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Solar sails also called light sails or photon sails are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the s.
A useful analogy to solar sailing may be a sailing boat; the light exerting a force on the mirrors 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 long operating lifetimes. Since they have few moving parts and use no propellant, they can potentially be used numerous times for 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 Sailboat Vector Simple Group 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 s. Solar pressure also affects the orientation of a spacecraft, a factor that must be included in spacecraft design. The total force exerted on an by metre solar sail, for example, is about 5 newtons 1.
Johannes Kepler observed that comet tails point away from the Sun and suggested that the Sun caused the effect. In a Sailboat Vector Simple Model letter to Galileo in , he wrote, "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.
James Clerk Maxwell , in �, published his theory of electromagnetic fields and radiation, which 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 , 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 , [4] published in , 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 Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in with a torsional balance; [6] Ernest Nichols and Gordon Hull conducted a similar independent experiment in using a Nichols radiometer.
Svante Arrhenius predicted in the possibility of solar radiation pressure distributing life spores across interstellar distances, providing one means to explain the concept of panspermia.
He apparently was 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 and suggested, "using tremendous mirrors of very thin sheets to utilize the pressure of sunlight to attain cosmic velocities".
Friedrich Zander Tsander published a technical paper in 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 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".
Bernal wrote in , "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. Carl Sagan , in the s, 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 at Jet Propulsion Laboratory for a proposed mission to rendezvous with Halley's Comet.
Many people believe that spacecraft using solar sails are pushed by the Solar winds just as sailboats and sailing ships are pushed by the winds across the waters on Earth. The momentum of a photon or an entire flux is given by Einstein's relation : [14] [15]. The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun unless extremely close to the Sun [17] , and by the square of the cosine of the angle between the sail force vector and the radial from the Sun, so.
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. A sail craft has a characteristic acceleration, a c , 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. 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 s.
The third, the lattice sailer, might represent about the best possible performance level. The dimension for heliogyro is blade tip to blade tip. An active attitude control system ACS is essential for a sail craft to achieve and maintain a desired orientation. The required sail orientation changes slowly often less than 1 degree per day in interplanetary space, but much more rapidly in a planetary orbit. The ACS must be capable of meeting these orientation requirements.
Attitude control is achieved by a relative shift between the craft's center of pressure and its center of mass. This can be achieved with control vanes, movement of individual sails, movement of a control mass, or altering reflectivity. Holding a constant attitude requires that the ACS maintain a net torque of zero on the craft. The total force and torque on a sail, or set of sails, is not constant along a trajectory.
The force changes with solar distance and sail angle, which changes the billow in the sail and deflects some elements of the supporting structure, resulting in changes in the sail force and torque. Sail temperature also changes with solar distance and sail angle, which changes sail dimensions. The radiant heat from the sail changes the temperature of the supporting structure.
Both factors affect total force and torque. To hold the desired attitude the ACS must compensate for all of these changes. In Earth orbit, solar pressure and drag pressure are typically equal at an altitude of about km, which means that a sail craft would have to operate above that altitude. Sail craft must operate in orbits where their turn rates are compatible with the orbits, which is generally a concern only for spinning disk configurations.
Sail operating temperatures are a function of solar distance, Sailboat Vector Simple 2020 sail angle, reflectivity, and front and back emissivities. A sail can be used only where its temperature is kept within its material limits. Generally, a sail can be used rather close to the Sun, around 0.
Potential applications for sail craft range throughout the Solar System , from near the Sun to the comet clouds beyond Neptune. The craft can make outbound voyages to deliver loads or to take up station keeping at the destination. They can be used to haul cargo and possibly also used for human travel. For trips within the inner Solar System, they can deliver loads and then return to Earth for subsequent voyages, operating as an interplanetary shuttle.
For Mars in particular, the craft could provide economical means of routinely supplying operations on the planet according to Jerome Wright, "The cost of launching the necessary conventional propellants from Earth are enormous for manned missions. Solar sail craft can approach the Sun to deliver observation payloads or to take up station keeping orbits.
They can operate at 0. They can reach high orbital inclinations, including polar. Solar sails can travel to and from all of the inner planets.
Trips to Mercury and Venus are for rendezvous and orbit entry for the payload. Trips to Mars could be either for rendezvous or swing-by with release of the payload for aerodynamic braking. Minimum transfer times to the outer planets benefit from using an indirect transfer solar swing-by.
However, this method results in high arrival speeds. Slower transfers have lower arrival speeds. For Saturn, the minimum trip time is 3. The Sun's inner gravitational focus point lies at minimum distance of AU from the Sun, and is the point to which light from distant objects is focused by gravity as a result of it passing by the Sun.
This is thus the distant point to which solar gravity will cause the region of deep space on the other side of the Sun to be focused, thus serving effectively as a very large telescope objective lens.
It has been proposed that an inflated sail, made of beryllium , that starts at 0. Such proximity to the Sun could prove to be impractical in the near term due to the structural degradation of beryllium at high temperatures, diffusion of hydrogen at high temperatures as well as an electrostatic gradient, generated by the ionization of beryllium from the solar wind, posing a burst risk.
A revised perihelion of 0. Robert L. Forward has commented that a solar sail could be used to modify the orbit of a satellite about the Earth. In the limit, a sail could be used to "hover" a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits such that they are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a " statite ". This is possible because the propulsion provided by the sail offsets the gravitational attraction of the Sun.
Such an orbit could be useful for studying the properties of the Sun for long durations. In his book The Case for Mars , Robert Zubrin points out that the reflected sunlight from a large statite, placed near the polar terminator of the planet Mars, could be focused on one of the Martian polar ice caps to significantly warm the planet's atmosphere.
Such a statite could be made from asteroid material. Minor errors are greatly amplified by gravity assist maneuvers, so using radiation pressure to make very small corrections saved large amounts of propellant.
In the s, Robert Forward proposed two beam-powered propulsion schemes using either lasers or masers to push giant sails to a significant fraction of the speed of light.
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.
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