Sunlight pushes gently on spacecraft with large, lightweight, mirrored solar sails. They enable travel throughout the solar system and beyond without rocket propellant.
Solar sails/ light sails/ photon sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds.
In 2010, IKAROS was the world's first spacecraft designed to use solar sailing propulsion to be successfully launched.
HOW IT WORKS??
There are two sources of solar forces. The first is radiation pressure, and the second is due to solar wind. The radiation pressure is much stronger than the wind pressure.
In 1924, the Russian space engineer Friedrich Zander proposed that, since light provides a small amount of thrust, this effect could be used as a form of space propulsion requiring no fuel. Einstein proposed – and experiments confirm – that photons have a momentum p=E/c, hence each light photon absorbed by or reflecting from a surface exerts a small amount of radiation pressure. This results in forces of about 4.57x10−6 N/m2 for absorbing surfaces perpendicular to the radiation in earth orbit, and twice as much if the radiation is reflected. This was proven experimentally by Russian physicist Peter Lebedev in 1900, and independently by Nichols and Hull at Dartmouth in 1901 using a Nichols radiometer.
Charged particles from the solar wind are able to knock out power grids on Earth, and point the tails of comets away from the sun. The solar wind averages 6.7 billion tons per hour at 520 km/s with "slow" low energy coronal ejections reaching 400 km/s and "fast" higher energy ejections averaging 750 km/s. At the distance of the earth, this results in average solar wind pressure of 3.4×10−9 N/m2, three orders of magnitude less than the photonic radiation pressure. Still the solar wind dominates many phenomena because its interaction cross section with gases and charged particles is about 109 times larger than that of the photons.
Both of these forces are small and decrease with the inverse square distance from the sun. Even large sails produce minute acceleration, but over time, sails can build up considerable speeds. Because the force on the sails and the force of gravity from the sun both vary as inverse square functions, solar sail vessels can be rated by the ratio of the sail's force divided by the gravitationalforce. Solar sail vessels with the same rating are able to follow the same trajectories.
Changing course trajectories can be accomplished in two ways. First, tilting the sail with respect to the light source changes the direction of acceleration because the force on a sail from reflected radiation and wind acts in a direction perpendicular to its surface. Smaller auxiliary vanes can be used to gently pull the main sail into its new position. Second, gravity from a nearby mass, such as a star or planet, will alter the direction of a spaceship. When orbiting a star or planet, sails can be used to slow down and spiral inward, or to increase the velocity and spiral outward. If the planet has moons or the star has planets, these techniques can be used to achieve slingshots around these bodies.
Now lets go into a little calculations.
First we need to find the net force on our sail. We will certainly have to deal with gravitational forces (which will slow us down) :
Fg = -GM.m/r2
where big M is the mass of the sun and little m is the mass of the sail. Now we need to find the radiation force on the sail. Since force is just rate of change of momentum, we can find the change of momentum of one photon per unit time, then find how many photons are hitting our sail. So for one elastic collision of a photon with the sail, the change in momentum will be
Δp = 2hν/c
and by conservation of momentum, this will also be the momentum gained by the sail.
SAIL MATERIALS
The material developed for the Drexler solar sail was a thin aluminum film with a baseline thickness of 0.1 micrometres, 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.
The most common material in current designs is aluminized 2 µm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side. The sails of Cosmos 1 were made of aluminized PET film (Mylar).
Research by Dr. Geoffrey Landis in 1998-9, 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.
In 2000, Energy Science Laboratories developed a new carbon fiber material which might be useful for solar sails. 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 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 g/m²,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 g/m², aluminized Kapton films have a mass as much as 12 g/m², and Energy Science Laboratories' new carbon fiber material masses 3 g/m².
APPLICATIONS
Robert L. Forward pointed out that a solar sail could be used to modify the orbit of a satellite around 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 about the Sun that 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 potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations.
Such a spacecraft could conceivably be placed directly over a pole of the Sun, and remain at that station for lengthy durations. Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity.
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 focussed on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material.
TRAJECTORY CORRECTIONS
The MESSENGER probe en route to Mercury is using light pressure reacting against its solar panels to perform fine trajectory corrections. By changing the angle of the solar panels relative to the sun, the amount of solar radiation pressure can be varied to adjust the spacecraft trajectory more delicately than is possible with thrusters. Minor errors are greatly amplified by gravity assist maneuvers, so very small corrections before lead to large savings in propellant afterward.
INTERSTELLAR FLIGHT
In the 1980s, 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 Flight of the Dragonfly, Forward described a light sail propelled by superlasers. 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. Second, they would demand more energy than the Earth currently consumes. Third, Forward's own solution to the electrical problem requires enormous solar panel arrays to be built at or near the planet Mercury. Fourth, a planet-sized mirror or fresnel lens would be needed several dozen astronomical units from the Sun to keep the lasers focused on the sail. Fifth, 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, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design would use a maser to drive it. Masers spread out more rapidly than optical lasers owing to their longer wavelength, and so would not have as long 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. 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, designs have considered the use of a large zone plate. This would be placed at a location between the laser or maser and the spacecraft. The plate could then be propelled outward using the same energy source, thus maintaining its position so as to focus the energy on the solar sail.
Additionally, it has been theorized by da Vinci Project contributor T. Pesando that solar sail-utilizing spacecraft successful in interstellar travel could be used to carry their own zone plates or perhaps even masers to be deployed during flybys at nearby stars. Such an endeavor could allow future solar-sailed craft to effectively utilize focused energy from other stars rather than from the Earth or Sun, thus propelling them more swiftly through space and perhaps even to more distant stars. However, the potential of such a theory remains uncertain if not dubious due to the high-speed precision involved and possible payloads required.
Another more physically realistic approach would be to use the light from the home star to accelerate. The ship would first orbit continuously away around the home star until the appropriate starting velocity is reached, then the ship would begin its trip away from the system using the light from the star to keep accelerating. Beyond some distance, the ship would no longer receive enough light to accelerate it significantly, but would maintain its course due to inertia. When nearing the target star, the ship could turn its sails toward it and begin to orbit inward to decelerate. Additional forward and reverse thrust could be achieved with more conventional means of propulsion such as rockets.
FUTURE APPROACHES
Despite the losses of Cosmos 1 and NanoSail-D (which were due to failure of their launchers), scientists and engineers around the world remain encouraged and continue to work on solar sails. While most direct applications created so far intend to use the sails as inexpensive modes of cargo transport, some scientists are investigating the possibility of using solar sails as a means of transporting humans. This goal is strongly related to the management of very large (i.e. well above 1 km²) surfaces in space and the sail making advancements. Thus, in the near/medium term, solar sail propulsion is aimed chiefly at accomplishing a very high number of non-crewed missions in any part of the solar system and beyond.
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