Solar sail

Solar sails (also called light sails or photon sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is about 10^-5 Pa at Earth's distance from the Sun [ [http://www.tsinghua.edu.cn/docsn/lxx/mainpage/a/Web/index_files/Page567.htm Solar Sail ] ] and decreases by the square of the distance from the light source (e.g. sun), but unlike rockets, solar sails require no reaction mass. Although the thrust is small, it continues as long as the light source shines and the sail is deployed. In theory a lightsail (actually a system of lightsails) powered by an Earth-based laser could even be used to decelerate the spacecraft as it approaches its destination.

Solar collectors, temperature-control panels and sun shades are occasionally used as expedient solar sails, to help ordinary spacecraft and satellites make minor attitude control corrections and orbit modifications without using fuel. This conserves fuel that would otherwise be used for maneuvering and altitude control. A few have even had small purpose-built solar sails for this use. For example, EADS Astrium's Eurostar E3000 geostationary communications satellites use solar sail panels attached to their solar cell arrays to off-load transverse angular momentum, thereby saving fuel (angular momentum is accumulated over time as the gyroscopic momentum wheels control the spacecraft's attitude - this excess momentum must be offloaded to protect the wheels from overspin). Some unmanned spacecraft (such as Mariner 10 [ [http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1973-085A] NASA's Mariner 10 Mission Description] ) have substantially extended their service lives with this practice.

The science of solar sails is well-proven, but the technology to manage large solar sails is still undeveloped. Mission planners are not yet willing to risk multimillion dollar missions on unproven solar sail unfolding and steering mechanisms. This neglect has inspired some enthusiasts to attempt private development of the technology, such as the Cosmos 1.

The concept was first proposed by German astronomer Johannes Kepler in the seventeenth century. It was again proposed by Friedrich Zander in the late 1920s and gradually refined over the decades. Recent serious interest in lightsails began with an article by engineer and science fiction author Robert L. Forward in 1984.

How they work

The spacecraft arranges a large membrane mirror which reflects light from the Sun or some other source. The radiation pressure on the mirror provides a small amount of thrust by reflecting photons. Tilting the reflective sail at an angle from the Sun produces thrust at an angle normal to the sail. In most designs, steering would be done with auxiliary vanes, acting as small solar sails to change the attitude of the large solar sail (see the vanes on the illustration labeled "Cosmos 1", below). The vanes would be adjusted by electric motors. [cite
author = Jerome Wright
date = 1992
title = Space Sailing
publisher = Gordon and Breach Science Publishers, chapter 1]

In theory a lightsail driven by a laser or other beam from Earth can be used to decelerate a spacecraft approaching a distant star or planet, by detaching part of the sail and using it to focus the beam on the forward-facing surface of the rest of the sail. cite journal | author=Forward, R.L. | title=Roundtrip Interstellar Travel Using Laser-Pushed Lightsails | journal=J Spacecraft | volume=21 | issue=2 | pages=187–195 | date=1984 | doi=10.2514/3.8632 ] In practice, however, most of the deceleration would happen while the two parts are at a great distance from each other, and that means that, to do that focusing, it would be necessary to give the detached part an accurate optical shape and orientation.

Sails orbit, and therefore do not need to hover or move directly toward or away from the sun. Almost all missions would use the sail to change orbit, rather than thrusting directly away from a planet or the sun. The sail is rotated slowly as the sail orbits around a planet so the thrust is in the direction of the orbital movement to move to a higher orbit or against it to move to a lower orbit. When an orbit is far enough away from a planet, the sail then begins similar maneuvers in orbit around the sun. [Wright, ibid., chapter 2]

The best sort of missions for a solar sail involves a dive near the sun, where the light is intense, and sail efficiencies are high. Going close to the Sun may be done for different mission aims: for exploring the solar poles from a short distance, for observing the Sun and its near environment from a non-Keplerian circular orbit the plane of which may be shifted some solar radii, for flying by the Sun such that the sail gets a very high speed.

An unsuspected feature, until the first half of the 1990s, of the solar sail propulsion is to allow a sailcraft to escape the solar system with a cruise speed higher or even much higher than a spacecraft powered by a nuclear electric rocket system. The spacecraft mass-to-sail area ratio does not need to achieve ultra-low values, even though the sail should be an advanced all-metal sail. This flight mode is also known as fast solar sailing. Proven mathematically (like many other astronautical items well in advance of their actual launches), such sailing mode has been considered by NASA/Marshall as one of the options for a future precursor interstellar probe exploring the near interstellar space beyond the heliosphere. [(NASA/CR 2002-211730, chapter IV) ]

Most theoretical studies of interstellar missions with a solar sail plan to push the sail with a very large laser Beam-powered propulsion Direct Impulse beam. The thrust vector (spatial vector) would therefore be away from the Sun and toward the target.

Limitations of solar sails

Solar sails don't work well, if at all, in low Earth orbit below about 800 km altitude due to erosion or air drag. [ [http://www.launchloop.com/isdc2002energy.pdf Launchloop's discussion of launch altitudes] ] Above that altitude they give very small accelerations that take months to build up to useful speeds. Solar sails have to be physically large, and payload size is often small. Deploying solar sails is also highly challenging to date.

Solar sails must face the sun to decelerate. Therefore, on trips away from the sun, they must arrange to loop behind the outer planet, and decelerate into the sunlight. [Wright, ibid. pg 15, 'Braking by Solar Pressure']

There is a common misunderstanding that solar sails cannot go towards their light source. This is false. In particular, sails can go toward the sun by thrusting against their orbital motion. This reduces the energy of their orbit, spiraling the sail toward the sun, see Tack (sailing). [Wright, ibid, chapter 2]

Investigated sail designs

"Parachutes" would have very low mass, but theoretical studies show that they will collapse from the forces placed by shrouds. Radiation pressure does not behave like aerodynamic pressure. [Wright, ibid., p71, last para]

The highest thrust-to-mass designs known (2007) were theoretical designs developed by Eric Drexler. [Drexler, K.E., (1977) Design of a High Performance Solar Sail System, MS Thesis, Dept. of Aeronautics and Astronautics, Massachusetts Institute of Techniology, Boston] He designed a sail using reflective panels of thin aluminum film (30 to 100 nanometres thick) supported by a purely tensile structure. It rotated and would have to be continually under slight thrust. He made and handled samples of the film in the laboratory, but the material is too delicate to survive folding, launch, and deployment, hence the design relied on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer accelerations an order of magnitude higher than designs based on deployable plastic films.

The highest-thrust to mass designs for ground-assembled deployable structures are square sails with the masts and guy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guide wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the sun. This form can therefore go quite close to the sun, where the maximum thrust is present. Control would probably use small sails on the ends of the spars.

In the 1970s JPL did extensive studies of rotating blade and rotating ring sails for a mission to rendezvous with Halley's Comet. The intention was that such structures would be stiffened by their angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure. So the difference in the thrust-to-mass ratio was almost nil, and the static designs were much easier to control.

JPL's reference design was called the "heliogyro" and had plastic-film blades deployed from rollers and held out by centripetal forces as it rotated. The spacecraft's altitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cycle and collective pitch of a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design.

JPL also investigated "ring sails" (Spinning Disk Sail in the above diagram), panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Weights in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large manned structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to the gravity on the surface of Mars.

A solar sail can serve a dual function as a high-gain antenna. Designs differ, but most modify the metallization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest, including visible light.

Pekka Janhunen from FMI has invented a type of solar wind sail called the electric solar wind sail [http://www.space.fmi.fi/~pjanhune/Esail/] . It has little in common with the solar wind sail design externally, bacause the sails are substituted with straigthened conducting tethers (wires) which are placed radially around the host ship. The wires are electrically charged and thus an electric field is created around the wires. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Because the solar wind electrons react on the electric field similarly as on a concrete solar wind sail, the function radius of the wires is based on the electric field that is generated around the wire rather than the actual wire itself. This fact also makes it possible to maneuver a ship with electric solar wind sail by regulating the electric charge of the wires. A full-sized functioning electric solar wind sail would have 50-100 straightened wires with a length of about 20 km each.

ail testing in space

NASA has successfully tested deployment technologies on small scale sails in vacuum chambers. [http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html]

No solar sails have been successfully used in space as primary propulsion systems, but research in the area is continuing. It is noteworthy that the Mariner 10 mission, which flew by the planets Mercury and Venus, demonstrated use of solar pressure as a method of attitude control, in order to conserve attitude-control propellant.

On February 4, 1993, Znamya 2, a 20-meter wide aluminized-mylar reflector, was successfully tested from the Russian Mir space station. Although the deployment test was successful, the experiment only demonstrated the deployment, not propulsion. A second test, Znamaya 2.5, failed to deploy properly.

On August 9, 2004 Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film. The experiment was purely a test of the deployment mechanisms, not of propulsion. [http://www.planetary.org/news/2004/0810_Japanese_Researchers_Successfully_Test.html]

A joint private project between Planetary Society, Cosmos Studios and Russian Academy of Science launched "Cosmos 1" on June 21, 2005, from a submarine in the Barents Sea, but the Volna rocket failed, and the spacecraft failed to reach orbit. A solar sail would have been used to gradually raise the spacecraft to a higher earth orbit. The mission would have lasted for one month. A suborbital prototype test by the group failed in 2001 as well, also because of rocket failure.

A 15-meter-diameter solar sail (SSP, solar sail sub payload, "soraseiru sabupeiro-do") was launched together with ASTRO-F on a M-V rocket on February 21, 2006, and made it to orbit. It deployed from the stage, but opened incompletely. [ [http://www.skyrocket.de/space/doc_sdat/ssp.htm SSSat 1, 2 ] ]

A team from the NASA Marshall Space Flight Center (Marshall), along with a team from the NASA Ames Research Center, developed a solar sail mission called NanoSail-D which was lost in a launch failure aboard a Falcon 1 rocket on 3 August 2008. [ [http://www.nasaspaceflight.com/content/?cid=5482 NASASpaceflight.com - SpaceX Falcon I FAILS during first stage flight] ] [cite web |url=http://science.nasa.gov/headlines/y2008/26jun_nanosaild.htm?list1097511 |title=NASA to Attempt Historic Solar Sail Deployment |publisher=NASA |date=2008-06-26] The primary objective of the mission had been to test sail deployment technologies. The spacecraft might not have returned useful data about solar sail propulsion, according to Edward E. Montgomery, technology manager of Solar Sail Propulsion at Marshall, "The orbit available to us in this launch opportunity is so low, it may not allow us to stay in orbit long enough for solar pressure effects to accumulate to a measurable degree." [cite web |url=http://science.nasa.gov/headlines/y2008/31jul_solarsails.htm?list994055 |title= A Brief History of Solar Sails |date=2008-07-31 |publisher=NASA] The NanoSail-D structure was made of aluminum and plastic, with the spacecraft weighing less than convert |10|lb|kg. The sail has about convert|100|sqft|m2 of light-catching surface.

ail materials

The best known material is thought to be a thin mesh of aluminium with holes less than ½ the wavelength of mostFact|date=September 2008 light. Nanometre-sized "antennas" would emit heat energy as infrared. Although samples have been created, it is too fragile to unfold or unroll with known technology.

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.

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 aluminum or Kapton films. [http://www.niac.usra.edu/files/studies/final_report/4Landis.pdf]

In 2000, Energy Science Laboratories developed a new carbon fiber material which might be useful for solar sails. [http://www.space.com/businesstechnology/technology/carbonsail_000302.html SPACE.com Exclusive: Breakthrough In Solar Sail Technology ] ] The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same weight. 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 ½ the wavelength of light impinging on the sail. While such materials have as-of-yet only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could weigh less than 0.1 g/m² [ [http://www.physorg.com/news5890.html Researchers produce strong, transparent carbon nanotube sheets ] ] making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick Mylar sail material weighs 7 g/m², aluminized Kapton films weighs up to 12 g/m², [http://solarsails.jpl.nasa.gov/introduction/design-construction.html] and Energy Science Laboratories' new carbon fiber material weighs in at 3g/m².

Applications

tatites

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.

Interstellar Flight

Robert Forward proposed the use of lasers to push solar sails, providing beam-powered propulsion. Given a sufficiently powerful laser and a large enough mirror to keep the laser focused on the sail for long enough, a solar sail could be accelerated to a significant fraction of the speed of light. To do so, however, would require the engineering of massive, precisely-shaped optical mirrors or lenses (wider than the Earth for interstellar transport), incredibly powerful lasers, and more power for the lasers than humanity currently generates.

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 thanks 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. [ [http://www.space.com/businesstechnology/technology/technovel_sail_050211.html SPACE.com - Earth To Mars in a Month With Painted Solar Sail ] ] 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 endeavour 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.

Future Visions

Despite the loss of "Cosmos 1" (which was due to a failure of the launcher), 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.

Misunderstandings

Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. However, when in Earth orbit, the majority of mass on most interplanetary missions is taken up by fuel. A robotic solar sail could therefore multiply an interplanetary payload by several times by reducing this significant fuel mass, and create a reusable, multimission spacecraft. Most near-term planetary missions involve robotic exploration craft, in which the directness of the course is unimportant compared to the fuel mass savings and fast transit times of a solar sail. For example, most existing missions use multiple gravitational slingshots to reduce necessary fuel mass, in order to save transit time at the cost of directness of the route.

There is also a misunderstanding that solar sails capture energy primarily from the solar wind high speed charged particles emitted from the sun. These particles would impart a small amount of momentum upon striking the sail, but this effect would be small compared to the force due to radiation pressure from light reflected from the sail. The force due to light pressure is about 5,000 times as strong as that due to solar wind. A much larger type of sail called a magsail would employ the solar wind.

It has been proposed that momentum exchange from reflection of photons is an unproven effect that may violate the thermodynamical Carnot rule. This criticism was raised by Thomas Gold of Cornell, leading to a public debate in the spring of 2003. [ [http://www.newscientist.com/news/news.jsp?id=ns99993895 Solar sailing 'breaks laws of physics' - 04 July 2003 - New Scientist ] ] This criticism has been refuted by Benjamin Diedrich, pointing out that the Carnot Rule does not apply to an open system. Further explanation of lab results demonstrating is provided. [http://www.ugcs.caltech.edu/~diedrich/solarsails/newscientistletter.html] James Oberg has also refuted Dr. Gold's analysis: "But ‘solar sailing’ isn’t theoretical at all, and photon pressure has been successfully calculated for all large spacecraft. Interplanetary missions would arrive thousands of kilometers off courseif correct equations had not been used. The effect for a genuine ‘solar sail’ will be even morespectacular." [http://www.jamesoberg.com/sunbeam_ast.pdf]

One way to see the conservation of energy is not a problem is to note that when reflected by a solar sail, a photon undergoes a Doppler shift; its wavelength increases (and energy decreases) by a factor dependent on the velocity of the sail, transferring energy from the sun-photon system to the sail. [ [http://blag.xkcd.com/2008/02/15/the-laser-elevator/ "The Laser Elevator" by Randall Munroe ] ] This change of energy can easily be verified to be exactly equal (and opposite) to the energy change of the sail.

Mathematical survey

The Extended Heliocentric Reference Frame

* In the 1991-92 the classical equations of the solar sail motion in the solar gravitational field were written by using a different mathematical formalism, namely, "the lightness vector" fully characterizing the sailcraft dynamics. In addition, solar-sail spacecraft has been supposed to be able to reverse its motion (in the solar system) provided that its sail were sufficiently light that "sailcraft sail loading" (σ) is not higher than 2.1 g/m². This value entails a high-performance technology indeed, but much probably within the capabilities of emerging technologies.
* For describing the concept of fast sailing and some related items, we need to define two frames of reference. The first is an inertial Cartesian coordinate system centred on the Sun or a heliocentric inertial frame (HIF, for short). For instance, the plane of reference, or the XY plane, of HIF can be the mean ecliptic at some standard epoch such as J2000. The second Cartesian reference frame is the so-called heliocentric orbital frame (HOF, for short) with the origin in the sailcraft barycenter. The x-axis of HOF is the direction of the Sun-to-sailcraft vector, or position vector, the z-axis is along the sailcraft orbital angular momentum, whereas the y-axis completes the counterclockwise triad. Such definition can be extended to sailcraft trajectories, including both counterclockwise and clockwise arcs of motion, such a way HOF is always a continuous positively-oriented triad. The sail orientation unit vector (defined in sailcraft), say, n can be specified in HOF by a pair of angles, e.g. the azimuth α and the elevation δ. Elevation is the angle that n forms with the xy-plane of HOF (-90° ≤ δ ≤ 90°). Azimuth is the angle that the projection of n onto the HOF xy-plane forms with the HOF x-axis (0 ≤ α < 360 °). In HOF, azimuth and elevation are equivalent to longitude and latitude, respectively.

* The sailcraft lightness vector L = [λ_r , λ_t , λ_n] depends on α and δ (non-linearly) and the thermo-optical parameters of the sail materials (linearly). Neglecting a small contribution coming from the aberration of light, one has the following particular cases (irrespective of the sail material):
# α = 0 , δ = 0 ⇔ [λ_r , 0 , 0] ⇔ λ=|L|=λ_r
# α ≠ 0 , δ = 0 ⇔ [λ_r , λ_t , 0]
# α = 0 , δ ≠ 0 ⇔ [λ_r , 0 , λ_n]

A Flight Example

Conventional strategy

* Now suppose we have built a sailcraft with an all-metal sail of Aluminium and Chromium such that σ = 2 g/m². A launcher delivers the (packed) sailcraft at some million kilometers from the Earth. There, the whole sailcraft is deployed and begins its flight in the solar system (here, for the sake of simplicity, we neglect any gravitational perturbation from planets). A conventional spacecraft would move approximately in a circular orbit at about 1 AU from the Sun. In contrast, a sailcraft like this one is sufficiently light to be able to escape the solar system or to point to some distant object in the heliosphere. If n is parallel to the local sun-light direction, then λ_r = λ = 0.725 (i.e. 1/2 < λ < 1); as a result, this sailcraft moves on a hyperbolic orbit. Its speed "at infinity" is equal to 20 km/s. Strictly speaking, this potential solar sail mission would be faster than the current record speed for missions beyond the planetary range, namely, the Voyager-1 speed, which amounts to 17 km/s or about 3.6 AU/yr (1 AU/yr = 4.7404 km/s). However, three kilometers per second are not meaningful in the context of very deep space missions.
* As a consequence, one has to resort to some L having more than one component different from zero. The classical way to gain speed is to tilt the sail at some suitable "positive" α. If α= +21°, then the sailcraft begins by accelerating; after about two months, it achieves 32 km/s. However, this is a speed peak inasmuch as its subsequent motion is characterized by a monotonic speed decrease towards an asymptotic value, or the cruise speed, of 26 km/s. After 18 years, the sailcraft is 100 AU away from the Sun. This would mean a pretty fast mission. However, considering that a sailcraft with 2 g/m² is technologically advanced, is there any other way to increase its speed significantly? Yes, there is. Let us try to explain this effect of non-linear dynamics.

Optimal Strategy

* The above figures show that spiralling out from a circular orbit is not a convenient mode for a sailcraft to be sent away from the Sun since it would not have a high enough excess speed. On the other hand, it is known from astrodynamics that a conventional Earth satellite has to perform a rocket maneuver at/around its perigee for maximizing its speed at "infinity". Similarly, one can think of delivering a sailcraft close to the Sun to get much more energy from the solar photon pressure (that scales as 1/R²). For instance, suppose one starts from a point at 1 AU on the ecliptic and achieves a perihelion distance of 0.2 AU in the same plane by a two-dimensional trajectory. In general, there are three ways to deliver a sailcraft, initially at R₀ from the Sun, to some distance R < R₀:
** using an additional propulsion system to send the folded-sail sailcraft to the perihelion of an elliptical orbit; there, the sail is deployed with its axis parallel to the sun-light for getting the maximum solar flux at the chosen distance;
** spiralling in by α slightly negative, namely, via a slow deceleration;
** strongly decelerating by a "sufficiently large" sail-axis angle negative in HOF.

: The first way - although usable as a good reference mode - requires another high-performance propulsion system.

: The second way is ruled out in the present case of σ = 2 g/m²; as a matter of fact, a small α < 0 entails a λ_r too high and a negative λ_t too low in absolute value: the sailcraft would go far from the Sun with a decreasing speed (as discussed above).

: In the third way, there is a "critical negative" sail-axis angle in HOF, say, α_cr such that for sail orientation angles α < α_cr the sailcraft trajectory is characterized as follows:

:# the distance (from the Sun) first increases, achieves a local maximum at some point M, then decreases. The orbital angular momentum (per unit mass), say, H of the sailcraft decreases in magnitude. It is suitable to define the scalar "H" = H•k, where k is the unit vector of the HIF Z-axis;:# after a short time (few weeks or less, in general), the sailcraft speed "V" = |V| achieves a local minimum at a point P. "H" continues to decrease; :# past P, the sailcraft speed increases because the total vector acceleration, say, A begins by forming an acute angle with the vector velocity V; in mathematical terms, d"V" / dt = A • V / "V" > 0. "This is the first key-point to realize"; :# eventually, the sailcraft achieves a point Q where "H" = 0; here, the sailcraft's total energy (per unit mass), say, "E" (including the contribution of the solar pressure on the sail) shows a (negative) local minimum. "This is the second key-point"; :# past Q, the sailcraft - keeping the negative value of the sail orientation - regains angular momentum by reversing its motion (that is H is oriented down and "H" < 0). R keeps on decreasing while d"V"/dt augments. "This is the third key-point";:# the sailcraft energy continues to increase and a point S is reached where "E"=0, namely, the escape condition is satisfied; the sailcraft keeps on accelerating. "S is located before the perihelion". The (negative) "H" continues to decrease;:# if the sail attitude α has been chosen appropriately (about -25.9 deg in this example), the sailcraft flies-by the Sun at the desired (0.2 AU) perihelion, say, U; however, differently from a Keplerian orbit (for which the perihelion is the point of maximum speed), past the perihelion, "V" increases further while the sailcraft recedes from the Sun.:# past U, the sailcraft is very fast and pass through a point, say, W of local maximum for the speed, since λ < 1. Thus, speed decreases but, at a few AU from the Sun (about 2.7 AU in this example), both the (positive) "E" and the (negative) "H" begin a plateau or cruise phase; "V" becomes practically constant and, the most important thing, takes on a cruise value considerably higher than the speed of the circular orbit of the departure planet (the Earth, in this case). This example shows a cruise speed of 14.75 AU/yr or 69.9 km/s. At 100 AU, the sailcraft speed is 69.6 km/s.

H-reversal sun flyby trajectory

: The Figure below shows the mentioned sailcraft trajectory. Only the initial arc around the Sun has been plotted. The remaining part is rectilinear, in practice, and represents the cruise phase of the spacecraft. The sail is represented by a short segment with a central arrow that indicates its orientation. Note that the complicate change of sail direction in HIF is very simply achieved by a constant attitude in HOF. That brings about a net non-Keplerian feature to the whole trajectory.

Some remarks are in order.

* As mentioned in point-3, the strong sailcraft speed increase is due to both the solar-light thrust and gravity acceleration vectors. In particular, d"V" / dt, or the along-track component of the total acceleration, is positive and particularly high from the point-Q to the point-U. This suggests that if a quick sail attitude maneuver is performed just before "H" vanishes, α → -α, the sailcraft motion continues to be a direct motion with a final cruise velocity equal in magnitude to the reversal one (because the above maneuver keeps the perihelion value unchanged). The basic principle both sailing modes "share" may be summarised as follows: "a sufficiently light sailcraft needs to lose most of its initial energy for subsequently achieving the absolute maximum of energy compliant with its given technology".

* The above 2D class of new trajectories represents an ideal case. The realistic 3D fast sailcraft trajectories are considerably more complicated than the 2D cases. However, the general feature of producing a fast cruise speed can be further enhanced. Some of the enclosed references contain strict mathematical algorithms for dealing with this topic. Recently (July 2005), in an international symposium an evolution of the above concept of fast solar sailing has been discussed. A sailcraft with σ = 1 g/m² could achieve over 30 AU/yr in cruise (by keeping the perihelion at 0.2 AU), namely, well beyond the cruise speed of any nuclear-electric spacecraft (at least as conceived today). Such paper has been published on the Journal of the British Interplanetary Society (JBIS) in 2006.

olar sails in fiction

* "On the Waves of Ether (По волнам эфира)" by B. Krasnogorsky 1913, spacecraft propelled by solar light pressure.
* "Sunjammer" by Arthur C. Clarke, a short story (in The Wind from the Sun anthology) describing a solar sail craft Earth-Moon race. It was originally published under the name "Sunjammer" but when Clarke learned of the short story of the same name by Poul Anderson, he quickly changed it.
* "Dust of Far Suns", by Jack Vance, also published as "Sail 25", depicts a crew of space cadets on a training mission aboard a malfunction-ridden solar sail craft.
* "The Mote in God's Eye" (1975) by Larry Niven and Jerry Pournelle depicts an interstellar alien spacecraft driven by laser-powered light sails.
* Both "Green Mars" and "Blue Mars", by Kim Stanley Robinson, contain a solar reflecting mirror called a soletta made of earth to mars solar powered shuttles.
* "Rocheworld" by Robert L. Forward, a novel about an interstellar mission driven by laser-powered light sails.
* In the movie Tron, the characters Tron, Flynn and Yori are using a computer model of a solar sailer to escape from the MCP.
* Solar sails appeared in "", in which Count Dooku has a combination hyperdrive and starsail spacecraft dubbed the Solar Sailer. In episode 6 of the animated TV series , Count Dooku also appears travelling on a space ship equipped with a solar sail.
* In the film "", an officer aboard a crippled spaceship discusses a plan to construct a solar sail to take his ship to the nearest port.
* A solar sail appears in the "" episode "Explorers", as the primary propulsion system of the "". The vessel inadvertently exceeds the speed of light by sailing on a stream of tachyons.
* "The Lady Who Sailed The Soul" by Cordwainer Smith, a short story (part of "The Rediscovery Of Man" collection) describing journeys on solar sail craft.
* In David Brin's Heaven's Reach, sentient machines are using solar sails to harvest carbon from a red giant star's atmosphere to repair a Dyson Sphere-like construct.
* The book "Accelerando" by Charles Stross depicts a solar sail craft powered by a series of very powerful lasers being used to contact alien intelligences outside of our solar system.
* The GSX-401FW Stargazer, a primarily unmanned Gundam mobile suit from the Cosmic Era timeline of the Gundam Seed metaseries, employs a propulsion system dubbed "Voiture Lumière" which utilizes a nano-particle solar sail.
* The R.L.S. "Legacy", seen in the Disney movie "Treasure Planet", was powered entirely by solar sails.
* A solar sail appears in an early episode of the most recent incarnation of "The Outer Limits" (season one, "The Message"). [ [http://www.theouterlimits.com/episodes/season1/111.htm The Outer Limits - The Message ] ] The description refers to it as a planet, perhaps to avoid being a "spoiler".
* The 1983 "Doctor Who" serial Enlightenment depicts a race through the solar system using solar sail ships.

* 1985 Japanese Original Video Animation title ", directed by Eiichi Yamamoto and Takeshi Shirado, features a space ship that travels on beams of light, with its massive sails, over great distances in space.

* The 2006 science-fiction novel Le Papillon Des Étoiles (lit. "The Butterfly Of The Stars"), by Bernard Werber, tells the story of a community of humans who escape from Earth and set off towards a new habitable planet aboard a large spaceship pulled by a gigantic solar sail (one million square kilometers large when deployed).

* A space yacht rigged with solar sails is described in the science-fiction novel "Planet of The Apes" by Pierre Boulle (original 1963 work).

ee also

* Magnetic sail
* Electric sail
* Cosmos 1
* Spacecraft propulsion
* optical tweezers
* Crookes radiometer

References

*G. Vulpetti, L. Johnson, G. L. Matloff, Solar Sails: A Novel Approach to Interplanetary Flight, Springer, August 2008
* "Space Sailing" by Jerome L. Wright, who was involved with JPL's effort to use a solar sail for a rendezvous with Halley's comet.
* "Solar Sailing, Technology, Dynamics and Mission Applications" - [Colin R. McInnes] presents the state of the art in his book.
* "NASA/CR 2002-211730, the chapter IV" - [http://nuke.giovannivulpetti.it/SolarSailing/tabid/56/Default.aspx] presents the theory and the optimal NASA-ISP trajectory via the H-reversal sailing mode
*G. Vulpetti, The Sailcraft Splitting Concept, JBIS, Vol.59, pp. 48-53, February 2006
*G. L. Matloff, Deep-Space Probes: to the Outer Solar System and Beyond, 2nd ed., Springer-Chichester, UK, 2005
*T. Taylor, D. Robinson, T. Moton, T. C. Powell, G. Matloff, and J. Hall, Solar Sail Propulsion Systems Integration and Analysis (for Option Period), Final Report for NASA/MSFC, Contract No. H-35191D Option Period, Teledyne Brown Engineering Inc., Huntsville, AL, May 11, 2004
*G. Vulpetti, Sailcraft Trajectory Options for the Interstellar Probe: Mathematical Theory and Numerical Results, the Chapter IV of NASA/CR-2002-211730, “The Interstellar Probe (ISP): Pre-Perihelion Trajectories and Application of Holography”, June 2002
*G. Vulpetti, Sailcraft-Based Mission to The Solar Gravitational Lens, STAIF-2000, Albuquerque (New Mexico, USA), 30 Jan - 3 Feb, 2000
*G. Vulpetti, General 3D H-Reversal Trajectories for High-Speed Sailcraft, Acta Astronautica, Vol. 44, No. 1, pp. 67-73, 1999
*C. R. McInnes, Solar Sailing: Technology, Dynamics, and Mission Applications, Springer-Praxis Publishing Ltd, Chichester, UK, 1999
*Genta, G., and Brusa, E., The AURORA Project: a New Sail Layout, Acta Astronautica, 44, No. 2-4, pp. 141-146 (1999)
*S. Scaglione and G. Vulpetti, The Aurora Project: Removal of Plastic Substrate to Obtain an All-Metal Solar Sail, special issue of Acta Astronautica, vol. 44, No. 2-4, pp. 147-150, 1999
*J. L. Wright, Space Sailing, Gordon and Breach Science Publishers, Amsterdam, 1993

External links

* [http://www.nasa.gov/mission_pages/smallsats/nanosaild.html NASA Mission Site for NanoSail-D]
* "NanoSail-D" mission: Dana Coulter, [http://science.nasa.gov/headlines/y2008/26jun_nanosaild.htm?list1097511 "NASA to Attempt Historic Solar Sail Deployment"] , NASA, June 28th 2008
* [http://solarsail.jpl.nasa.gov/introduction/how-sails-work.html How Sails Work] from NASA
* [http://www-spof.gsfc.nasa.gov/stargaze/Solsail.htm Far-out Pathways to Space: Solar Sails] from NASA
* [http://www.solarsails.info Solar Sails] Comprehensive collection of solar sail information and references, maintained by Benjamin Diedrich. Good diagrams [http://wiki.solarsails.info/index.php?title=Tacking_Solar_Sails] showing how light sailors must tack.
* [http://www.u3p.net/ U3P] Multilingual site with news and flight simulators
* [http://www.ec-lille.fr/~u3p/bd/bda.html The Wind from the Sun] in graphic novel form
* [http://www.isas.jaxa.jp/e/snews/2004/0809.shtml ISAS Deployed Solar Sail Film in Space]
* [http://science.howstuffworks.com/solarsail.htm How Solar Sail Technology Works] from HowStuffWorks
* [http://www.planetary.org/programs/projects/solar_sailing/ Planetary society site for solar sailing projects]
* [http://www.solar-thruster-sailor.info/figs/fig13.html Suggestion of a solar sail with roller reefing, hybrid propulsion and a central docking and payload station.]
* [http://www.andybrain.com/extras/solar-sail.htm Interview with NASA's JPL about solar sail technology and missions]
* [http://nuke.giovannivulpetti.it/SolarSailing/tabid/56/Default.aspx Website with technical pdf-files about solar-sailing, including NASA report and lectures at Aerospace Engineering School of Rome University]
* [http://72.14.253.104/search?q=cache:qwgqedUymfwJ:www.niac.usra.edu/files/studies/final_report/4Landis.pdf+physics+solar+sails+dopler+shift&hl=en&ct=clnk&cd=41&gl=us Advanced Solar- and Laser-pushed Lightsail Concepts]
* cite web | url=http://www.andrews-space.com/images/videos/PAPERS/Pub-InterstellarTransportation(200307).pdf
title=Interstellar Transportation using Today’s Physics | author=Andrews, D.G. | journal=AIAA Paper 2003-4691 | date=2003 | publisher=American Institute of Aeronautics and Astronautics