Starwisp
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Starwisp is a hypothetical unmanned interstellar probe design proposed by Robert L. Forward. It utilizes beam-powered propulsion in the form of a maser driving a solar sail.
The Starwisp probe would consist of a mesh of extremely fine wires about a kilometer across, with the wires spaced the same distance apart as the wavelength of the microwaves that will be used to push it. Light weight is the key feature of Starwisp; the wire mesh would have a mass of 12 grams, and the remainder of the probe's microcircuitry and sensors would mass 4 grams for a total mass of only 16 grams. Constructing such a delicate probe would be a significant challenge. One proposed method would be to "paint" the probe and its circuitry onto an enormous sheet of plastic which degrades when exposed to ultraviolet light, and then wait for the sheet to evaporate away under the assault of solar UV after it has been deployed in space.
Using a microwave laser producing 10 gigawatts of power, Starwisp could be accelerated at 115 Gs (1130 m/s²) and reach 20% of the speed of light in only a matter of days. Since masers cannot be easily focused at long ranges, Starwisp could not be driven much faster than this. The probe would cruise without power at 20% c for decades until it finally approaches the target star, at which point the maser which launched it would again target its beam on Starwisp. This would be done when the Starwisp was about 80% of the way to its destination, so that the maser beam and Starwisp would arrive there at the same time. At such extreme long range the maser would be unable to provide any propulsion, but Starwisp would be able to use its wire sail to collect and convert some of the microwave energy into electricity to operate its sensors and transmit the data it collects back home. Starwisp would not slow down at the target star, performing a high-speed flyby mission instead.
Since the maser is only required for a few days at any particular Starwisp's launch and a few days several decades later to power it while it passes its target, Starwisp probes would probably be mass-produced and launched by the maser every few days. In this manner, a continuous stream of data could be collected about distant solar systems even though any given Starwisp probe only spends a few days travelling through it. Alternately, the launching maser could be used in the interim to transmit power to Earth for commercial use, as with a solar power satellite. Indeed, given the existence of a solar power satellite, launching Starwisps with it could be a minor side benefit provided in addition to its regular role as a power source.
A major problem this design would face would be the radiation encountered en route. Travelling at 20% of light speed ordinary interstellar hydrogen would become a significant radiation hazard, and the Starwisp would be without shielding and likely without active self-repair capability. Another problem would be keeping the acceleration of the Starwisp even enough across its area that its delicate wires would not tear or be twisted out of shape. Distorting the shape of the Starwisp even slightly could result in a runaway catastrophe, since one portion of the Starwisp would be reflecting microwaves in a different direction than the other portion and be thrust even farther out of shape. Such delicate and finely-balanced control may prove impossible to realize.