Orbital Decay, Space Debris Risks
Summary: Server sky will greatly reduce the space debris problem. Fully populated, server sky may someday place hundreds of billions of objects in space, but these objects will all be continuously controlled to high precision, precisely located to micrometers, and will be in a 6411 kilometer altitude near-circular equatorial orbit, devoid of other assets, far above low earth orbit, and out of the flight path of launch vehicles aimed at higher orbits.
Server sky arrays can be configured as radars, and continuously track small objects in LEO and above. With precise characterization of millions of third party debris objects, debris collisions will be easy to avoid by active satellites. Collision risks between derelicts can be predicted with high precision, and missions to capture or deflect the derelicts can be prioritized.
Ultrathin sunlight harvesting systems such as Server Sky and Space Solar Power Satellites must be massive enough to maintain stable orbits in spite of light pressure perturbations. Rather than increase launch cost, ultralight systems can be deployed, with captured space debris mass added in orbit. Presuming orbital maneuvering stages to capture debris, and some means to cut it up into appropriately sized ballast masses, debris now becomes a valuable asset, and the combined value of useful mass and threat reduction will make the capture process more valuable than mass launched from the ground.
Space debris is the uncontrolled garbage resulting from space missions, which threatens other space missions.
Uncontrolled: space debris, from shrouds and upper stages to failed satellites, does not have control or thrust, and remains in slowly decaying orbits for a very long time. It is also poorly tracked - radars have limited visibility of the sky through distorting atmospheres, and sensitivity is limited by the inverse fourth power of distance.
Threatened: with poor knowledge of where space debris is, it is difficult for controlled satellites to move out of the way. Space is vastly big, so collisions are actually quite rare, but with millions of large and small chunks of mass in wildly varying orbits, impacts can come at any time from many directions.
Server sky thinsats maneuver with light pressure, and are highly redundant, capable of remaining under control and in formation even after multiple punctures by meteorites or space debris. They will rapidly grow obsolete due to Moore's law, but old thinsats will be useful as ballast for new thinsats. Should a thinsat unexpectedly stop functioning or behave erratically, it can be sandwiched in between two other thinsats and shepherded to a recycling satellite, where it can be cut and folded and attached as ballast to new thinsats.
Thinsats that leave constellations unexpectedly can be retrieved by other solar sailing "space tugs", gram-weight devices designed to collect stragglers. At the very slow rate orbits evolve at 6000+ kilometer altitudes, we have years to plan and schedule these retrievals. More precisely, the robots will - this process will be highly automated. The stragglers will have high radar cross sections and their orbits will remain very well characterized. This is the "high accuracy physics zone"; except for collisions with meteoroids and space debris, drag is low and orbits will be very precisely predictable. For example, we know where the LAGEOS laser geodesy satellites are within fractions of a millimeter.
If an out-of-control thinsat escapes all these constraints, its orbit will deviate from the rest of the constellation in a predictable way, and perigee will descend into a slightly higher drag region of the neutral atmosphere. That equatorial orbit will very gradually decay over hundreds of years, mostly descending in periods of high solar activity. Because of a thinsat's large area, it will remain easy to track, and drag will be relatively high. Once perigee dips below 3000 kilometers, remaining time in orbit drops below 100 years; below 2000 kilometers, less than 10 years.
At 1000 kilometers height, decay time drops to a few months; at 500 kilometers height, a few days. At 400 kilometer ISS altitude, space station altitude, decay time is 12 hours, and descent rates are tens of meters per second. Below 300 kilometers, thinsats will drop into the atmosphere after less than one orbit.
These high descent rates occur because the area-to-mass ratio of thinsats is much higher than other satellites, 5 square meters per kilogram. This makes their lifetime as potential debris objects much lower than more traditional "big iron" satellites. It also means that they deposit far less energy per area in a collision; they might damage a millimeter or two of spacecraft surface in a collision, but they will not penetrate or disable a traditional satellite. Because they are in an equatorial orbit, satellites in inclined orbits will encounter them only when those satellites cross the orbital plane, and they will do so at lower velocities than two satellites with a large difference between their inclined orbital planes.
Overall, the risk and damage to other satellites will be far smaller than the risk from meteoric material, and even this small risk will only occur if all thinsat operators are very careless, wasteful, and inefficient, not only letting their own valuable mass escape, but ignoring the riches abandoned by their competition.
While we can wish that everyone did the right thing in space because of elevated moral responsibility, we are more likely to get good results if the right thing is both profitable and favored by physics. The server sky concept is in its infancy, and will either improve into an even better and safer system, or degrade due to careless neglect by potential contributors, and shortsighted stupidity by incompetent operators. We hope there will be many contributors to teach competence to the operators.
One of the downsides of the rapid advance of semiconductor technology is that new high-tech geosynchronous communication satellites are replacing older satellites at an astounding rate. The older satellites are moved into higher altitude "graveyard" orbits and abandoned. They will supposedly last a long time in these orbits, but they will not last forever; eventually, perhaps after many decades, their orbits will interfere with active GEO satellites.
Fortunately, geosynchronous space solar power satellites (SSPS) will also need ballast; they will have large sail-to-mass ratios as well, because they will have extremely large areas of photovoltaics or mirrors facing the sun. Their orbits must be very tightly constrained to avoid interference with other communication satellites. Adding ballast mass, perhaps as long projecting tidal anchors, will be helpful for many SSPS designs, and will also reduce the chances of damaging collisions.
Work in progress, MoreLater