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Corals may be triggered to incorrectly spawn with as little as GUESS 10% of full moon illumination (1E-5 full sun), or 1E-6 of full sun illumination in the night sky. If GUESS 10% of the tumbling server-sat illumination ends up in the night sky, then the server-sats can intercept only 1E-5 of sunlight. This works out similarly to the thermal discussion above: Corals may be triggered to incorrectly spawn with as little as 3% of full moon illumination (1E-5 full sun), or 3E-7 of full sun illumination in the night sky. If GUESS 10% of the tumbling server-sat illumination ends up in the night sky, then the server-sats can intercept only 3E-6 of sunlight. This works out similarly to the thermal discussion above:
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|| orbit || radius || capture area || 1E-5 area || number of server-sats ||
|| m288 || 12789 km || 5.1E14 m^2^ || 5.1E09 m^2^ || 250 billion ||
|| m720 || 20295 km || 1.3E15 m^2^ || 1.3E10 m^2^ || 650 billion          ||
|| GEO || 42164 km || 5.6E15 m^2^ || 5.6E10 m^2^ || 2.8 trillion
||
|| lunar || 384400 km || 4.6E17 m^2^ || 4.6E12 m^2^ || 230 trillion ||
|| asteroid || 400E6 km || 2.0E21 m^2^ || 2.0E16 m^2^ || 1.0 quintillion ||
|| orbit || radius || capture area || 3E-6 area || number of server-sats ||
|| m288 || 12789 km || 5.1E14 m^2^ || 1.5E09 m^2^ ||  75 billion ||
|| m720 || 20295 km || 1.3E15 m^2^ || 3.9E09 m^2^ || 200 billion ||
|| GEO || 421
64 km || 5.6E15 m^2^ || 1.7E10 m^2^ || 850 billion ||
|| lunar || 384400 km || 4.6E17 m^2^ || 1.4E12 m^2^ || 70 trillion ||
|| asteroid || 400E6 km || 2.0E21 m^2^ || 6.0E15 m^2^ || 300 trillion ||
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This is much lower than the thermal limit. If each server-sat, configured as a power-sat, could deliver 1 watt each to the electric grid, then "Power Sky" could deliver only 3 Terawatts from GEO distances. To provide a full planet 50 Terawatts ( the Smalley Terawatt challenge for 2100 ) we will have to go further out.   This is much lower than the thermal limit. If each server-sat, configured as a power-sat, could deliver 1 watt each to the electric grid, then "Power Sky" could deliver only 850 gigawatts from GEO distances. To provide 50 Terawatts ( the Smalley Terawatt challenge for 2100 ) we will have to go out to the moon.

Biological and Environmental Effects of Server Sky

Server-sats may stay in orbit a long time - perhaps millions of years. We cannot assume they will always be under beneficent control, and need to include the worst case in our design.

The extreme case biological doomsday. For an information system, that could be as simple as providing very bad information to credulous and lethally-armed surface-dwellers. But as Justice Brandeis once said, the cure for bad speech is more speech. As long as there are many channels of information, independently controlled, Server Sky merely increases the bandwidth, and intelligent people will hesitate before launching Armageddon. Anxious people will still be anxious, and some may choose to redirect their anxieties at Server Sky. But that does not make it lethal.

Leaving aside the dangers (and anti-dangers) posed by information, the rest of this page will focus on the physical aspects of Server Sky, a collection of objects that have some mass and emit or reflect heat, light, and microwave radiation.

Mass risks

Server satellites are very thin - unless a cylinder of server-sats fails to deploy, or a cylinder is remade out of removed-from-service server-sats, they will be far too thin to survive reentry. Chances are, charging effects will push server-sats apart, so they will not tend to clump before they potentially reenter. When they do reenter, they will probably turn into flakes and sand.

The major risk posed posed by server sat mass is collision with other space objects, perhaps some inhabited. If space objects have value, we will be active in space and managing the server-sats.

Heat risks

The constellation of server satellites will not be dense enough reflect or emit enough heat to be a problem in the foreseeable future. If server-sats (at 75C or about 350K ) are dense enough to be a significant infrared source (slightly heating the globe) then they will probably reflect far larger amounts of sunlight, which we will discuss in the next section.

Since the Earth is cooled by black body radiation, proportional to temperature to the 4th power, a 1% increase in incoming energy will cause only a 0.25% increase in absolute temperature. If an acceptable Earth temperature rise is only 0.1 Kelvins above the average black body temperature of 255K, the temperature rise is caused by 4E-4 of the solar input. Assuming the Earth is surrounded by a sparse shell of server-sats emitting collected solar radiation isotropically as thermal radiation, then the density of radiated energy is 25% of the incoming radiation. Working through the math, it turns out that the shell must capture 1.6E-3 of the sun's energy passing through the shell to re-radiate enough infrared in the direction of the Earth to heat it by 0.1K . That is a LOT of server-sats. Here is the number of 15 cm-solar-cell server-sats (0.02m2) at various distances that can capture 1.6E-3 of the sun's energy:

Earth Heating Limits (0.1K)

orbit

radius

capture area

1.6e-3 area

number of server-sats

m288

12789 km

5.1E14 m2

8.2E11 m2

41 trillion

m720

20295 km

1.3E15 m2

2.1E12 m2

105 trillion

GEO

42164 km

5.6E15 m2

9.0E12 m2

450 trillion

lunar

384400 km

4.6E17 m2

7.4E14 m2

37 quadrillion

asteroid

400E6 km

2.0E21 m2

3.2E18 m2

160 quintillion

There are no near-term thermal limits to server-sat constellation size, assuming larger constellations can move further out.

Light Risks

The big problem with accidental light reflection is undesirable illumination of the dark night sky on Earth. With the server-sats intact, properly optically designed, and under good control, they will put very little light into the night sky. We don't want to deal with rampaging astronomers, do we? In the worst case, we may not be too worried about astronomers, but there are other reasons to keep the night sky dark.

Assume it is a long time from now, and the constellation contains many abandoned server-sats. In the very long term, Earth-orbiting server-sats will be oriented flat in the equatorial plane, edge-on to Earth, with only diffuse reflection reaching higher latitudes. But unless they are actively controlled, they will spend a long time tumbling in random orientations, and will reflect some light on the earth. Let's assume the average server-sat albedo is 0.5 and for ease of computation the server-sats are mirrors, all specular reflection and no diffuse reflection. This assumption is justified by assuming that each small portion of a server-sat surface behaves this way, and summing all those little bits.

The server-sats in between the earth and the sun will not reflect any light into the night sky. The server-sats in the Earth's shadow will not receive any light to reflect. Only the server-sats in the side portions of the orbit can potentially scatter light into the night sky.

FINISH THIS CALCULATION AND REPLACE THE SERVERSAT ILLUMINATION GUESS BELOW

MORE LATER

Corals

leafcoral.png

MORE LATER

Corals may be triggered to incorrectly spawn with as little as 3% of full moon illumination (1E-5 full sun), or 3E-7 of full sun illumination in the night sky. If GUESS 10% of the tumbling server-sat illumination ends up in the night sky, then the server-sats can intercept only 3E-6 of sunlight. This works out similarly to the thermal discussion above:

MORE LATER

Night Illumination

orbit

radius

capture area

3E-6 area

number of server-sats

m288

12789 km

5.1E14 m2

1.5E09 m2

75 billion

m720

20295 km

1.3E15 m2

3.9E09 m2

200 billion

GEO

42164 km

5.6E15 m2

1.7E10 m2

850 billion

lunar

384400 km

4.6E17 m2

1.4E12 m2

70 trillion

asteroid

400E6 km

2.0E21 m2

6.0E15 m2

300 trillion

This is much lower than the thermal limit. If each server-sat, configured as a power-sat, could deliver 1 watt each to the electric grid, then "Power Sky" could deliver only 850 gigawatts from GEO distances. To provide 50 Terawatts ( the Smalley Terawatt challenge for 2100 ) we will have to go out to the moon.

Note that this same limit applies to any other kind of space solar power system with the same efficiencies Space solar power systems may need to be at lunar distances to provide large amounts of power in normal operation, while avoiding the "coral problem" after failure.

lightscale.png

MORE LATER

Microwave risks

Unlike the risks above, the microwave risks are worst during improperly controlled "normal" operation; hijacked arrays used as microwave beam weapons. In normal conditions, communication arrays are providing enough energy to move bits. With a 17dB carrier to noise ratio (quite high!) and a noise temperature of 50K, each bit uses 50kT or 3.5E-20 joules. A 10 Gbps signal captured by a 1m2 antenna array equates to a power density of 3.5E-10 W/m2 . If there are 1000 such signals impinging on any particular area of the surface at once (spatially and frequency and code multiplexed) then the power level is 0.25 microwatts per square meter - no discernable biological effect.

Power beams will be a different story. Although some SSPS proposals posit a low energy density and a large rectenna, they are merely spreading out the problem in hopes of exceeding some small average dose. It may be better to focus the beam down onto a number of 10km diameter rectennas at very large power densities, perhaps 10 KW/m2 peak. Assuming 50% utilization and 80% collection efficiency, that translates to 12500 km2 of rectennas, or 160 of these 10km diameter arrays scattered around the globe.

However,larger areas nearby will be subject to sidelobe radiation, and rectennas will probably be surrounded with a kilometer of "no fly zone". When birds enter, they are scared away. If they get too close, the beam is turned off. Turnoff takes about 3 seconds, given the speed of light delay to and from lunar distances.

A few kilometers around the array may be covered with wire mesh, with low plants growing underneath, like at the Arecibo radio observatory. Farther out, the sidelobe microwave power will still exceed radiation guidelines for unshielded people, but people in vehicles or wearing protective clothing will do fine. The land can be used for agriculture, or as wildlife preserves. The abundance of life in the exclusion zones around Chernobyl demonstrate that wild nature thrives in radiation zones that scare away humans. Again, the beams can be turned off while people are in the unshielded sidelobe areas.

So an entire "rectenna reservation" may be 50km across, for a total area of 320000 km2. The land surface of the Earth is 149 million km2, so the land usage is 0.2% of the total land surface. Much smaller than high efficiency PV solar arrays, and vastly smaller than biofuel crop arrays. Most of the reservation area will be useful for life, if not inhabited, unlike solar arrays.

MORE LATER

Question : how much power is needed to vaporize clouds, or blow them away with the heat column rising from a hot rectenna?

MORE LATER

Weaponized microwave power beams

Transmitters with focuses capable of sharp power dropoff at the edge of the rectenna will be able to form spot beams as sharp as the dropoff. Potentially all the power-sat arrays facing a particular small area on earth will be able to dump all their beam power into a sharp spot. If 25 Terawatts is focused on 5 square kilometers, the beam power is 5 Megawatts per square meter, or 5000 suns, or 1 kiloton of nuclear weapon equivalent per second per square kilometer. This would vaporize people and torch cities.

This cannot be allowed to happen, of course. There are many system controls that can be added, from the political down to the operational. Perhaps the best way to protect cities from such treatment is to make the transmitters respond only to ground guide beams, perhaps beaming up "E-cash" messages to buy the power from the array. Otherwise, the arrays do not get proper phasing information and cannot focus.

At 2.8 cents per kilowatt hour, or 1 cent per megajoule, it would cost 250 thousand dollars per second to pay for a 25 TW beam (and probably lots more to outbid all the other arrays competing for the same power). Chances are, the source of the guide beam will get blown out or scrambled by all the incoming power, so the burst would be uncomfortable, but brief. So even if all the interlocks fail, economics could save the day. :-)

Actually, if the E-cash messages are cryptographically signed with receiver location information and time, then there are more defenses still; don't issue E-cash for weaponized use.

Another protection for cities might involve "scrambling" beams. The transmitter hardware can be designed to decohere when focus is attempted on a scrambler. While this may lead to sabotage and denials of service, if designed into the hardware it will be very difficult to get around.

Obviously, before we get to power levels that can do this level of harm, we will need to design many levels of defense in depth, relying on design and physics to protect us. And before these systems are turned on, we must insist that the designs are open and inspectable, and the server-sats coming off the manufacturing lines adhere to the safe designs, before we permit them to be launched.

MORE LATER.

BiologicalEffects (last edited 2013-02-17 02:30:55 by KeithLofstrom)