easiest methods of performing space-to-ground experiments. Therefore, this additional experiment could provide added incentive for pursuing a laser approach. The microwave Astro-SPAS advanced demonstrator would, by contrast, place emphasis on developing operational-type concepts for maintaining the Powersat in close formation with a target The use of ion or arc-jet thrusters is considered mandatory here. In addition, this demonstrator would test a full- scale development model of an actual size space station rectenna design - an experimental version of which is included in the ASAP demonstrator. The use of an inflatable structure seems ideal for this type of application. Most everywhere else, the microwave Astro-SPAS would be essentially a small-scale version of an operational Powersat. Astro-SPAS was chosen over other options because it exists, it allows a very large payload, it enables flexible design arrangements, and, above all, it is the least expensive option available. Its fundamental disadvantage is that it relies on the Shuttle for launch, and the mission will only last a few days. The only other alternative that could provide an equivalent capability is the use of a comsat bus launched on Ariane 5. This option was not studied in detail, although a rough estimate placed its cost at around a factor of 2 or more higher than that of Astro-SPAS. The value of international cooperation should be studied in greater detail. It is very clear that the US, Japan and CIS have much stronger interests in Powersats and SPS systems than Europe. There is considerable commonality between the requirements of the reference concepts proposed here and on-going activities in the US and Japan in particular. US organisations have already indicated their interest in the ASAP demonstrator because of problems they are encountering with their Shuttle-dependent experiment Availability of Astro- SPAS might also provide an alternative for US experiments and reduce the cost
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