I recently discovered an old hoax saying that Rolling Rock was going to turn the moon into a giant billboard by projecting their logo onto it with a laser. Even though this was a hoax, the idea of advertising in space is not new. It is a common enough idea that it even warrants its own wiki page. I wanted to consider, from a physics perspective, some of the logistics that would be necessary to project an advertisement onto the moon.
Projecting onto the Moon
The only feasible way of projecting an advertisement onto the moon would be with an extraordinarily powerful laser. Not only would the laser have to be very powerful, but it is just impossible with current technology to make it powerful enough to be able to see it wherever the Sun is shining on the moon (mainly due to the Moon’s albedo). That means that there would be no advertising during a full moon. The best time to advertise on the moon using this technique would be during a crescent moon. This would provide the biggest area possible to use as an advertisement. If you think this idea is a bit far-fetched, there was a proof of concept experiment done way back in 1968 that consisted of shining a laser onto a sattellite that was orbiting the moon. The two spots in the following image are actually lasers emitted from observatories on Earth.
This experiment was done with a 2 Watt Argon laser. With today’s technology, commercially available lasers can reach powers of up to 3000 Watts. Such commercial carbon dioxide lasers would definitely be powerful enough to be seen from Earth when it is reflected off of the Moon, but the next question that would need to be answered is: Is the Moon big enough to display an advertisement?
To answer this question, we first need to figure out how big the Moon is as seen from Earth and compare that to how well the human eye can see. The first part of this question is very easy to look up, measure (with a fairly precise telescope), or calculate. If you look at one side of the moon and then look at the other, the angle you have to turn you head is considered the angular diameter of an object. This is often the most useful measure of the size of an object in astronomy. The Moon has an angular diameter of about one half of a degree. To give some perspective, the width of your thumb, held at arm’s length, is about four times that. So, look at your thumb at arm’s length, mentally divide that into quarters, and imagine an advertisement that size.
To become a bit more precise than just using our thumbs, optometrists have created a standard which we can use to help us. The Snellen Chart is used to judge eyesight by optometrists. Using this chart, optometrists have decided that 20/20 vision means that you should be able to read letters that subtend 5 minutes of arc (1/12th of a degree).
That means that you could fit a total of six letters across the moon at its widest point. That doesn’t make for much of an advertisement.
So, in conclusion, I think that our Moon is safe from unsightly advertisements. It just does not provide a big enough object to justify the cost of projecting such a powerful laser onto the moon. There are other proposals for advertising in space, but they suffer from the same size limitations as what is discussed above. Space is just so far away that things look very small unless they are exorbitantly large but that introduces other logistical problems.