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Wednesday, April 09, 2008

 

Let a Million Laserpointers Bloom

Back in the cold war there was a joke that ran something like this:

An aide runs into the office of the US president and pants "Mr. President, the Russians are painting the Moon red!". The president just nods and keeps on working. A little while later the aide runs in again "Mr. President, the Russians have painted half the Moon red!" The president just nods and keeps on working. Finally the aide runs in "Mr. President! The Russians have painted the entire Moon red!" The president looks up and says "Now get those NASA boys on the phone and get then to take up some white Paint and write "Cocoa-Cola" up there."

This joke came back to me as I was watching a news item about the local Adelaide idiot who was arrested and sent to jail for nearly three years for shining laser light into a helicopter pilots eyes. This comes on top of a series of laser pointer attacks on aircraft coming in to land at Sydney airport. Uh, why you ask, does this remind me of that joke, there is nothing remotely funny about this? What is the connection? Now, my mind runs in all sorts of tangents, but there is a connection.

The connection is the idea that you could try and paint the Moon with laser pointers. Of course, it turns out to be impractical, as almost every person on Earth would need around aim 100 million laser pointers at the full Moon to colour it. This in turn bought to mind the US beer company Rolling Rock's "Moonvertising". Yes, I know it can't work as they state it (come on, it's a PR stunt for a beer company for goodness sakes! They can't even get their diagram of the Moon right, so even if they had a ginormous laser, they couldn't aim to write anything, and yes, I know the Bad Astronomer has already blogged this, but let me have my moment in laser illumination).

Now, the idea of advertising using space objects isn't new, it has been used by Arthur C Clarke in "Watch This Space" and Issac Asimov's "Buy Jupiter", and there has been the odd April fools joke before, and an Advertise on the Moon site (could be a hoax that one). Is it feasible?

Yes, for a suitably expensive definition of "feasible". The Rolling Rock, Advertise on the Moon and Laser Pointer scenarios all use the full Moon for a target. The full Moon has a number of advantages. It's visible all night, it is uniformly illuminated, so you could use the whole face of the Moon without contrast breaking things up.

On the downside, the illuminated Moon is bright, your laser has to overcome the light already reflected by the Moon. Also the Moon is SMALL, around 0.5 degrees wide, you can cover it's image with your thumb. So you are not going to be able to get any sophisticated image or text up there.
(Darlek Laser image copyright Peter Ward)

Under the "Paint the Moon" scenario you need to cover the entire Moon surface, and Julian calculated (under simplifying assumptions) that you would need 6.6 × 1017 watts to do the trick (That is nearly an exawatt). The most powerful laser available is 500 Terawatts formed from combining 192 laser beams. This is about 100 times too weak to do the job, even if all the laser beams weren't plugged into an experimental fusion reactor.

But for an ad we don't need to paint the entire Moon. We just need to illuminate a smaller spot. To illuminate a single dot on the Moon that is visible from Earth is a lot easier. Especially if we aim our laser at the unilluminated part of a crescent Moon. Patricia Daukantas at the Optics and Photonics News Blog calculated that you would need (depending on your assumptions) between a 1 to 100 Giggawatt laser to produce a visible dot on the unilluminated part of a crescent Moon. This is a lot more feasible than a Terawatt laser, and there are a few gigawatt lasers lying around. They just produce these levels for less than a picosecond (that's really, really short).

If we want to make an advertising message on the Moon, we need to drive that spot along the surface, to form a raster image like on a television screen. So the dot has to last a lot longer than a picosecond. You could get around this by firing a lot of gigawatt lasers in sequence, but as need around 2.5% of the total US energy consumption to power one of these lasers, you are looking at quite an energy bill.

So the answer to "is it feasible" is "yes, sort of", if you are willing to hijack a whole bunch (possibly the entire Earth's supply) of major experimental lasers and have the GDP of a small European nation in disposable cash to pay for the power, for an add that can only be seen on a handful of days when the Lunar crescent is sufficiently small to not interfere with the visiblity of the dot on the dark side, for a few hours before the crescent Moon sets. (new Moon and one day old Moon's won't work as they too near the Sun, and the spot will be lost in the sky brightness). Good luck with that.

Of course, seeing as the "missile killer" lasers are only around the megawatt range, the local aviation authorities might object to firing gigawatt lasers anywhere near flight paths (and the Birdwatchers probably wouldn't be too happy with all the fried wildfowl generated). Still, they are minor difficulties compared to what you will have to go through to get the lasers. It had better be a good logo.

Anyway, as you are reading this, lasers are bouncing off the Moon. They are bouncing off the mirror cubes (disco is good for something) left behind by the Apollo astronauts. Of course, you need a 3.5 meter telescope and really good photomultiplier's to detect these laser beams, and no one is making beer adds with them, but it is nice to know that they are there.

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