solar sails
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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 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) 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.
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. 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.
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.
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. 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, see Tack (sailing).
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.
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.
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.
Pekka Janhunen from FMI has invented a type of solar wind sail called the electric solar wind sail. 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.
Sail testing in space
NASA has successfully tested deployment technologies on small scale sails in vacuum chambers.
No solar sails have been successfully used in space as primary propulsion systems, but research in the area is continuing. It is noteworthy that both the Mariner 10 mission, which flew by the planets Mercury and Venus, and the MESSENGER mission to Mercury 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.
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.
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.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." The NanoSail-D structure was made of aluminum and plastic, with the spacecraft weighing less than 10 pounds (4.5 kg). The sail has about 100 square feet (9.3 m2) of light-catching surface.
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