Even though the interstellar message plaques currently in flight have been controversial, it is nearly certain that future extrasolar ventures will be equipped with such devices. Future message plaque designers may well decide to further broaden the base of participation, perhaps to include input from thousands of people. What is the best medium to contain such a comprehensive message, without exceeding the stringent mass limitations on the payload?

It should be a medium that could encompass creative input from thousands and it should be easily decipherable by extraterrestrials who intercept it. Future plaques should also be as low mass as possible, since mass budgets on near-term interstellar ventures will be extremely limited. Also, since a message plaque must survive for eons in interstellar space, it must be able to withstand the rigors of the space environment. And wouldn't it also be nice if the message plaque could serve a dual function such as assisting with spacecraft propulsion or steering?

Substantial progress toward creation of such a miracle message plaque began in the summer of 2000, when artist C Bangs curated an exhibition in conjunction with an International Academy of Astronautics symposium on deep-space exploration, in Aosta, Italy. About 35 visual artists contributed work to this "Messages from Earth'' exhibition, which investigated visual concepts for future interstellar message plaques.

One of the symposium participants who viewed this exhibition was noted space scientist Robert Forward, who suggested to Bangs that white-light holography would be an excellent medium for future interstellar message plaques. Subsequent discussions between Forward and NASA managers resulted in a commission to Bangs to create a prototype holographic message plaque for the NASA Marshall Space Flight Center (MSFC).

It was soon apparent that creation of a hologram is labor intensive and not inexpensive. Very few artists will produce holograms in their studios, without the assistance of a skilled team of technologists.

It was first necessary to create some three-dimensional and two-dimensional images for incorporation into the holographic images. This aspect of the work was performed by C Bangs, with the assistance of computer artists David Wister Lamb and Lajos Szobozlai. The creation of the hologram itself was accomplished at the facilities of The Center for Holographic Arts, in Long Island City, New York. Prior to its delivery to NASA MSFC, the 40 by 50 centimeter hologram plate was framed by Simon Liu, Inc. in Brooklyn, New York.

All holograms are produced by the interference of two collimated light beams (helium—neon laser beams in this case). These monochromatic beams are usually produced by splitting the output of a single laser into two beams. One beam interacts with the object to be imaged. The second reference beam is then recombined with the first. A transparent photograph of the interference pattern is then used to produce the three-dimensional holographic image.

A traditional monochromatic hologram can only be viewed by inserting the transparent plate in the laser beam. A white-light hologram can be viewed in polychromatic, or natural, light.

More care must be taken with white-light holograms than with their monochromatic cousins. If a monochromatic transparent holographic plate is dropped and broken, each fragment can serve as a monochromatic hologram. If the same unhappy fate befalls your white-light hologram, all you have is a lot of broken glass.

The type of white-light hologram selected for the NASA prototype holographic message plaque was a Benton or rainbow hologram. Three-dimensional imagery is best incorporated in this variety of white-light hologram.

A non-holographic color image of the full Earth taken from an Apollo spacecraft in cis-lunar space serves as a backdrop in the prototype holographic message plaque. Holographic images include three-dimensional sculpted representations of male and female figures, two-dimensional line drawings of the same figures, a rendering of a hypothetical spacecraft trajectory and some of the relevant equations of solar-sail radiation-pressure acceleration.

View angle and illumination angle are critical in image visibility. It is generally necessary to shift view position to shift among images on the hologram. Under certain illumination conditions, two superimposed holographic images are visible.

Research revealed that rainbow holograms have an enormous information capability. According to Dr john Caulfield of Alabama A&M University, a well-designed hologram can contain hundreds of images, since a viewer angular shift of one degree or less is sufficient to shift between images. Since each image can be three-dimensional and contain many independent components, the work ofmany thousands ofartists can be incorporated in a single message plaque.

As Caulfield pointed out, a space-qualified white-light hologram can be exposed on a photographic emulsion less than a micron in thickness. Thus, even a large message plaque need not add a great deal to the mass budget of a future extrasolar probe.

NASA propulsion engineers realized the possible utility of holographic films on solar sails. Since holographic film reflectance can be theoretically varied greatly by rotating a sail slightly from a reflecting surface's image to an absorbing surface's image, a holographic sail would allow thrust to be easily varied. Tests at the MSFC Space Environments Facility on commercial white-light holograms revealed that maximum hologram reflectance compares favorably with the reflectance of conventional sail films. Also, experiments revealed that holographic images are essentially immune to very large levels of simulated space radiation.

We see, therefore, that white-light holograms can be applied to both message plaques and propulsion. Settling the solar system need not be the sole province of engineers and scientists—artists will likely be right beside them! Creation of this prototype message plaque was a very interesting exercise on the boundaries of art and science.

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