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For one of the inspirations for Server Sky (there are many), read [[ http://www.amazon.com/exec/obidos/ASIN/1884989128/ | Ivan Bekey's 2003 book "Advanced Space System Concepts and Technologies". Also look for it on [[ http://www.worldcat.org/search?q=1884989128 | Worldcat ]] |
Brin2012Mar17
A response to comments to David Brin's Startram post
The Startram article by Powell and Maise makes some "interesting" claims. The JHAPL folks assume repulsive magnetic levitation between a floating conductor and a ground conductor - closing the loops to make coils (paging Dr. Kirchoff!) results in a huge inductance, on the order of Henrys, and energy storage on the order of gigawatt-years.
See also:
Startram is not interesting. It won't work, because it is based on misunderstandings of launch, orbits, coilguns, and superconductors. Enough of that ...
Most of this page is to respond to some thoughtful questions raised about Server Sky around mid April in the comments. You will find a lot more detail in published papers and the pages on this site. Not complete, of course, this is a work in progress.
For one of the inspirations for Server Sky (there are many), read Ivan Bekey's 2003 book "Advanced Space System Concepts and Technologies". Also look for it on [[ http://www.worldcat.org/search?q=1884989128
... a straight road to the Kessler syndrome ...
Donald Kessler refers to the accumulation of uncontrolled mass in low earth orbit, where it collides and makes small debris, which can disable active satellites and render them uncontrolled as well. This can grow exponentially. Bhe accumulation of penetrating debris is the problem, in orbits with high relative inclination (hence high closing rates) at intensely populated altitudes.
So, don't do that! The Kessler syndrome will not occur if objects are in orbits with very small relative inclination and velocity, remain maneuverable until they are removed, are thin and robust and redundant enough to tolerate millimeter scale penetrators, can radar-track every object that comes near their orbit, and have enough delta V to maneuver out of harm's way. If the objects are distributed arrays, then damage to one element of the array does not kill the whole array - a collider may make a few grams of shrapnel, but it does not create a multi-ton derelict.
Thinsats are not quick - light pressure acceleration is continuous but it is slow. A large unexpected collider can't be avoided in a tiny fraction of an orbit, but if it has been observed for days or weeks, it can be avoided and tagged for removal.
The fraction of thinsat collision debris that stays in orbit will be small. If debris orbits are more elliptical, their perigees will be close enough for rapid decay and reentry - the ballistic parameter (kg/m2) is tiny. If perigee drops to below 1000 km altitude, they will reenter in weeks.
Thinsats can be made with current laptop screen manufacturing technology with a power/mass ratio of about a watt per gram. Watts times "newness" equals function per launch weight - far higher than big iron satellites, with heavy structure and old technology. Don't think of them as a bunch of independent satellites - think of them as a multi-ton satellite divided into cooperating components. Ant colonies, not elephants.
Thinsats can be made much thinner, but light pressure orbit perturbations get too large. In time, it should be possible to attach a small chunk of ballast to a micron thick, 50 watt per gram thinsat, bringing the whole assembly back to a watt per gram. Where does that mass come from? Obsolete thinsats, in the beginning. Thinsats will be "Moore's Law-ed" out of usefuless in a few years. But their mass can be attached to the back of other thinsats.
In the longer term, orbital debris, kilotons of it. Chopping an empty third stage (mostly thin aluminum) into gram-weight chunks is as easy as space manufacturing can get.
Collecting the mass? Jerome Pearson ( author of first technical space elevator paper in Acta Astronautica, 1975 ) thinks it can be done with tethers. I think it can also be done with laser ablative thrusters, or VASIMR electric engines, or ... Which is to say, I don't know yet. In any case, we first need to know where the objects are, precisely, so we can maneuver to them.
Radar: The ability to make narrow kilowatt beams of time-coded millimeter wave coherent radar energy, from a precisely located look-down source, will be helpful. It may be possible to choose frequencies that are stopped by the atmosphere, and an intermodulation harmonic of the receive band of existing radar satellites, so that those satellite's lower frequency receivers will respond to this normally-out-of-band signal. Receive signal efficiency of a thinsat will be poor compared to a plain old parabolic dish, so we probably want to use big iron satellites for this.
So, thinsat arrays can be the solution to Kessler syndrome - a high paying use for the debris that is up there, and an element of the collection process.
Search this site for more information - obviously, there are books worth of analysis to do, and it will take a while to generate.
... Active collision warning and avoidance ...
Granted, the almost random way we launch satellites to all orbits, inclinations, and orbital elements, and abandon them in fractions of a century, makes them hazards. Server sky must not be deployed that way, or it will be hazardous, too. Instead, server sky arrays will be deployed in very similar orbits near the equatorial plane, with very small inclinations relative to each other. Using geometry and radio signal time of flight, we can measure the relative spacing relationships down to a fraction of a micron, and calibrate them out.
Two satellites with the same orbital elements (except for different "anomalies" or angle around the orbit) cannot collide - they follow each other in formation. But we want to launch three dimensional arrays. How?
Orbits with the same semimajor axis ( half the distance from apogee to perigee ) have the same orbit time. If we draw a very thin torus around that orbit, we can map slightly elliptical orbits on it, with perigee on the inner edge and apogee on the outer edge, with the "semi latus rectum" (halfway up the ellipse) canted to side, kissing the top and the bottom of the toroid. Those orbits will have the same semimajor axis, and will appear to make one circle around the minor radius of the toroid as they make one orbit around the major radius. As the torus gets bigger, the cross section becomes noncircular, but the main point remains - these orbits never intersect. By nesting torii, it is possible to fill a large toroidal space with trillions of non-intersecting objects.
Should an object get slightly perturbed into a collision with a neighbor, its closing velocity will be small, about a meter per second for a 5 kilometer perturbation. For a neighbor 10 meters away, about 2 millimeters per second. Such collisions result in bounces at best, not breakage, and they take an hour to reach their neighbor. With 2 micrometer per second squared acceleration, a neighboring thinsat can move 12 meters out of the way in thirty minutes. Relative to other controlled (or recently out of control) objects in the array, there is plenty of time to move.
more at ToroidalOrbits
The big threat is unexpected colliders coming through the orbit. Centimeter-scale micrometeoroids are very rare - the chance of one hitting a particular thinsat is tiny, and you end up with more ballast. Other thinsats can maneuver into the same position and nudge the dead one along, keeping it (or its fragments) out of the way until it can be collected. The collection will be made by a heavier object with a much larger ballistic parameter - since the light pressure modified orbits of the thinsats change over the course of a year, rendezvous between a particular spot in the toroidal system and a particular collector may occur only once per year, but there can be many collectors.
The collectors will probably be the biggest collision hazards - we can assume they are mostly passive and will run out of fuel. Another fuelless passive object in an orbit near m288 is Lageos - it is dense enough to stay in orbit for millions of years. I expect we will be able to service the collectors within a decade or so.
Lageos is also a demonstration of the accuracy of satellite tracking for a satellite properly designed for it. Covered with retroreflectors, we measure the position of Lageos from hundreds of laser optical observatories on the ground. Even through a refractive atmosphere, we can measure the position within fractions of a micrometer over tens of thousands of kilometers. The accuracy is so good that we can characterize the earth's gravity field to extreme accuracy, test relativity, and measure continental drift. If we can do that do a big dumb sphere of brass through an atmosphere and over thousands of kilometers, we can measure meters and kilometers between two actively cooperating thinsats in continuous view of each other to micrometers.
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