) and the collective effect over time is great enough to be considered a potential manner of propelling spacecraft.
History of concept Johannes Kepler observed that comet tails point away from the Sun and suggested that the Sun caused the effect. In a letter to Galileo in 1610, he wrote, "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.""Da naves aut vela coelesti aurae accommoda, eruntqui ne ab illa quidem vastitate sibi metuant." - Dissertatio cum Nuncio Sidereo He might have had the comet tail phenomenon in mind when he wrote those words, although his publications on comet tails came several years later.Johannes Kepler (1604) Ad vitellionem parali pomena, Frankfort; (1619) De cometis liballi tres , Augsburg
The theory of electromagnetic fields and radiation, first published by James Clerk Maxwell in 1861–1864, shows that light has momentum and thus can exert pressure on objects. Maxwell's equations provide the theoretical foundation for sailing with light pressure. So by 1864, the physics community and beyond knew sunlight carried momentum that would exert a pressure on objects.
Jules Verne, in From the Earth to the Moon,Jules Verne (1865) De la Terre à la Lune (From the Earth to the Moon) published in 1865, wrote "there will some day appear velocities far greater than these [of the planets and the projectile], of which light or electricity will probably be the mechanical agent ... we shall one day travel to the moon, the planets, and the stars."Chris Impey, Beyond: Our Future in Space, W. W. Norton & Company (2015) This is possibly the first published recognition that light could move ships through space.
Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in 1899 with a torsional balance;P. Lebedev, 1901, "Untersuchungen über die Druckkräfte des Lichtes", Annalen der Physik, 1901 Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using a Nichols radiometer.
Svante Arrhenius predicted in 1908 the possibility of solar radiation pressure distributing life spores across interstellar distances, providing one means to explain the concept of panspermia. He was apparently the first scientist to state that light could move objects between stars.Svante Arrhenius (1908) Worlds in the Making
Konstantin Tsiolkovsky first proposed using the pressure of sunlight to propel spacecraft through space in 1921 and suggested "using tremendous mirrors of very thin sheets to utilize the pressure of sunlight to attain cosmic velocities".Urbanczyk, Mgr., "Solar Sails-A Realistic Propulsion for Space Craft", Translation Branch Redstone Scientific Information Center Research and Development Directorate U.S. Army Missile Command Redstone Arsenal, Alabama, 1965.
Friedrich Zander (Tsander) published a technical paper in 1925 that included technical analysis of solar sailing. Zander wrote of "applying small forces" using "light pressure or transmission of light energy to distances by means of very thin mirrors".Friedrich Zander's 1925 paper, "Problems of flight by jet propulsion: interplanetary flights", was translated by NASA. See NASA Technical Translation F-147 (1964), p. 230.
JBS Haldane speculated in 1927 about the invention of tubular spaceships that would take humanity to space and how "wings of metallic foil of a square kilometre or more in area are spread out to catch the Sun's radiation pressure".JBS Haldane, The Last Judgement, New York and London, Harper & Brothers, 1927.
J. D. Bernal wrote in 1929, "A form of space sailing might be developed which used the repulsive effect of the Sun's rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune's orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the Sun."J. D. Bernal (1929) The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul
Science fiction authors used the solar sail in their writing, such as Russell Saunders's 1951 article Clipper Ships of Space in Astounding Science Fiction, and the short story Sunjammer by Arthur C. Clarke, originally published in the March 1964 issue of Boys' Life Short Stories . Arthurcclarke.net, 2007-2011, retrieved June 22, 2011 depicting a yacht race between solar sail spacecraft.
Carl Sagan, in the 1970s, popularized the idea of sailing on light using a giant structure which would reflect photons in one direction, creating momentum. He brought up his ideas in college lectures, books, and television shows. He was fixated on quickly launching this spacecraft in time to perform a rendezvous with Halley's Comet. Unfortunately, the mission didn't take place in time and he would never live to finally see it through.
The first formal technology and design effort for a solar sail began in 1976 at Jet Propulsion Laboratory for a proposed mission to rendezvous with Halley's Comet.
Types Reflective Most solar sails are based on reflection. The surface of the sail is highly reflective, like a mirror, and light reflecting off of the surface imparts a force. A key limitation, which in practice affects the efficiency of the trajectories flown by the sail, is that the direction and magnitude of the thrust are coupled, hence cannot be controlled independently.
Diffractive
In 2018, diffraction was proposed as a different solar sail propulsion mechanism. A material with an adjustable diffractive grating would be used to direct incoming light at a desired angle. This arrangement would remove the coupling of direction and magnitude of the thrust, and allow this thrust to be adjusted more rapidly than with a reflective sail (which needs to be tilted to different angles to achieve control). However, the technology readiness level of the sail material is not as mature as in the case of reflective sails.
Solar photon thruster The coupling of direction and thrust seen in a reflective solar sail has also been proposed to be addressed with a solar photon thruster. This also uses a reflective membrane, but unlike a conventional solar sail, the membrane directs the incoming radiation onto a small collimating mirror. The collimating mirror directs this radiation onto a reflecting mirror, the orientation of which affects the direction of the outgoing beam of radiation and hence the direction of the thrust. Thus, the thrust direction is controlled not by turning the whole reflector, but by turning a small mirror on which this radiation has been focused. Whilst this does not fully remove the coupling of the magnitude and direction of the thrust, it means that thrust is proportional to the cosine, rather than the cosine squared of its direction, an alteration which increases the magnitude of the thrust available at all angles.
Alternatives Electric solar wind
Pekka Janhunen from FMI has proposed a type of solar sail called the electric solar wind sail. Mechanically it has little in common with the traditional solar sail design. The sails are replaced with straightened conducting tethers (wires) placed radially around the host ship. The wires are electrically charged to create an electric field around the wires. The electric field extends a few tens of metres into the plasma of the surrounding solar wind. The solar protons are reflected by the electric field (like the photons on a traditional solar sail). The radius of the sail is from the electric field rather than the actual wire itself, making the sail lighter. The craft can also be steered by regulating the electric charge of the wires. A practical electric sail would have 50–100 straightened wires with a length of about 20 km each.
Electric solar wind sails can adjust their electrostatic fields and sail attitudes.
Magnetic
A magnetic sail would also employ the solar wind. However, the magnetic field deflects the electrically charged particles in the wind. It uses wire loops, and runs a static current through them instead of applying a static voltage.
All these designs maneuver, though the mechanisms are different.
Magnetic sails bend the path of the charged protons that are in the solar wind. By changing the sails' attitudes, and the size of the magnetic fields, they can change the amount and direction of the thrust.
Physical principles for reflective sails
Solar radiation pressure
The force imparted to a solar sail arises from the momentum of photons. The momentum of a photon or an entire flux is given by Einstein's relation:Wright, Appendix A
where p is the momentum, E is the energy (of the photon or flux), and c is the speed of light. Specifically, the momentum of a photon depends on its wavelength
Solar radiation pressure can be related to the irradiance (solar constant) value of 1361 W/m2 at 1 AU (Earth-Sun distance), as revised in 2011:
perfect absorbance: F = 4.54 μN per square metre (4.54 μPa) in the direction of the incident beam (a perfectly inelastic collision) perfect reflectance: F = 9.08 μN per square metre (9.08 μPa) in the direction normal to surface (an elastic collision)
An ideal sail is flat and has 100% specular reflection. An actual sail will have an overall efficiency of about 90%, about 8.17 μN/m2, due to curvature (billow), wrinkles, absorbance, re-radiation from front and back, non-specular effects, and other factors.