# Could sunlight be imported to New York from Tokyo?

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#### arumalpra

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##### Could sunlight be imported to New York from Tokyo?
« on: 28/12/2012 10:58:32 »
Could somebody help me on this to know whether this could be done theoretically?

New York is in dark. Tokyo day time. Import light from Tokyo. And vice versa.

1. Beam collected sunlight to orbiting mirror from Tokyo and reflect it to New York. And Vice versa.

2. Place mirror down in the ocean, reflect light from Tokyo to New York.

3. Transport light from Tokyo to New York by fiber optic.
« Last Edit: 28/12/2012 15:03:34 by chris »

#### CliffordK

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##### Re: Importing light from Tokyo to New York
« Reply #1 on: 28/12/2012 12:55:19 »
I can imagine creating an artificial sun over NYC or Tokyo would be controversial, but I could imagine the benefits of using renewable energy for street lights.

If one had, say a 1 square mile parabolic reflector, it may overwhelm a fiber optic system with heat, and one may have significant power transmission losses.  Then on the distribution end, one would still have a complex light distribution network to build.  The advantage, of course, is that it could be distributed inside and outside of buildings.

For a satellite based system, assuming dim light, it may not be sufficient for inside buildings, but the distribution network would be part of the satellite system.  A satellite based system could go in one of two ways.
Ground --> Satellite --> Satellite --> Ground
Sun --> Satellite --> Ground

The advantage of a ground based collector is that a large collector would be relatively cheap to build, and the satellites could be relatively small, and thus cheaper.  For example, one could put mirrors on rooftops.  I presume that on a sunny day, frequencies of light that makes it to the ground could be reflected back to a satellite with minimal additional power loss.

A couple of issues would include loss of power on cloudy days, both on the collector and delivery side.  Since Tokyo and NYC are both in the Northern Hemisphere, they would also both experience shortening of the daylight hours in the winter, so alone, they wouldn't be able to provide adequate sunlight to each city for the full night without additional collectors elsewhere.

And, of course, complex solar tracking collectors might be better suited for local solar-electric generation.

A Sun --> Satellite --> Ground system would not have the issues with clouds on the collector side, as barring rare eclipses (which might be able to be compensated for), the collector would always be in full sunlight.  However, the space based collector might have to be huge, perhaps a few square miles to illuminate a city like NYC.

Night time clouds at the receiving end would still be a problem, but one would still get a significant amount of diffuse illumination by shining on top of the clouds.

Much of the night, the satellite would be at nearly an optimum angle between the Sun and city being illuminated.  A geostationary satellite would be above the equator, and always send light to NYC at an angle.  However, one might be able to have a geosynchronous orbit that would put the satellite more overhead at night, and farther south during the day.

#### evan_au

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #2 on: 28/12/2012 22:36:30 »
A system which passes light through the atmosphere only once would be more efficient than one which passed light through the atmosphere 3 times, so a sun-satellite-earth system would be better.

Midday illumination levels at the equator are about 1kW/m2; indoor illumination levels are around 1W/m2 and useful street lighting may be around 0.1-0.01W/m2.

The Sun is a diffuse source of light, about half a degree across at the distance of the Earth. It is hard to focus the sunlight down to less than half a degree across from the location of a mirror (or lens). A mirror at geosynchronous orbit would produce an image of the Sun at least 300km wide on the Earth. A mirror in lower orbit could be smaller, and produce a smaller patch of light, but only for a short time.

The satellite mirrors would need to be very large, and steerable. It would need to be resilient to withstand micrometeorites and meteor showers, and would pose a significant hazard to orbiting spacecraft.

One concern is that someone could take control of it, and turn it into a weapon to overheat, freeze or starve someone they didn't like - or worse still, to cut off their satellite TV reception! Perhaps fortunately, this is far beyond our current capabilities.

Current research and trials are taking place on solar sails for planetary exploration, which would still be very useful in a much smaller size.

#### CliffordK

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #3 on: 28/12/2012 23:58:29 »
For the mirror satellite, the majority of "steering" would be maintaining the proper orientation between Earth and the Sun which could be done with gyroscopes and solar energy, and not requiring fuel.  However, torque and rigidity of the system would be an issue.

I'm trying to envision whether one could build the system with a primary mirror and a secondary mirror located in separate satellites, somewhat like an enormous telescope.  But, I think for it to be effective, one would actually have to mount the primary and secondary mirrors in a rigid orientation (one satellite).  However, this would allow one to optimize the collector angle which might reduce the need to operate the primary collector at an acute angle.

I can see that non-parallel light paths are a problem for parabolic mirrors.

The ability to weaponize the system would be dependent on the focusing ability.  If, as you suggest, the focusing system would be incapable of focusing the sun to more than a few hundred km, then the weaponizing would be negligible without physically improving the focus with a new focusing satellite, or laser pumping system.

While solar sail research may aid with the design of large lightweight mirrors, the solar wind, and the "solar sail" effects might be more problematic than beneficial with blowing the satellite off course.  Unless, of course, one designed the system to slowly drift, with multiple satellites to cover major cities, SF, LA, Denver, Chicago, NYC, Madrid, London, Moscow, Tokyo, etc.  A solar sail would also be less concerned with precise focus.

#### arumalpra

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #4 on: 30/12/2012 09:16:47 »
After going through replies, thought to make theoretical and practical model which is simple, quick and workable . Consideration is only on the intensity of light collected on the ground and reflected by the satellite reflector. Nothing else considered as everything else is depends on feasibility of passing enough light over entire path.

Only considered sun--satellite--earth option as putting up collector in the sky is hard do discuss. Drawback of passing light 2 extra times through the atmosphere could be countered by increasing the amount of light collected and carefully selecting the collector site.

1. Objective :  When the city is in dark after sun sets, give 0.1-0.01W/m2 or so illumination over small area (1000 square meter or so) for little duration of time (3 hours or so ). Amount of illumination, area and duration is really not a matter.
2. Using satellite with 100 square meter reflector.
3. Other conditions are perfect.

Q1. How much of light need to be reflected from the satellite mirror toward the earth?
Q2. How much of light need to be sent from the earth to the satellite in the sky?
Q3. Would simulating this in dark room filled with smoke be helpful for understanding?

Could somebody help me to find answers to these two questions? It may be difficult to find answers in numbers. Some explanation would be helpful.
« Last Edit: 30/12/2012 09:42:05 by arumalpra »

#### CliffordK

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #5 on: 30/12/2012 13:39:41 »
100 square meters (10m x 10m) sounds very small.
Compare to the International Space Station that has 8 solar arrays, each array is 35m x 12m, or 420 m2 per array, or a total solar array size of about 3,360 m2 for the ISS.

1000 m2 is about 31m x 31m, or quite a bit less than a city block.

If doing direct solar illumination, it generally wouldn't be time dependent.  However, it is likely that the maximum illumination would be at midnight.  The mirror would be at about a 45° angle at dawn and dusk, reducing the effective size exposed to the sun by a factor of about $$\frac{1}{\sqrt{2}}$$, or I think about 71% of the maximum exposure area.

If the 100m2 array could be focused onto a 1000m2 area, then it would focus about 1/10 of the incoming light.

If the average sunlight to ground level is about 445 watts of visible light.  Dividing that by a factor of 10, and one gets 45 watts...  which is still quite intense.

One could shine the 100m2 array over an area of about 100m(array size) * 445W/m2 (estimated incoming) / 0.01W/m2 (target) = 4,450,000 m2, or about 2km x 2km.

HOWEVER, as Evan mentioned, it may not be possible to directly focus the sunlight to generate a 2km x 2km focal patch from the 10m x 10m mirror.  He suggested that one may in fact get about a 300km x 300km area of illumination, or about 90,000,000,000 m2, at a very weak power.

Or, one would need to increase the power to about 20,000 suns.

The size of the array to illuminate a 90 Billion m2 area to about 0.01W would be about:
90B m2 * 0.01 w/m2 (target) / 450 w/m2 (solar), and it results in about 2,000,000 m2, or about 1.5km x 1.5km (a square mile or so).

(am I off by a factor of 10 somewhere in my calcs?  I'll look at it later).

Reflecting light from the ground to the satellite would likely be affected by the same diffusion problems unless one managed to convert it to a laser.

#### arumalpra

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #6 on: 31/12/2012 09:36:43 »
Clearly impossible...
Many thanks for great insight...

#### CliffordK

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #7 on: 31/12/2012 10:56:13 »
I wouldn't say impossible.
Just not cheap.
And at least a space solution would be sub-optimal for cloudy nights.

A city like Cairo or Mexico City might have significant benefits for such a project over NYC, London, or Tokyo, at least for early deployment.

Assuming my calculations above were correct, and a mirror about 2 million m2, (just under 1.5km x 1.5km) would adequately illuminate a metro area (ignoring angles and shadows).

Solar sail material is between 0.1g/m2 (theoretical?) and 12g/m2

Then, material at say 2 g/m2 would weigh about 4,000 kg, and would be well within the launch capabilities of a single rocket.  Even 12g/m2 would bring one up to 24,000 kg, and might take 2 to 4 Delta IV launches, but it is within reason.

If one could design a 20 year life before maintenance, and perhaps being able to extend it longer with maintenance.

Acting like a solar sail might be a problem.  However, it might also be an advantage as one might be able to use daytime sunlight/solar wind for positioning the satellite without requiring fuel.

Then the question is whether the launch cost, and energy in both building and launching, and later maintaining the satellite would be less than the cost of the electricity and maintenance and upgrades of the current system.  Perhaps ½ Billion Dollars for the first satellite.

#### FuzzyUK

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #8 on: 31/12/2012 23:46:02 »
We already have a device which exports reflected sunlight to Tokyo and New York at night time. It's called the moon.

#### evan_au

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #9 on: 01/01/2013 00:22:23 »
Some more orbital constraints:
• For street illumination, you would prefer light coming down vertically from the zenith. For a reflector in geosynchronous orbit, this happens naturally for cities close to the equator like Mexico City or Cairo.
• For cities far from the equator (like New York at 40° North, or London at 51° North), light from a geosynchronous reflector would come in at an angle equal to the latitude; even low-rise buildings would cast significant shadows across the street.
• Another constraint is the Earth's shadow, which is in the shape of a cone. The cone of totality is as wide as the Moon at the distance of Earth's Moon; at geosynchronous orbit it takes almost 10 minutes to pass through the shadow, which happens for a few days near the equinoxes, with a period of partial eclipse occurring much more frequently. This might lead to a crime spree in spring and fall/autumn.
• For satellites in low Earth orbit, the satellite enters Earth's shadow soon after sunset, which would not provide good night-time illumination.
• A solution may be to use satellites in a Molniya-type orbit (frequently used by Russian satellites), which would provide at least 6 hours illumination per night, at a high inclination so it spends most of its time outside Earth's shadow. http://en.wikipedia.org/wiki/Molniya_orbit
• In the skyscraper canyons of Manhattan, you would need a string of satellite reflectors to provide good illumination at ground level.

Some more material constraints:
• You want it to be as reflective as possible for light (the moon only reflects about 12% of the light that falls on it)
• But transparent at microwave/communications frequencies used by satellites

Note: Astronomers hate suggestions like this - they have enough problems with light from the Moon, and cities several hundred km away, without adding another night-time illuminator in the sky! Even when it is not reflecting light on the astronomers, this satellite will be blocking objects that the astronomers want to examine in the night sky...

#### CliffordK

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##### Re: Could sunlight be imported to New York from Tokyo?
« Reply #10 on: 01/01/2013 02:51:35 »
A geosynchronous orbit does not have to be above the equator.  It can be put at an angle as in this diagram.

In which case, I believe the satellite slowly forms a figure 8 over the course of a day.

Does a round orbit create a narrow "8", and an elliptical orbit create a wide "8"?  Of course, you might need somewhat of an ellipse to avoid bumping into stuff at the equator.

Unfortunately, while the satellite would return to its city every day, over the course of the year, it would go from being in the North during the night to being in the south during the night due to the orbit around the sun.

By setting it at a slightly faster (lower) orbit (I think), essentially on a sidereal day, one could keep it over the North at night, but it would slowly drift, Eastward (I think).  It should also minimize eclipses.

If tall buildings were reflective, the shadows might be minimized.  Or, one might choose a higher illumination level to minimize the shadows.

Unfortunately, one might not want to spend a half billion dollars (or more) on a streetlight mirror satellite, and still need to maintain a redundant backup system for cloudy days, eclipses, catastrophic failures, shadows, etc.

Or, to have multiple satellites to illuminate a single city, unless one would benefit from illuminating from different angles, but with smaller area mirrors, so the total mirror size would be the same, but the city would have broader coverage, at perhaps a similar cost.