needed in small quantities, but cumulatively they could represent a market for several hundred launches per year. Some uses, such as packet-switching low orbit communications networks, could involve hundreds or thousands of microsats. Obviously, there are also possible strategic defense applications, particularly in connection with recent suggestions for swarms of small space-based interceptors (‘Brilliant Pebbles’). The number of applications grows enormously if some form of assembly in space is possible. It is currently impractical to assemble anything in space from 20 kg modules using either human or robotic labor; there is not even a practical way to collect such pieces and bring them together. However, the technology needed to build small autonomous spacecraft capable of rendezvous and docking maneuvers is rapidly being developed. At the simplest level, satellites of moderate size could be launched as modules, starting with a maneuverable guidance and command unit. This unit could, over several days, collect and join together independent modules (power, communications, scientific experiment, booster) to form, for example, an interplanetary probe. A larger-scale application of this concept would be efficient resupply of Space Station Freedom. Supplies (food, water, tools, spare parts, etc.) could be delivered to orbit perhaps 100 km from the space station (to keep the station safe from both laser beams and packages at high relative velocity). A very small (<100 kg) retriever vehicle would collect these supply packages and return them to a suitable airlock on the station. Astronaut time would be needed only to unpack and store the supplies, and perhaps to monitor the final approach of the retriever to the station. Even chemical fuels, oxygen and batteries could be delivered—a particularly direct way of ‘beaming’ power, and one which could be used over arbitrary distances, since the laser can easily launch payloads to escape velocity. The laser system cannot launch to a given non-equatorial orbit at any time; the laser is precisely in the orbital plane only twice a day. However, the laser has some crossrange capability—the vehicle can be steered in a ‘dogleg’ trajectory which results in an orbit that does not pass over the laser. Even a 100 km crossrange capability (out of a laser range of 1000 km or more) could allow at least eight payloads per day to reach the space station. Eight payloads per day would be over 50 tons—two shuttle loads—per year. The limited size of each payload would be somewhat offset by the promptness of delivery; a tool or spare part could be delivered to the station with, in many cases, less than a day’s delay. As Federal Express has demonstrated, overnight delivery frequently commands a premium price, and is sometimes truly invaluable. Uses for a Sub-scale Laser Facility Although a true launch-to-orbit system requires a 20 MW system, there are some propulsion applications for considerably smaller lasers. Perhaps the most important of these is orbital maneuvering propulsion. A laser as small as 1-2 MW can give considerable impulse to a satellite passing overhead. To keep the beam projector size and cost within reason, the satellite must deploy a crude reflector (essentially a beach umbrella of aluminized Mylar) to concentrate the laser beam. However, with such a concentrator the satellite can get thrust with triple the specific impulse of solid rockets, or twice that of H2/O2 rockets, from a completely safe and stable block of inert propellant. The laser can only track a given satellite in low orbit for a few minutes each day; exactly how much time depends on the details of the satellite’s orbit and the laser range. (Orbiting mirrors would greatly increase this, but would cost much more than the laser system.) That is sufficient to allow a 2 MW laser to maintain or raise the
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