Solar sail
Solar 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 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 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.
The total force exerted on an solar sail, for example, is about at Earth's distance from the Sun, making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines, but as it uses no propellant, that force is exerted almost constantly and the collective effect over time is great enough to be considered a potential manner of propelling spacecraft.
History of concept
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." 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.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, published in 1865, wrote "there will some day appear velocities far greater than these , of which light or electricity will probably be the mechanical agent... we shall one day travel to the moon, the planets, and the stars." 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; 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.
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".
Friedrich Zander 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".
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".
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."
Arthur C. Clarke wrote Sunjammer, a science fiction short story originally published in the March 1964 issue of Boys' Life 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.Diffractive
In 2018, diffraction was proposed as a different solar sail propulsion mechanism, which is claimed to have several advantages.Alternatives
Electric solar wind
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 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. 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:where p is the momentum, E is the energy, 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 value of 1361 W/m2 at 1 AU, as revised in 2011:
- perfect absorbance: F = 4.54 μN per square metre in the direction of the incident beam
- perfect reflectance: F = 9.08 μN per square metre in the direction normal to surface
The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun, and by the square of the cosine of the angle between the sail force vector and the radial from the Sun, so
where R is distance from the Sun in AU. An actual square sail can be modelled as:
Note that the force and acceleration approach zero generally around θ = 60° rather than 90° as one might expect with an ideal sail.
If some of the energy is absorbed, the absorbed energy will heat the sail, which re-radiates that energy from the front and rear surfaces, depending on the emissivity of those two surfaces.
Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure of about 3 to 4 nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.
Sail parameters
Sail loading is an important parameter, which is the total mass divided by the sail area, expressed in g/m2. It is represented by the Greek letter σ.A sail craft has a characteristic acceleration, ac, which it would experience at 1 AU when facing the Sun. Note this value accounts for both the incident and reflected momentums. Using the value from above of 9.08 μN per square metre of radiation pressure at 1 AU, ac is related to areal density by:
Assuming 90% efficiency, ac = 8.17 / σ mm/s2
The lightness number, λ, is the dimensionless ratio of maximum vehicle acceleration divided by the Sun's local gravity. Using the values at 1 AU:
The lightness number is also independent of distance from the Sun because both gravity and light pressure fall off as the inverse square of the distance from the Sun. Therefore, this number defines the types of orbit maneuvers that are possible for a given vessel.
The table presents some example values. Payloads are not included. The first two are from the detailed design effort at JPL in the 1970s. The third, the lattice sailer, might represent about the best possible performance level. The dimensions for square and lattice sails are edges. The dimension for heliogyro is blade tip to blade tip.
| Type | σ | ac | λ | Size |
| Square sail | 5.27 | 1.56 | 0.26 | 0.820 |
| Heliogyro | 6.39 | 1.29 | 0.22 | 15 |
| Lattice sailer | 0.07 | 117 | 20 | 0.840 |