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Space Mirror for City Illumination

Many years ago the Russians experimented with space mirrors.  The idea was to place a very large mirror into earth orbit and have it redirect sunlight down onto the earth’s surface below.  The mirror would be positioned so the sunlight would illuminate a whole city wide area during nighttime.  Just how practical is this concept?  How bright would the light be?  If it worked, then a city could use the mirror instead of street lights, which could be a real energy saver.
One of the first Russian attempts was a 20 meter (65 foot) diameter mirror, which was designed to project a spot 5KM in diameter.  But, that device failed. A second attempt used a mirror 25 meters (82 feet) in diameter.

It partly worked and illuminated a 7KM diameter spot on earth to a brightness 5 to 10 times that of a full moon.  I think to be practical; a lot more light will need to be projected onto a larger surface area.

Space Mirror  
Russian Space Mirror    Russian Space Mirror
After some digging on the Internet, I learned that typical moon light intensity is 0.27 lux.  One lux is a lumen per square meter.  Typical clear sunlight is 130,000 lux.  This sunlight intensity would be higher in space but once it made its way through the earth’s atmosphere, it would be back to the same surface level.
What should a target light level be for nighttime illumination in a city?  I think something around 50 lux might be reasonable.  I also think that a target area should be about a 10 mile (16KM) diameter circle. That should cover a medium size city with a surface area of 78 square miles.  If you divide 130,000 lux by 50 you get 2600.  So, the area of the mirror would need to be 78 square miles divided by 2600 or 0.03 square miles.  That would put the mirror diameter at about 1000 feet.  That is big but I think it might be possible.  It is not known how this mirror would be kept in the right position to track the sun, so the light spot would remain stationary.  Aluminum coated Mylar plastic is fairly light weight but we don’t have experience is an orbiting object with such a large surface area. I would expect some problems with small forces such as solar wind on such a large surface area.
Would the general public in a city approve of this kind of wide area lighting?  I’m not sure.  Some may object to a second sun up in the sky at night.  If the light is enough, it could mean many street lights would not have to be turned on.  But, the mirror would not help much during heavy overcast sky conditions, so street lights may still be needed.  I think this concept should be tried again with a larger mirror, using perhaps a 100 meter diameter mirror.  What do you think?


The intensity of moonlight varies greatly depending on the lunar cycle but even the full moon typically provides only about 0.2 lux illumination, so the full moon is about 500,000 times fainter than the sun.

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux as a measure of the total "amount" of visible light present, and the illuminance is a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area.

In standard units, luminous flux is measured in lumen.  One lux is equal to one lumen per square meter.




10−4 lux

Total starlight,  overcast sky

0.002 lux

Moonless clear night sky with airglow

0.01 lux

Quarter moon

0.27 lux

Full moon on a clear night

1 lux

Full moon overhead at tropical latitudes

3.4 lux

Dark limit of civil twilight under a clear sky

0 lux

Family living room

80 lux


100 lux

Very dark overcast day

320–500 lux

Office lighting

400 lux

Sunrise or sunset on a clear day.

1,000 lux

Overcast day; typical TV studio lighting

10,000–25,000 lux

Full daylight (not direct sun)

32,000–130,000 lux

Direct sunlight

Issue 2 -2011     February

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What the World
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