Solar Sails

ABSTRACT

Hundreds of space missions have been launched since the last lunar mission, including several deep space probes that have been sent to the edges of our solar system. However, our journeys to space have been limited by the power of chemical rocket engines and the amount of rocket fuel that a spacecraft can carry. Today, the weight of a space shuttle at launch is approximately 95 percent fuel. What could we accomplish if we could reduce our need for so much fuel and the tanks that hold it? International space agencies and some private corporations have proposed many methods of transportation that would allow us to go farther, but a manned space mission has yet to go beyond the moon. The most realistic of these space transportation options calls for the elimination of both rocket fuel and rocket engines -- replacing them with sails. Yes, that's right, sails. Solar-sail mission analysis and design is currently performed assuming constant optical and mechanical properties of the thin metalized polymer films that are projected for solar sails. More realistically, however, these properties are likely to be affected by the damaging effects of the space environment. The standard solar-sail force models can therefore not be used to investigate the consequences of these effects on mission performance. The aim of this paper is to propose a new parametric model for describing the sail film's optical degradation with time. In particular, the sail film's optical coefficients are assumed to depend on its environmental history, that is, the radiation dose. Using the proposed model, the optimal control laws for degrading solar sails are derived using an indirect method and the effects of different degradation behaviors are investigated for an example interplanetary mission.

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CONTENTS 1. Introduction

2. Solar Sail Concept 3. Solar Sail Construction 4. Solar Sail Dynamics and control 4.1 Cruising by Sunlight

5. Solar Sail Material 5.1 Aluminium as Material 5.1.1Titainum as reinforcing material 5.1.2Siliconmonoxide as reinforcing material 5.1.3Boron as reinforcing material

6. Solar Sail Launch 7. Investigated Sail Design 8. Cosmos-1 Spacecraft Design 9. Advantages 10. Limitations 11. Misunderstandings 12. Future Outlook 13. References

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1. INTRODUCTION

Hundreds of space missions have been launched since the last lunar mission, including several deep space probes that have been sent to the edges of our solar system. However, our journeys to space have been limited by the power of chemical rocket engines and the amount of rocket fuel that a spacecraft can carry. Today, the weight of a space shuttle at launch is approximately 95 percent fuel. What could we accomplish if we could reduce our need for so much fuel and the tanks that hold it? International space agencies and some private corporations have proposed many methods of transportation that would allow us to go farther, but a manned space mission has yet to go beyond the moon. The most realistic of these space transportation options calls for the elimination of both rocket fuel and rocket engines -- replacing them with sails. NASA is one of the organizations that has been studying this amazing technology called solar sails that will use the sun's power to send us into deep space.

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2. SOLAR SAIL CONCEPT Nearly 400 years ago, as much of Europe was still involved in naval exploration of the world, Johannes Kepler proposed the idea of exploring the galaxy using sails. Through his observation that comet tails were blown around by some kind of solar breeze, he believed sails could capture that wind to propel spacecraft the way winds moved ships on the oceans. While Kepler's idea of a solar wind has been disproven, scientists have since discovered that sunlight does exert enough force to move objects. To take advantage of this force, NASA has been experimenting with giant solar sails that could be pushed through the cosmos by light. There are three components to a solar sail-powered spacecraft: •

Continuous force exerted by sunlight

•

A large, ultrathin mirror

•

A separate launch vehicle

A solar sail-powered spacecraft does not need traditional propellant for power, because its propellant is sunlight and the sun is its engine. Light is composed of electromagnetic radiation that exerts force on objects it comes in contact with. NASA researchers have found that at 1 astronomical unit (AU), which is the distance from the sun to Earth, equal to 93 million miles (150 million km), sunlight can produce about 1.4 kilowatts (kw) of power. If you take 1.4 kw and divide it by the speed of light, you would find that the force exerted by the sun is about 9 newtons (N)/square mile (i.e., 2 lb/km2 or .78 lb/mi2). In comparison, a space shuttle main engine can produce 1.67 million N of force during liftoff and 2.1 million N of thrust in a vacuum. Eventually, however, the continuous force of the sunlight on a solar sail could propel a spacecraft to speeds five times faster than traditional rockets.

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3. SAIL CONSTRUCTION

The strategy for near-term sail construction is to make and assemble as much of the sail as possible on earth. Thus, while the delicate films of the sail must be made in space, all other components are made on earth. The sail construction system consists of the following elements: a scaffolding (to control the structure's deployment), the film fabrication device, a panel assembly device, and a "crane" for conveying panels to the installation sites. The scaffolding structure rotates at a rate within the operational envelope of the sail itself, to facilitate the sail's release. Six compression members define the vertical edges of the hexagonal prism. Many tension members parallel to the base link these compression members to support them against centrifugal loads. Ballast masses flung further from the axis provide additional radial tension and rigidity near the top of the scaffolding. Other tension members triangulate the structure for added rigidity. Tension members span the base of the prism, supporting a node at its center. The interior is left open, providing a volume for deploying and assembling the sail. The top space is left open, providing an opening for removing it. The face of the sail is near the top of the scaffolding, and the rigging below. If the scaffolding is oriented properly, the sun will shine on the usual side of the sail, making it pull up on its attachment point at the base of the prism. The total thrust of the said is then an upper bound on the axial load supported by the compression members. It is clearly desirable to make the scaffolding a deployable structure. The sail's structure consists of a regular grid of tension members, springs, and dampers, and a less regular three-dimensional network of rigging. This is a very complex object to assemble in space. Fortunately, even the structure for a sail much larger than described herein can be deposited in the Shuttle payload bay in deployable form. Since the sail is a pure tension structure, its structural elements can be wound up on reels. Conceptually, the grid structure can be shrunk into a regular array of reels and a plane. With each node in the lid represented by housings containing three reels. The rigging can be sunken into a less regular array, and the nodes containing its reels stacked on top of those of the grid.

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The structure will be deployed by pulling on cords attached to certain nodes. Deployment may be controlled by a friction brake in the hubs of the reels. By setting the brakes properly, positive tension must be applied for deployment and certain members may be made to deploy before others. Further control of the deployment sequence, if needed, may be introduced by a mechanism which prevents some elements from beginning to deploy until selected adjacent elements have finished deploying. If detailed external intervention is deemed desirable, brakes could be rigged to release when a wire on the housing is severed by laser pulse. The film fabrication device produces a steady stream of film triangles mounted to foil spring clusters at their corners. The panel fabrication device takes segments of the stream and conveys them along a track to assembly stations. Each segment is fastened to the previous segment and to the edge tension members that will frame the finished panel. This non-steady process of panel assembly requires a length of track to serve as a buffer with a steady film production process. At the assembly station, the segments are transferred to fixtures with a lateral transport capability. During transfer, each segment is bonded to the one before along one edge. While the next segment is brought into position, the last segment is indexed over a one strip width, completing the cycle. Special devices bearing the edge tension members travel on tracts and place foil tabs on the panel structure. The foil tabs linking the segments may be bonded to one another in many ways, including ultrasonic welding, spot welding, and stapling. Attachment and conveyance may be integrated if the foil tabs are hooked over pins for conveyance. The panel assembly cycle ends with a pause, as the completed panels, now held only by their corners, are lured into a storage region and new edge members are loaded into position. At this point the sail's structure is deployed within scaffolding, and panels are being produced and stored at a panel fabrication module. The stored panels are initially loaded at a node suspended on tension members above the center of the sail. A crane is likewise suspended, but from tension members terminated in actively controlled reels mounted on devices free to move around the top of the scaffolding. This makes it possible to position the crane over any aperture in the grid.

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Once panel installation is complete and the operation of various reels has been checked, the sail is ready for release and use. It is already spinning at a rate within its operational envelope, and is already under thrust, hence, this task is not difficult. First, the sail's path must be cleared. To do this, the film fabrication device, its power supply, the panel assembly device, and the crane are conveyed to the sides of the scaffolding in a balanced fashion. The top face is cleared of objects and tension members. Then, the members holding the corners of the sail are released, and the remaining restraint points are brought forward to carry the sail out of the scaffolding. Finally, all restraints are released, and the sail rises free.

A four quadrant, 20-meter solar sail system is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio.

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4. SOLAR SAIL DYNAMICS AND CONTROL There are essentially two modes for operation and control of the solar sail. In the first mode, the tilting of panels produces control forces. Each panel has a mass of some 0.3 to 1.1 kilograms. This first mode is conceived of as a semi-passive control mode for interplanetary cruising (where only slow changes of attitude are needed). It is of importance to consider the stability of a passive sail set at various angles to the sun. In the ideal sail approximation (planar, perfectly reflecting), thrust will be normal to the sail and act through its center of area, that is, along the axis of symmetry. In an absorbing sail, its thrust is divided into purely reflective and purely absorptive components. The former produces no torque, while the latter produces a torque. To counter this torque, light pressure must be increased on the far side of the sail from the sun relative to that on the near side. Making the sail concave toward the payload accomplishes this purpose. Since torques can be balanced at all sail angles of interest, small perturbing torques can shift the sail from one attitude to another, or change its rotation rate. Since heliocentric orbit times are typically months, spin-up and spin-down times of ten days and precession rates of 0.1 radian/day seem reasonable targets. Tilting a panel by about twenty degrees changes the force on it--both normal to the sail and parallel to it--by about thirty percent of the panel's maximum thrust. Sail operation in this first mode configuration is characterized by torques that may be ballasted by a few statically positioned trim panels 100, permitting an entirely passive cruise mode. Slow changes in the sail's attitude and spin rate may be made, from time to time, by cyclic variation of panel tilt to produce perturbing torques. The passivity of cruise mode and the ease of providing redundant tiltable panels recommend this mode for reliable interplanetary transportation. In the second mode of sail configuration, the payload mass is assumed to be large compared to the sail mass, and the sail is considered as a separate object linked to it by actively controlled shroud lines 202 and 204. In the second mode, the tilting of the panels 200 controls the spin rate. However, in this mode precession is effected by varying the tension exerted by the shrouds 202 and 204 on different parts of the sail. This is accomplished by reeling and

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unreeling the shrouds in a coordinated fashion as the sail turns. For the sail discussed above, and the probable range of sail performances, this arrangement implies precession rates of 13 to 26 rad/100 minutes, when the sail is flat with respect to the sun. This provides a generous margin in turn rate, even from maneuvers in low earth orbits. This active control permits damping of nutation. This is important, since nutation would otherwise be initiated by rapid changes in precession rate. It should be noted that during precession the payload is offset from the axis of rotation in a direction fixed in inertial space. For missions involving both interplanetary cruise and circumplanetary maneuvering, a vehicle able to operate in both modes is desirable. The first mode has a decisive advantage near planets (because of its maneuverability), but cannot enter a passive cruise mode. The greater distance between the payload and sail in this mode precludes balancing the torque on the sail resulting from absorbed light with a reasonable amount of concavity, as is done in the first mode. Instead, the torque must be countered in the same manner as the sail is precessed: by active manipulation of shroud tension. While control of shroud tension might be made redundant by placing reels at both ends of the lines, reliability still favors a passive system on long missions. Fortunately, interconversion seems simple. The second mode control can be maintained as the shroud lines 202 and 204 are reeled in, so long as the sail is properly ballasted for mode one. While the payload reaches the mode one position, the reel can be locked and mode one control begun.

4.1 Cruising by Sunlight Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft. By changing the angle of the sail with respect to the sun, you change the direction of the force exerted by sunlight. When the spacecraft is in orbit around the Earth or sun, it is traveling in a circular or elliptical path at a given speed and distance. To go to a higher orbit (travel farther away from the object), you angle the solar sail with respect to the sun so that the pressure generated by sunlight is in the direction of your orbital motion. The force accelerates the spacecraft, increases the speed of its orbit and the spacecraft moves into a higher orbit. In contrast, if you want to go to a lower orbit (closer to the object), you angle the sail with respect to the sun so that the pressure generated by the sunlight is opposite the direction of your orbital motion. DEPT OF EEE, SCE

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The force then decelerates the spacecraft, decreases the speed of its orbit and the spacecraft drops into a lower orbit. The pressure of sunlight decreases with the square of the distance from the sun. Therefore, sunlight exerts greater pressure closer to the sun than farther away. Future solar-sail spacecraft may take advantage of this fact by first dropping to an orbit close to the sun -- a solar fly-by -- and using the greater sunlight pressure to get a bigger boost of acceleration at the start of the mission. This is called a powered perihelion maneuver.

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5. SOLAR SAIL MATERIALS While solar sails have been designed before (NASA's had a solar sail program back in the 1970s), materials available until the last decade or so were much too heavy to design a practical solar sailing vehicle. Besides being lightweight, the material must be highly reflective and able to tolerate extreme temperatures. The giant sails being tested by NASA today are made of very lightweight, reflective material that is upwards of 100 times thinner than an average sheet of stationery. This "aluminized, temperature-resistant material" is called CP-1. Another organization that is developing solar sail technology, the Planetary Society (a private, non-profit group based in Pasadena, California), supports the Cosmos 1, which boasts solar sails that are made of aluminum-reinforced Mylar and are approximately one fourth the thickness of a one-ply plastic trash bag.

Aluminium being manufactured for the Solar Sail.

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The reflective nature of the sails is the key. As photons (light particles) bounce off the reflective material, they gently push the sail along by transferring momentum to the sail. Because there are so many photons from sunlight, and because they are constantly hitting the sail, there is a constant pressure (force per unit area) exerted on the sail that produces a constant acceleration of the spacecraft. Although the force on a solar-sail spacecraft is less than a conventional chemical rocket, such as the space shuttle, the solar-sail spacecraft constantly accelerates over time and achieves a greater velocity.

5.1 Aluminum as Solar Sail Material

The thin metal film, according to the preferred embodiment of this invention, is an aluminum film. Aluminum films have high reflectivity, low density, a reasonable melting point, and a very low vapor pressure. The reflectivity and transmissivity of aluminum film is a function of its thickness. Generally, reflectivity for short wave lengths falls off faster with decreasing film thickness than for longer wave lengths. Consequently, any aluminum film thick enough to reflect well in the visible wave lengths should reflect even better in the infrared, where roughly half the sun's power output lies. Even in the visible wave length, aluminum's reflectivity remains near its bulk value down to a thickness of 30 nm, and remains above 0.8down to about 15 nm. The reflectivity of aluminum films varies with the deposition conditions. Over a range of at least 300 degrees to 473 degrees Kelvin, reflectivity increases with decreasing substrate temperatures. High deposition rates, near-normal vapor incidence, and a good vacuum favor high reflectivity. In general, poor deposition conditions reduce reflectivity with a shorter wave length more than for a longer wave length, and thicker films are more sensitive to vapor incidence angle than are thin films. Since most of the sun's power output is at comparatively long wave lengths, and since the films are to be quite thin, poor deposition conditions should not greatly affect sail performance. Above some temperature, thin metal films fail by agglomeration. This occurs because thin films have an enormous ratio of surface to volume, permitting them to substantially reduce the surface energy by forming droplets. Above the melting point, the material rearranges swiftly, like a soap bubble bursting. At temperatures somewhat below the melting point, agglomeration into droplets occurs far more slowly, through surface diffusion. Thin films made from silver, with a melting point of 1235degrees Kelvin agglomerate at less than 500 DEPT OF EEE, SCE

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degrees Kelvin. However, the analogous temperature for aluminum is a mere 378 degrees Kelvin. Nevertheless, aluminum films have survived fifteen minute anneals at 673 degrees Kelvin, and two hour anneals at700 degrees Kelvin. The reason for this discrepancy is the presence of an oxide layer on the aluminum, which armors the surface with a rigid, refractory skin, thereby inhibiting surface diffusion and preventing changes of shape. Since the film is to be hot and mounted under tension, creep is of concern. The interior of a small droplet will be in compression, because of its surface energy and resulting force of surface tension. In like fashion, the interior of a thin film will be in compression, unless the mounting tension exceeds its surface tension. Considering the oxide-coated film, elongation not only breaks the oxide skin (which may be very strong), but also creates a fresh, uncoated aluminum surface. To shrink, on the other hand, it must somehow crush or destroy the outside surface, which it clearly cannot do. In fact, shrinkage would manifest itself as agglomeration, as discussed above. The strength of a variety of thin metal films and thicker vapor deposited sheets has been measured experimentally. Metals in thin films have mechanical properties differing from those of the bulk material, because of the close proximity of all parts of the film to the surface. The yield and fracture stresses of aluminum film increase as the film gets thinner. Aluminum films show substantial ductility, and a variable degree of deformation before failure. Aluminum films of the minimum thickness required for reflectivity may prove too weak to support the stresses imposed upon them during fabrication and operation, or may creep under load at elevated temperatures. If so, it is possible to strengthen them, not by adding further aluminum, but by adding a reinforcing film of a stronger, more refractory material. A good reinforcing film should be strong, light, and easy to deposit. It need not be chemically compatible with aluminum, since a few nanometers of some other material can serve as a barrier to diffusion. A reinforcing film is apt to have a high modulus such that it will act as the sole load bearing element in the composite film. The aluminum film could help contribute tear resistance, however. The use of a metal as a reinforcing film could reduce the amount of aluminum needed to give good reflectance. Some metals, such as nickel, may reflect well enough to be of interest by themselves.

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5.1.1 Titanium as reinforcing material

Films of pure titanium from 150 to 2,000 nanometers thick were found to have strengths of 460 to 620 NPa, while vapor deposited foils of Pi-6Al-4V from 40,000 to 2,000,000 nanometers thick had tensile strengths of 970 to 1200 NPa. Titanium has enough strength and temperature tolerance to make it an attractive choice as a reinforcing film.

5.1.2 Nickel as reinforcing material

The strength of nickel film exceeds 2,000 NPa at a thickness of 70 nanometers or less, dropping to 1500 NPa on annealing. Nickel’s density is a disadvantage for use in sails of the highest performance, which should prove acceptable for bulk transport sails.

5.1.3 Silicon Monoxide as reinforcing material

Silicon monoxide is a popular thin film material with many uses. On aluminum, these films have found extensive use as satellite thermal control coatings, and have demonstrated their stability in the space environment. Mounted on fine metal meshes, unbacked SiO films as thin as 2.5 nanometers have found use as specimen supports in electron microscopy; such films are described as having "great strength," and are so stable at high temperatures that they may be cleaned by passing them rapidly through a flame. Since silicon monoxide is easy to evaporate, is refractory, has a low density, is apparently of high strength in extremely thin film form, and is of known space compatibility, silicon monoxide shows promise as a reinforcing film material.

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5.1.4 Boron as reinforcing material

Vapor deposited boron film has a strength of 620 MPa. Since it is light and refractory, boron may prove desirable as a reinforcing material. Carbon forms amorphous films of "exceptional strength;" those used in electron microscopy are made as thin as 4 nanometers. Since carbon is strong, light, refractory, and easy to deposit, it is a promising material for reinforcing film. For a wide variety of reasons, the sail surface will not be one big piece of film, but rather many smaller sheets mounted on a structure. Since the fabrication device, as described hereinafter, will produce strips, natural choices for the shapes of the sheet include long strips, shorter rectangles or squares cut from strips, and triangles cut from the strips. The sheets must be tensioned, and should be planar. Since a triangular sheet will be planed if tensioned at its corners, and since triangular sheets will fit well into a fully triangulated structure, they will be used as a basis for further design. 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 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.

Tears are a critical concern in the use of thin films for solar sails. While even sheets of extremely thin material have adequate strength to support the load expected during fabrication and operation in the absence of stress concentrations, the inevitability of manufacturing flaws and micrometeoroid damage makes this a small comfort. A means of limiting the spread of tears would be desirable, as it would allow a thinner sheet to tolerate greater damage without failure. The most obvious method of limiting tears is to mount the film on a supporting mesh. However, differing coefficients of thermal expansion and differing temperature between the mesh and the film are apt to make the film become slack and lose its flatness, or become taut and possibly tear. Further, the mesh adds mass to the sail and, because it must be fabricated, transported into space and attached to the film, adds cost as well.

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A more natural approach to tear-stopping is to subdivide the film, convert it from a continuous sheet to a redundant network of small, load-bearing elements. In such a structure, a large manufacturing flow or a grazing micrometeoroid impact is free to initiate a tear--but the tear will cause the failure, not of an entire sheet, but of a small piece of film, perhaps 25 square millimeters in area. Patterns of cuts and wrinkles can de-tension areas of film to isolate stress to smaller regions. Each wrinkled region is fabricated with enough extra material to avoid being stretched flat as the film is tensioned. Stress isolation is aided by slits extending perpendicular to the boundary. The slits are terminated at their stress bearing ends in a way that avoids initiation of tears. This approach to tear resistance appears superior to that of mounting the films on a metal mesh. It involves the fabrication of no additional elements and the addition of no extra mass. By taking advantage of the natural strength of the films, it avoids slackness due to differential expansion and yields a flatter sail.

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6. SOLAR SAIL LAUNCH With just sunlight as power, a solar sail would never be launched directly from the ground. A second spacecraft is needed to launch the solar sail, which would then be deployed in space. Another possible way to launch a solar sail would be with microwave or laser beams provided by a satellite or other spacecraft. These energy beams could be directed at the sail to launch it into space and provide a secondary power source during its journey. In one experiment at NASA's Jet Propulsion Laboratory (JPL), sails were driven to liftoff using microwave beams, while laser beams were used to push the sail forward. Once launched, the sails are deployed using an inflatable boom system that is triggered by a built-in deployment mechanism.

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7. INVESTIGATED SAIL DESIGNS

The highest thrust-to-mass designs known (2007) were theoretical designs developed by Eric Drexler. 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.

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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 centrifugal 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.

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8. COSMOS-1 SPACECRAFT DESIGN The first solar-sail spacecraft, called Cosmos-1, has been developed, built and tested by The Planetary Society, a private, non-profit organization whose goal is to encourage the exploration of our solar system. The Planetary Society contracted a Russian space organization, the Babakin Space Center, to build, launch and operate the spacecraft. The cost of the project is about $4-million and is funded by Cosmos Studios, a new science-based media company. The spacecraft itself weighs 88 lb (40 kg) and can sit on a tabletop. After a first-phase test launch, the spacecraft will be launched into Earth orbit -- 522 mi (840 km) perigee and 528 mi (850 km) apogee. The spacecraft systems include:

Solar sail • • • •

made of aluminized Mylar thickness of 0.0002 inches (5 microns) area of 6,415 square feet (600 square meters) arranged in eight triangular blades: • each about 49 ft (15 m) long • consist of inflatable plastic tubes that support the sail (a foam may be used inside the tubes to hold them rigid once inflated) • can be pivoted (like a helicopter blade) by electric motors to change its angle relative to the sun

The Planetary Society One solar-sail blade

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• • •

Solar-sail deployment - A pressurized gas-filling system inflates the plastic tubes. Power - A small array of solar cells supplies all of the electrical power. Navigation - It is essential for the spacecraft to know where it is and where the sun is at all times. 1. A sensor detects the position of the sun. 2. A global positioning system (GPS) receiver detects the spacecraft's position. (From the ground, the spacecraft orbit will be determined from Doppler tracking data with the aid of on-board accelerometers, which we'll discuss later.) 3. The information from the sun sensor and the GPS receiver are continuously relayed to the spacecraft's on-board computer. 4. The on-board computer operate the motors that turn the sail blades to maintain the proper orientation of the sail blades with respect to the sun. 5. The on-board computer can accept corrections or override commands from the ground.

Communications - Redundant radio systems are used to communicate with flight controllers on the ground. • one UHF band, 400 megahertz • one S-band, 2210 MHz

On-board computer •

• •

•

• •

•

Two 386EX series microprocessors • old, but reliable in the harsh environment of outer space • can be run in low-power modes, similar to laptop computers • programmed to operate the on-board systems, relay information to the ground and receive commands from the ground A software program assigns tasks to each microprocessor based on workload and performance (speed, delay). Each processor has its own small amount of read-only memory (ROM) -enough to boot the computer and load the operating system into randomaccess memory (RAM). Three re-writable ROMs contain the operating systems and programs. The copies of ROM are checked before use for errors caused by radiation in outer space. Three RAMs are present to receive the operating system. Again, the integrity of each RAM is checked for errors before loading. The ROM architecture allows programmers on the ground to update and re-boot the spacecraft's software at any time. It also allows the spacecraft to function in the case of severe radiation damage. Data are stored in two separate databases connected by serial and parallel systems.

Instruments • •

Two on-board imaging cameras (Russian and American) to document the mission On-board accelerometers to measure the acceleration of the spacecraft due to sunlight pressure (non-gravitational acceleration)

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9. ADVANTAGES

A solar sail is a spacecraft without a rocket engine. It is pushed along directly by light particles from the Sun, reflecting off its giant sails. Because it carries no fuel and keeps accelerating over almost unlimited distances, it is the only technology now in existence that can one day take us to the stars. The major advantage of a solar-sail spacecraft is its ability to travel between the planets and to the stars without carrying fuel. Solar-sail spacecraft need only a conventional launch vehicle to get into Earth orbit, where the solar sails can be deployed and the spacecraft sent on its way. These spacecraft accelerate gradually, unlike conventional chemical rockets, which offer extremely quick acceleration. So for a fast trip to Mars, a solar-sail spacecraft offers no advantage over a conventional chemical rocket. However, if you need to carry a large payload to Mars and you're not in a hurry, a solar-sail spacecraft is ideal. As for traveling the greater distances necessary to reach the stars, solar-sail spacecraft, which have gradual but constant acceleration, can achieve greater velocities than conventional chemical rockets and so can span the distance in less time. Ultimately, solar-sail technology will make interstellar flights and shuttling between planets less expensive and therefore more practical than conventional chemical rockets.

Solar sails will set new speed records for spacecraft and will enable us to travel beyond our solar system.

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10. 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. 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. 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.

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11. 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. 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. 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 course if correct equations had not been used. The effect for a genuine ‘solar sail’ will be even more spectacular." One way to see the conservation of energy as 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 sunphoton system to the sail. This change of energy can easily be verified to be exactly equal (and opposite) to the energy change of the sail.

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12. FUTURE SPACE TRAVEL Solar sail technology will eventually play a key role in long-distance NASA missions. NASA believes that the exploration of space is similar to the tale of the "Tortoise and the Hare," with rocket-propelled spacecraft being the hare. In this race, the rocket-propelled spacecraft will quickly jump out, moving quickly toward its destination. On the other hand, a rocket less spacecraft powered by a solar sail would begin its journey at a slow but steady pace, gradually picking up speed as the sun continues to exert force upon it. Sooner or later, no matter how fast it goes, the rocket ship will run out of power. In contrast, the solar sail craft has an endless supply of power from the sun. Additionally, the solar sail could potentially return to Earth, whereas the rocket powered vehicle would not have any propellant to bring it back. If NASA were to launch an interstellar probe powered by solar sails, it would take only eight years for it to catch the Voyager 1 spacecraft (the most distant spacecraft from Earth), which has been traveling for more than 20 years. By adding a laser or magnetic beam transmitter, NASA said it could push speeds to 18,600 mi/sec (30,000 km/sec), which is one-tenth the speed of light. At those speeds, interstellar travel would be an almost certainty. Solar sailing is a way of moving around in space by allowing sunlight to push a spacecraft. In everyday experience, we do not feel any kind of force or pressure from sunlight. This is because sunlight is so gentle that all the other things in our environment - gravity, wind, and the strength of our own bodies - drown it out. However, in space, there is no air, and objects are freely falling through space instead of constantly fighting gravity. In this environment, sunlight can dominate and allow spacecraft to move at will, like sailing vessels on Earth's oceans. As it continues to be pushed by sunlight, the solar sail-propelled vehicle will build up speeds that rocket powered vehicles would never be able to achieve. Such a vehicle would eventually travel at about 56 mi/sec (90 km/sec), which would be more than 200,000 mph (324,000 kph). That speed is about 10 times faster than the space shuttle's orbital speed of 5 mi/sec (8 km/sec). To give you an idea how fast that is, you could travel from New York to Los Angeles in less than a minute with a solar sail vehicle traveling at top speed.

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13. REFERENCES

www.howstuffworks.com www.wikepedia.org www.answers.com

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