eburacum45":3vomjgsz said:
That's really nice; with a 1000 km diameter laser you could do a lot of really interesting things. Trouble is, you want to float these collectors on light pressure alone; this limits you to 0.78 grams of mass per square metre; a bit low.
And any light you use for power is not reflected, so can't be used to support the statite. These ultra lightweight light collectors/transmitters, so close to the star, would be affected by solar wind, which is variable and would mess up any collimation you tried to acheive. In short, statite power collection is going to be inefficient and present an unstable platform for focusing light beams.
Why not use more substantial, orbiting satellites; a few thousand or million solar satellites would soon add up to the area your scheme requires, and would be a more stable base for emitting collimated beams.
One scheme I have heard for converting light beams to particle beams is to use billions of tiny light sails which themselves impact the craft to transfer momentum. The tiny sails could even have a rudimentary steering mechanism so they consistently hit the target; this means the ship doesn't need hundreds of thousands of square kilometers of reflective sail, but a much smaller pusher plate or magnetic sail.
Its a matter of mass and effort. You are exchanging the mass of sails which may be reused, to the mass of intelligent particles which may not. What is the balance? I have looked at both and streams of small particles are interesting for interplanetary missions - especialliy if they may be reused. A system I call 'smart smoke' consists of collections of particles that act as emitters and reflectors to transfer momentum in a confined region. In many ways these are superior to macroscopic planes. A cellular approach also provides improved logistics and maintenance.
The size of the sail is a function of the size of the payload. A human being encased in a reflective suit need nothing larger than a parachute to sustain 1 gee of acceleration - as I mentioned elsewhere. Also, payloads accelerated by beam system AT the target star and AT the sun - need only project efficiently 0.054 lightyears - that's the distance a payload travels when accelerating to 1/3 light speed at 1 gee. So beams can be a bit tighter than shown here. Its only when you have a multi-staged sail system that you need to project the beam many light years.
Even so, the sun itself can act as a lens if you're greater than 660 AU from the sun. Some have proposed sensors at that distance to make pictures of remote star systems. Beaming energy to a satellite and then to remote star systems are also possble.
An orbiting cloud of free flying particles each with an active optical control system - can certainly be coordinated by a reference beam aboard the spacecraft. There are issues with delay and doppler effects to be worked out, but that appears possible.
http://www.youtube.com/watch?v=2QAUkt2VPHI
At 1 AU solar light pressure is 4.54 e-6 pascal. Pressure from solar wind ranges from 2e-9 to 3e-9 pascal depending on orbital position. At 3 million km this is 2500x greater. At 1 million km 22,500x greater
At 3 million km radius; 0.011325 Pa
At 1 million km radius 0.101925 Pa
Approximately 1 gram and 10 grams per sq km of force.
But because the acceleration varies with the square of the distance just like light, we have
At 3 million km radius acceleration is 1.50 gees
At 1 million km radius acceleration is 13.54 gees
That means your 0.78 grams per square meter is correct regardless of distance.
I mention this because some references (Wikipedia) speaks of diving into the sun to gain advantage.
A PV cell that is 50% efficient and has a reflecting optical bandpass filter can lift only 0.39 grams per square meter. The balance is absorbed, and then re-emitted, but it is emitted at far longer wavelengths. This transfers a net momentum equal to about 0.11 grams - for a total of 0.50 grams - no matter where we are - per square meter. A square cm masses 0.5 milligrams - 500 micrograms. - a device density of 2.5 grams per cubic centimeter means 2,500,000 micrograms per cubic centimeter. 1 mm thick layer reduces this sq cm to 250,000 micrograms. A 1 micron thick laer falls to 250 micrograms.
So we have a MEMS device made of a silicon like substance 1 cm on a side and 1.5 microns thick. It is highly relfective on one side, and capable of free flight. That's about 15,000 atoms thick - about a trillion atoms overall.
The device can accelerate away from the sun, and by turning its edge to the sun fall, and by angling its orientation to the sun, speed up or slow down along its orbit. One side is equipped with an optical bandpass filter that reflects light - and is highly controllable in its propulsive effects. The back side is equipped with a nonlinear optical window and laser setup that efficiently produces laser beams. It can also recieve and detect laser beams.
These devices are produced in a factory and ejected in an electromagnetic launcher into space. Early systems are built on Earth, later systems are built on asteroids between Mars and Jupiter. Once free flying in space they use sunlight and laser beams to navigate. They also communicate via laser beam. Since the same system that allows control of the laser beam and light reflections also segment the light across the device This allows the PV array to act as a camera system if necessary and modulation of the dwell point of the beam steering device modulates the beam to transmit information. Modulation of the reference beam sends control signals.
Orbiting cells do not have to touch to operate as the same optical emitter. A reference source coordinates the free flying system.
The light sail too can be a cloud of material ejected by the starship. Providing it is recycled back to the starship. A beam intercepts the cloud - which changes its momentum. The devices fly toward a thrust surface, and transfer momentum to that thrust surface - ejecting the particles back into the cloud that intercepts the beam. This great simplifies deployment - and reduces mass of the sail.
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