Starwisp
Encyclopedia
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.
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, unfortunately, 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.
The probe would cruise without power for decades until it finally approached the target star
, at which point the antenna 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 beam and Starwisp would arrive there at the same time. At such extreme long range the antenna 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 antenna is only required for a few days at Starwisp's launch and again for another few days several decades later to power it while it passes its target, Starwisp probes might 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. Alternatively, the launching transmitter could be used in the interim to transmit power to Earth for commercial use, as with a solar power satellite.
light, and then wait for the sheet to evaporate away under the assault of solar UV after it has been deployed in space.
Another proposed method noted that the Starwisp probe wires were of the same physical scale as wires and circuit elements on modern computer microchips
and could be produced by utilizing the same fabrication technologies as those computer chips. The probe would have to be built in sections the size of current chip fabrication silicon wafers and then connected together.
Interstellar probe
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, typically defined as the region beyond the heliopause...
design proposed by Robert L. Forward. It is propelled by a microwave sail, similar to a solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
in concept, but powered by microwaves from a man-made source.
Description
"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 Landis in 2000. The proposed device uses beam-powered propulsionBeam-powered propulsion
Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy...
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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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, unfortunately, 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
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
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.
The probe would cruise without power for decades until it finally approached the target star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
, at which point the antenna 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 beam and Starwisp would arrive there at the same time. At such extreme long range the antenna 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 antenna is only required for a few days at Starwisp's launch and again for another few days several decades later to power it while it passes its target, Starwisp probes might 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. Alternatively, the launching transmitter could be used in the interim to transmit power to Earth for commercial use, as with a solar power satellite.
Possible methods of fabrication
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 ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light, and then wait for the sheet to evaporate away under the assault of solar UV after it has been deployed in space.
Another proposed method noted that the Starwisp probe wires were of the same physical scale as wires and circuit elements on modern computer microchips
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
and could be produced by utilizing the same fabrication technologies as those computer chips. The probe would have to be built in sections the size of current chip fabrication silicon wafers and then connected together.
Technical problems
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 uniform enough across its sail 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.In fiction
- In a science fictionScience fictionScience fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
story Forward suggested that the beam from a solar power satellite could be used to push a Starwisp probe while the solar power satellite was being tested after construction. - Frederik PohlFrederik PohlFrederik George Pohl, Jr. is an American science fiction writer, editor and fan, with a career spanning over seventy years — from his first published work, "Elegy to a Dead Planet: Luna" , to his most recent novel, All the Lives He Led .He won the National Book Award in 1980 for his novel Jem...
's 1987 novel The Annals of the Heechee makes brief mention of starwisps left floating in deep space by a long-dead race of aliens. - Charles StrossCharles StrossCharles David George "Charlie" Stross is a British writer of science fiction, Lovecraftian horror and fantasy. He was born in Leeds.Stross specialises in hard science fiction and space opera...
, in his novel AccelerandoAccelerando (novel)Accelerando is a 2005 science fiction novel consisting of a series of interconnected short stories by British author Charles Stross. As well as normal hardback and paperback editions, it was released as a free e-book under the Creative Commons attribution-noncommercial-no derivatives license...
, propels a small payload to a hypothetical brown dwarfBrown dwarfBrown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
star, 3 light-years away, using a Starwisp propulsion system, as well as using starwisps as kinetic-kill missiles against space-based warships.