Starwisp
Starwisp is a hypothetical unmanned interstellar probe design proposed by Robert L. Forward. It is propelled by a microwave sail, similar to a solar sail in concept, but powered by microwaves from a man-made source.
"Starwisp" is a concept for an ultra-low-mass interstellar probe pushed by a microwave beam. It was proposed by scientist and author Robert L. Forward in 1985, and further work was published by Geoffrey A. Landis in 2000. The proposed device uses beam-powered propulsion in the form of a high-power microwave antenna pushing a sail. The probe itself would consist of a mesh of extremely fine carbon wires about 100 m across, with the wires spaced the same distance apart as the 3 mm wavelength of the microwaves that will be used to push it.
Forward proposed that the wires would incorporate nanoscale computer circuitry, sensors, microwave power collection systems and microwave radio transmitters fabricated on the wire surfaces, giving the probe data collection and transmission capability. Being distributed across the entire sail, no "rigging" is needed, as would be the case if the mission electronics were placed in a separate probe that was pulled by the sail.
The original Starwisp concept assumed that the microwaves would be efficiently reflected, with the wire mesh surface acting as a superconductor and nearly perfectly efficient mirror. This assumption is not valid. Landis showed that a grid will absorb a significant fraction of the power incident on it, and therefore cannot stay cool enough to be superconducting. The design is thermally limited, hence the use of carbon as the material in Landis's concept.
Low mass was the key feature of the Starwisp probe. In Landis's calculations, the mesh has a density of only 100 kg/km2, for a total mass of 1 kg, plus a payload of 80 grams.
Although the diffraction limit severely constrains the range of the transmitting antenna, the probe is designed to have an extremely high acceleration of 24 m/s2, so that it can reach a significant fraction of the speed of light within a very short distance, before passing out of range. The antenna uses a microwave lens 560 km in diameter, would transmit 56 GW of power, and would accelerate the probe to 10% of the speed of light.
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