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which is very large. The angle of the radial vector along the lens would be 0 ~ 2 r/R. which for n = 1.5 it is easily found this lens will have a thickness of ~ 60 m. Obviously this is not practical due to the huge mass of such a lens. However, a Fresnel lens will work. The material set into concentric Fresnel zones which act as a slice of the lens at some radius. The sum of these over all concentric zones will concentrate light in the same fashion that a single lens does [8.2]. This technique is used to direct large beams of light from lighthouses. Fresnel lenses can be stamped onto thin sheets of plastic, such as seen with the overhead projector. It should be possible to position a huge Fresnel lens in the environs of the inner solar system and to direct a collimated beam of solar radiation.

Large sail structures have to be masted, just as sails were masted on sailing ships. Without this slight fluctuations in photon pressure on these large areas will induce wave motion and cause the structure to flap and lose its shape. This requires of course added mass to the structure. This would generically consist of a large boom protruding from the center of the back face of the sail. From there guy wires attach to points on the sail. The photon pressure pushes the sail forward, where tension on these lines preserves the shape of the sail. This may be augmented by rotating the whole sail to maintain a baseline tension on the guy wires. For the sail ship this affair can be relatively modest, for in principle it can maintain a constant direction. So there is no need for any steering mechanism required. This is less the case for the Fresnel lens. It must maintain a photon beam at an angle that has wiggle tolerance of 1 x 10"13 rad, or the beam will start to drift away from the target photon sail. Hence the directionality of this huge structure is a critical issue. By shifting the center of mass of the craft from the center of area will induce a torque on the Fresnel lens. Yet this will be slow and sluggish. A more advanced approach might be to have the Fresnel zones adjust their geometry. This would require that the whole lens contain a matrix of "smart materials" and nanotechnologies that can adjust the optical properties or directionality of the lens.

Another difficulty with a huge Fresnel lens is that it is in orbit around the sun. This compounds the steering problem enormously. A further problem is that the sun's rays will only be normal to the lens for one time out of the year. Even with smart materials that adjust the geometry of the Fresnel lens there will be a narrowed period of time where this could be used to directly propel the spacecraft. So it appears that a direct use of a Fresnel lens to propel the craft is not practical. However, the Fresnel lens could still be used to power a solar sail indirectly. It could be designed to concentrate solar radiation on a power station on the moon, which could be part of a cis-lunar solar power generation grid [8.3]. If the lens were placed at the L1 lagrange point it would be comparatively stationary and could direct solar radiation to a power station at one of the lunar poles. Mirrors in lunar orbit might be needed to redirect this light to the right point on the lunar surface. This would then power a huge laser that directs photons at the starsail. Clearly care is needed to insure against steering the Fresnel lens towards the Earth.

A 100 km radius Fresnel lens could deliver around 1.0 x 1014 w of power to this power station. Recall that this amount of solar energy could sent a 2.7 x 107 kg craft with an acceleration g ^ .011 m/sec2. With the Fresnel lens this power could be converted to laser light, where we assume a 20% efficiency and directed to a 10 km solar sail with a mass of ~ 105 kg. The acceleration is then g ~ .22 m/sec2. This gives a performance that is somewhat more in line with what is desired. Directing photons from lasers fixed to the surface of the moon would cure the small vibration problems inherent with attempting to direct photons many light year distances to a structure in space.

The problems with photon sailing are nearly as daunting as the rel-ativistic rocket. This involves large scale construction in space, which is considerably beyond current capabilities. It also appears to require the construction of a power station on the moon which is able to generate power at a rate 1 to 2 orders of magnitude larger than all the energy currently generated on Earth. This power station would likely be as large and complex as the Clavius base depicted in the movie 2001 A Space Odyssey. Such things certainly do not come cheap.

Robert Forward proposed a photon propelled craft called Star Wisp. The sail craft is to be propelled by microwaves generated by a large solar power station in space. The sail consists of a web or mesh of microwave

Solar powered Microwave generator

Staiwisp microwave scattering sail

Fig. 8.3. Diagram of the proposed Star Wisp sail craft.

Detail of scattering mesh

Solar powered Microwave generator

Staiwisp microwave scattering sail

Fig. 8.3. Diagram of the proposed Star Wisp sail craft.

reflecting elements controlled by computer chips or smart materials. This sail is proposed to have a mass of 16 grams with a radius of 3 km. The microwaves are collimated into a beam by a microwave analogue of a Fresnel lens. This lens consists of rings of aluminum which act as Fresnel zones. Just as an optical Fresnel lens in a watchtower concentrate light in a direction this lens directs microwaves towards the sail craft. The Star Wisp is proposed to be accelerate at 115 gees (1130 m/s2), and is thought to only reach a fifth the speed of light, or 7 = 1.02. This limitation is due to the long wavelength of microwaves, which become redshifted relative to the frame of the craft, and that such radiation has lobes which make tight focusing difficult.

The starwisp sail would consist of a mesh of wires spaced apart a distance equal to the wavelength of the microwave radiation. This would mean that virtually all of the microwave radiation would interact with the sail, ideally with 100% of this radiation reflecting off. The dimensions of this craft are then comparatively modest. The Starwisp could in principle be constructed within a couple of decades. Robert Forward envisioned the sail as superconducting which is a perfect mirror. This assumption was overly optimistic. The mesh will absorb some of this radiation, and keeping it in a superconducting state might prove to be too difficult.

The microwires in this mesh are proposed to hold nanotechnology and nano-computer circuitry. Thus the whole sail will act as an integrated data collection system and data transmission system. These systems also are designed to adjust the tension in the sail at all times in order to keep it from responding to slight differences in photon pressure. Since the operations of the craft are distributed across the entire sail there is no rigging, or static strutting of the system. The sail would be an integrated network or neural-like net of processors which can perform all the data collection required and to maintain the trim of the sail as it is being accelerated. Of course at this time such technology does not exist completely.

The craft would have to be constructed in a lithographic manner. The wires and processors would have to be etched into some matrix of material. This material is either folded into a bundle and unfurled, or stored in pieces which are assembled in space. The sail would be contained in some epitaxial matrix which would then have to be removed. This could be some form of UV sensitive plastic which dissolves after exposure to solar radiation or some other form of evaporating material.

Space based conversion of solar radiation to microwaves has been proposed since the 1970s as a possible way to convert large amounts of solar radiation into electrical power. A 10 gigawatt microwave power generator is then proposed to be the energy source for the Starwisp. Ultra-thin and very long filaments of aluminum in a large series of concentric circles then compose a Fresnel lens that focuses this energy onto the sail. This Fresnel lens is expected to be 25,000 km in radius. Constructing this lens would be a formidable challenge. If it is constructed of ultralight materials it will still end up having a mass of 6 x 104 metric tons. Due to the limited range of microwave systems the Starwisp is propelled quickly to .2c, where it then cruises to its interstellar destination. The microwave beam would then be activated again on the craft as it flies by its target. The microwaves will be too weak to provide any propulsion, or braking thrust, but it could be used to power the systems on the craft.

The microwave power satellite would only need to propel the craft for several days. Thus a star system could be visited by a flotilla of such craft which probe particular regions of the stellar system. Such flyby missions would not be able to probe in detail the surface of planets in a star system, but as with planetary explorations the first missions to planets we later landed probes on have been flyby missions. The Starwisp could be used to explore planetary systems within about 10 lightyears. The missions could be accomplished within a program to place solar power stations in geosynchronous orbit.

The largest difficulty with the Starwisp is in maintaining the trim of the sail. A slight deviation in microwave pressure on one part of the sail could cause it to start flapping or twisting up. The beam profile of lasers and masers are not 100% uniform, where there are small deviations in the distribution of power through the beam. A very small deviation in the pressure on the sail could lead to a small deformation of its shape, which would then amplify the asymmetry of pressure on the sail as deformed regions reflect the radiation in non-uniform directions. This clearly requires that the tiny wires be dynamically held in place against such small perturbations. Another problem with such a light weight craft would be the presence of interstellar hydrogen. This will cause damage to nanoscale structures over time. Forward's Starwisp envisions no shielding against this assault, where obviously shielding will increase the mass of the craft considerably.

The Starwisp is a candidate for the first photon sail craft to be sent to the stars. It has the advantage of being light weight, far less massive than the optical photon craft considered above. However, it may leave little in the way of much detection and measurement apparatus on board. A craft that reaches another solar system will do so in order to make measurements and to perform astronomical observations of planets. In the case of a photon sail craft it will unlikely be able to visit every planet in the system. So it will have to take a decent telescope to observe the system. Weighting in at 16 grams the Starwisp could only at best give us a tantalizing glimpse of an extrasolar system. It is likely that the Starwisp would be scaled up sufficiently to carry the appropriate instrumentation to conduct research. Even with micro-miniaturization to electronics it is not possible to get around the need for mirror sizes and the mass required to conduct a reasonable scientific survey.

It is likely that the starwisp is the extreme light mass end of the photon sail, while the larger mass optical photon sail is at the large mass end of the scale. It is then likely a compromise will be arrived at so that photon driven starsail craft with a mass of ~ .1-10 tons will emerge as the realistic craft of choice with 7 ~ 1.2.

A starsail must by some means come to a halt as it approaches its target star [8.4]. It is likely that more advanced starsail craft will not just perform a quick flyby mission which would last a few days. The simplest way to accomplish this is to have the disk sail contain an inner disk that detaches from the main disk. Photons continue to reach the larger annulus. These photons reflect off of it and reach the smaller disk. This will then act to break the motion of the smaller craft relative to the star. The annulus will have to adjust its shape slightly to focus the photons onto the decelerating craft. This presumably could be accomplished with guy wires. The annulus

Fig. 8.4. The braking process for a photon sail.

will accelerate away as an expendable part of the craft, while the smaller central disk will slow down sufficiently to begin its exploration of the stellar system. The main sail, the annulus, would then accelerate onward or be sent into the star so as not to be some relativistic bullet flying through the galaxy. The sail craft would then use the light from the star to navigate around the star system.

If interstellar probes are indeed launched it is likely the photon sail will be the first to reach other stars. There are only about 10 stars that are accessible by this method. The relativistic rocket has about 100 stars accessible to it, as defined by the ability to deliver a message back to Earth within 50 years. It is possible that an energy architecture will involve converting solar energy collected from space into a form that can be used on earth. A lunar power station that accumulates concentrated energy from large Fresnel lenses might divert some of this energy to push photosail spacecraft to the stars [8.3]. Whether any of this comes to fruition is a future's guess. The possible impetus for this will be energy short falls here on Earth, which might in the future push our energy grid into the cis-lunar region. If this future energy pathway is able to produce sufficient energy for requirements here on Earth a possible excess might be used to propel spacecraft to the stars.

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