# The Ultimate Night Light

Is it theoretically possible to cover the near side of the moon with some kind of reflective material such as mylar in order to provide us with enough light to not need streetlamps for part of the month? How severely would it impact wildlife? How much light would it provide? would the long term savings in energy make it worth it? How much would it cost?

Simply wrapping the moon in mylar would cause less light to be reflected toward the earth (because the moon is roughly spherical - it’s not a flat disc) ; in the ideal case of turning the moon into a perfectly mirrored sphere, we would only see the specular highlight.

What would be required would be to set up an array of mirrors, each of them angled in such a way as to reflect light toward Earth; because the relative positions of the bodies change, the array would have to actively track the sun.

I shudder to think about what that kind of plan (the active mirror system, not the mylar) would do to the global climate, not to mention the flora and fauna…

A full moon reflects about 8.4% of the light hitting it toward the earth (visual geometric albedo at 5[sup]o[/sup] phase angle.)

If you increased that to 100% via Mangetout’s suggestion, you’d make the full moon about 12 times brighter.

The light intensity under a good streetlight is 2 footcandles, and you can (just) read a newspaper under that kind of light.

If the full moon was 12 times brighter, it would give a light intensity of about 0.36 footcandles.

So no, it wouldn’t be capable of replacing streetlighting even during a full moon.

Hmmm, depends Desmostylus; an active array could be focused, so you could actually be capturing the full output of the sun across a circular area the size of the moon’s diameter, then reflecting it all back to a very small area on the Earth’s surface; I imagine that would be quite a formidable amount of energy…

I’m just drawing some crude diagrams; I’ll post a link to them in a moment.

Whoo hoo! Death-ray coming up!

About 10 Petawatts (10[sup]16[/sup] Watts), would be the power available.

I should have specified a reflective substance that would only reflect the visible portion of the spectrum not UV or IR.

If you were able to focus the beam on a square area 100 kilometres on a side (10,000 square kilometres), you’d get an intensity of about 1 Megawatt per square metre, which is about 1,000 times as intense as full sunlight.

That should be able to turn everything into slag.

Wow, that really would be the ultimate weapon, no fallout and you could cook an area for hours at a time. Even underground bunkers wouldn’t be safe unless they had their own supply of cool air to breathe.

Ok, the moon currently reflects light something like this - incoming sunlight (parallel rays assumed for the sake of simplicity) is scattered by the irregular surface, so it looks pretty much the same brightness from any angle, but because of this, from any angle, only a small proportion of the light hitting the moon is reflected back at your eye.

Coating it with mylar to make a big mirrored sphere would result in reflections like this - light would simply bounce off at the same angle as it came in - because of this, the moon as viewed from Earth would appear mostly dark; only a small spot of bright light would appear where the angles were just right (look at the sun’s reflection in the back of a spoon).

Covering it with an active, focusable array of individual mirrors like this would allow us to direct all of the incoming light anywhere we choose, if we angled the mirrors in such a way that the outgoing rays were parallel, we would be able to project a moon-size patch of full strength daylight onto the Earth’s surface. If we made the rays converge, things could get really nasty (as Desmostylus says).
In this case though, viewing the moon from somewhere on Earth outside of the projected patch, the moon would appear dark because all of the light would be going somewhere other than your viewpoint.

Of course when the moon was not full, then some of the mirrors in the array would not be receiving light to reflect back; output would diminish accordingly.
In theory, the light from all of the mirrors in the array could be focused on a spot not much larger than the area of the largest individual mirror surface, in which case the results would be truly frightening; I don’t know what would actually happen.

(BTW, sorry that the diagrams are a bit crappy)

Hey, the diagrams are great. Good work, Mangetout.

Ah, don’t worry the diagrams are fine. How about the financial feasability? Assuming they were built out of lunar materials and the fact that even though it couldn’t light up a whole hemisphere to daylight levels it could spotlight smaller areas and probably replace a good deal of our nuclear weapons.

If it could be made from lunar materials, then the implementation cost could actually be quite small; imagine a solar-powered factory that manufactures smaller solar-powered vehicles, each equipped with a mirror, then despatches them to find their pre-allotted place in the array. The R&D cost would be huge though.

It would be better to build the array on the far side of the moon and use to defend the Earth against asteroid strike - assuming that we would have sufficient notice of the incoming asteroid, we could focus the array on one side of the incoming body, causing some of the asteroid material to be boiled away and ejected, altering the trajectory.

As a first order estimate, take the capital cost as \$1,000 per square metre. Total cost = \$10[sup]16[/sup]. About equal to 1,000 years worth of the current US GDP.

The Russians are quite interested in using orbital mirrors for illuminating cities in Siberia. They launched a controllable mirror, the Znamya 2.5, in 1999 but it failed to deploy. I don’t think there has been another attempt.

Too bad, the concept really had a lot of potential.

Astronomers would kill if that happened… we are already up in arms over light polluting street lamps.

I’m assuming it wold only be used to light cities, or other places with a high population where there is already too much light pollution from streetlamps and other sources to do any serious stargazing.