I think the calculations are extremely wasteful, since the writing being discussed would actually be visible from much of the solar system, while we only need it to be visible from Earth. The Earth subtends 1.2 millisteradians as seen from the moon, which is about 1/5000 of the whole half sphere. So if you place a mirror on the moon which throws a sunspot on the Earth, you can be 5000 times as efficient in emphasizing that patch of ground as if you simply paint it completely white (the reflectance of the moon itself is only 12%, and of course its light is diffuse and not concentrated on Earth).

Going with his surface area of 500,000 sq. kilometers, we would only need about 100 sq. kilometers of mirrors, spread randomly through the area to be emphasized, to produce a readable message on Earth. This is actually doable, although hard, and the message would only be visible at one particular angle (say, full moon directly overhead edit: depending on how much of the Earth you cover and how well, it may not need to be directly overhead), unless you want articulated mirrors.

In fact, I read, long ago, an article about a guy that performed this hack on Earth during the fairly early days of satellite photography. He drove around the desert in Southeast US, and placed small reflectors at angles calculated to reflect sunlight into a passing satellite. This resulted in huge overexposed letters in the satellite image, since each reflector exposed a very large pixel/grain of the fairly low-resolution satellite. I can't seem to find this article anywhere, although I remember it pretty clearly. If you can find it, I promise to upvote your submission!

You don't need big craters to create a shadow. Plough the moon, turning over the dust in 20 cm wide and 10 cm high piles. At a square km per machine per week (I'm making that up, but it is less than a hectare per hour, and the world record is over 4 square km/day (http://www.guinnessworldrecords.com/records-1/ploughing-area...), likely on way better ground, but not nuclear-powered, either), that's 'only' 10,000 machine-years. Turn 10,000 nuclear warheads into crude nuclear engines, and you can do it within a few years.

I might help spending time choosing an area where you can avoid deep craters and have enough dust to form the piles.

It seems like it would be much easier to finally make some self-replicating bots, or just self-repairing bots, or even just a small factory that can produce bots... And leave it there for a couple of decades with the task of writing that 'hello' message.

Given the low reflectivity of the moon, can't we "just" lay down a lot of mirrors in the shape of the letters we want?

Build a rocket ship that makes a flyby of the moon, drops some mirrors and returns. Maybe it could rendezvous with the ISS to get a fresh supply of mirrors and we send the mirrors to the ISS by conventional means.

And when I say mirrors, I am thinking of really thin ones. Really, just giant sheets of aluminium foil. I suspect they'll naturally drift around and degrade over time, giving people something to think about as a poignant metaphor of some artistry thing.

You know how the moon looks HUGE when it is low on the horizon? That is just perspective, but you should write on the moon then. Your brain does this built in Zoom thing when it has something to scale against, and you could use that to your advantage.

Rather than smoothing the surface to reduce shadows, Why not write in a color other than black? I'm thinking a laser in Green would work well and would contrast against black and white quite well.

More authoritative, and fun http://what-if.xkcd.com/13/

Fantastic!

As a side note: I wasn't aware how _little_ surface damage nuclear weapons actually inflict. I was always under the impression that a nuke would level entire cities + surrounding suburbs.

How about writing on the moon while it's dark? Reflect enough sunlight onto the dark side to make it about as bright as the light side. That would give a good contrast.

You would need an array of mirrors about the size of France in orbit about the earth. Thin film mirrors would be a lot lighter than asphalt. Precisely aiming the mirrors would be the challenge.

For those interested in the laser bit (hey, who isn't?), there's already a laser being fired at the moon from the Apache Point Observatory in New Mexico, measuring the earth-moon distance. Bonus cool photo: http://apod.nasa.gov/apod/ap140418.html

If we really has the ability to write on the moon, what will most people agree to write? hm....It's a difficult problem.

You just have to invent a way to change albedo of a surface using only local resources. You also need to think how exactly you're going to cover that territory of France - how heavy should be robots doing that, how many of them, how they'll withstand harsh environment - and require replacements...

OK, why are we making it darker? Can't we just LED the place up - sure, it'll require ongoing power, but it must be cheaper and more flexible.

Imagine you did this on Earth and how much you could save over nuclear bombing the place with paint. In fact, we already did it, as a side effect.

Great read. I thought that maybe it would be even more impossible to project the text on the moon with a geostationary moon projector. I heard that making a geostationary moon satellite is tricky, adding to the benefits of this solution.