Deployment Orbits

The first server-sat arrays will be deployed in a "4 hour" orbit, or more precisely a 23.9344696/6 = 3.989078 hour or 14360.7 second orbit (sidereal). This means it passes over the same spot on earth 5 times per day ( = 6-1, the earth is turning underneath once per sidereal day ). A 4 hour equatorial circular orbit has a radius of 12770 kilometers, and an altitude above the equator of 6399 kilometers. This puts it in a "thinner" part of the Van Allen belt, with an estimated unshielded radiation dose of 1Mrad/year [citation needed].

Relative to a position on the earth, a serversat will be visible in the same position 5 times per solar day, at intervals of 288 minutes. The constellation of associated orbits will therefore be called the m288 constellation. Of course, higher and lower orbit constellations are possible, though they will get more radiation. The m240 constellation will repeat 6 times per day, and the m360 constellation will repeat 4 times per day. For reasons that will become apparent, it is important that the orbital period is an integer fraction of a day.

The m288 central orbit can be seen at 58 degrees north and south latitude, at a distance of 10500 km. The round trip ping time is 70 milliseconds. The ground ping time through optical fiber across the United States is faster in theory, but ground networks are slowed by switches and indirect routes. Ping times from fat-pipe servers in Dallas Texas to mit.edu are 42 milliseconds , and to orst.edu are 49 milliseconds, so 70msec is not way out of line. However, much of the routing will travel "around the cloud", and without local caching in the "near" links, some pings may need as much as 200 milliseconds to hop from the far side of the orbit. Still, this is better than the 250+ millisecond ping time through a geosynchronous satellite.

Server-sats will actually be deployed in very slightly inclined and elliptical orbits, which map onto a nested set of torii ("torus-es") centered on the 4 hour, zero-inclination equatorial orbit. These are 4 hour orbits as well, and the largest torii are have major radii slightly offset inwards. A plane drawn perpendicularly to the center orbit (which intercepts the orbit in two places, and also intersects the center of the earth) will have "territories" marked on it for the regions around the various orbits, and the orbits passing through each "territory" can be treated as a property. This orbital property is further subdivided into angles around the orbit. Server-sats precisely positioned in each orbit will never intersect server-sats in other properties. Arrays in orbits in the same "property" will never intersect the orbits of arrays in different properties. This allows a large region of space to be filled with server-sats, potentially trillions of them. The density is limited by shading - at some point server-sats closer to the sun will reduce the daylight falling on the ones in the "back" of the orbit, and may begin to detectably reduce the sunlight falling on the earth.

Shading

Server-sats will be spaced perhaps 100 meters apart in an array - an array with 32768 server-sats will be 3.2km on a side. This puts them far enough apart that the shade area behind one 0.3 meter diameter server-sat will never completely block sunlight to the server-sat behind it. If the nested tori extend outwards to 500 km around the central orbit, that is a spatial volume of 3E10 cubic kilometers. Potentially, that is room for 30 trillion server-sats at a 100 meter spacing. This "fuzzy toroidal cloud" of server-sats will block some sunlight, both to the server-sats in the back of the toroidal cloud, and to the surface of the earth. With 30 trillion 300mm diameter server-sats, the light blockage to the ground would be about 7% at noontime near the equator, with the blockage zone following winter as shown above. The blockage of back server-sats near the sides of the orbit would be more severe - at the equinoxes, it could be as much as 77% for portions of a fully populated array. However, it is hard to imagine needing that many server-sats, even if they were operating mostly as space solar power satellites, beaming about 5 watts each to the ground, as that far exceeds the projected world demand for electricity. With a "mere" 10 trillion server-sats, the blockages would be 2% to the ground and 38% to the back of the array. Also, for power production (and one-way information broadcast) ping time is no longer an issue, so m360 and higher orbits can be used.

Elevation above the horizon, interference with geosynchronous satellites

Assume that the first torus to be filled with arrays is at a minor radius of 200km from the central orbit. Refraction will lift the apparent elevation of the central orbit a few degrees above the horizon, so a south-facing antenna at 60 degrees north or south may be able to see the edges of the torus. However, ground sites further north or south of the central band may need to relay through other satellites, or through landlines on the ground. For latitudes between 10 degrees north or south, the torus will have similiar ground antenna elevation angles as geosynchronous satellites. Assuming that the same satellite frequency bands are used for server-sky as for existing geosynchronous services, latitudes between 10 degrees and 60 degrees north and 10 degrees and 60 degrees south should be able to make direct use of server-sky.

Intentional gaps in the constellation

The orbits will not be completely filled - there will be large gaps in them to permit transit of launch vehicles. Establishing these "windows" will involve lots of negotiation, beyond the scope of this document.

If it ever proves practical to build space elevators, there will need to be other gaps in time and "property" so that the elevators and the server-sats never interact. A failing space elevator will collide with an undetermined number of server-sats (as well as destroying all the other space elevators), so space elevators and the permanent use of these orbits (or possibly any near-earth orbit) may prove incompatible.

In the short term, there will be millions, not trillions, of server-sats. They should still be assigned positions and orbits compatible with a much more crowded sky. It is good to know, going into this, that the region available for well behave server-sky orbits offers much room for growth.

Collisions

The server-sats have limited maneuvering capability, but they travel in a "pure physics" region - their position and their orbit can be tracked and predicted down to the millimeter level. They are far enough up that solar maximums will not cause increased drag, and well above most of the "dense" cloud of debris in lower orbits. What debris there is can be tracked, predicted, and avoided - in most cases, a server-sat need only change orbit by one or two meters to avoid this known debris, and will have many hours to do so. If a server-sat can accelerate at an average of 0.32 meters per hour per hour (less than 1% of theoretical solar sail maximum), it can move 10 meters out of its normal track in 8 hours.

The server-sat arrays have vast computational power. From a server-sat point of view, they are nearly motionless in space, and able to execute quintillions of instruction per orbit. With enough data on potential impactors, they can easily avoid thousands of objects per orbit. Most potential impactors will be in inclined orbits, and will only be a threat when those orbits come through the orbital plane. For known objects, this will be an easy problem to solve.

There are unknown objects traveling through the server-sky torus - these may occasionally take out a server-sat or two, and create a larger cloud of debris that may collide with more server-sats. In the longer term, it may be necessary to develop mechanisms that can rendezvous with and remove this debris, or the larger objects that cause them. The server-sat radios may potentially be re-purposed as phased array radar beam transmitters to aid in the search for and accurate tracking of these objects. Obsolete server-sats, still operable but with relatively diminished compute capacity, may be maneuvered to rendezvous with some of these objects. The server-sat and the target will likely have high voltage differences between them, and this will probably draw them together, and perhaps provide "arc welding energy" to lightly weld them together. Then the server-sat can act as a radio beacon to accurately locate the object. This same process can be used to mate an obsolete server-sat to a non-functioning one, eventually building "trash heaps" of server-sats for re-purposing or recycling, or merely as targets to absorb smaller bits of space debris.

The server-sat is thin enough that small impactors will travel straight through, creating a small hole, most likely through the solar cell. The impactor will keep going, and most of the solar cell will remain intact and usable. This will probably be a detectable electrical event, perhaps even locatable in the surface of the solar cell by some means (the shock will ripple out and affect other sections of the solar cell, and this may be measurable). Perhaps some clever inventor will figure out a way to estimate the track of the impactor and debris for later tracking - that might be a fun thing to think about.

Long term, everything capable of damaging a server-sat should be tracked, avoided, and removed if possible. As the number of server-sats in orbit grows, the risk grows, but so does the capability to reduce that risk. There may be millions of potential impactors, down to the size of a grain of sand, but it is not inconceivable that with enough time and observation, almost all of these objects can be found.

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