How strong a projector would be required to display an image on the moon
May 12, 2021 11:37 AM   Subscribe

Inspired by My Brother, My Brother, and Me's coverage of this Taco Bell promotion I had the thought that it would make sense for them to advertise on the moon. Would it be possible to project an image from earth, onto the moon, and have it visible back on earth?
posted by gzimmer to Technology (6 answers total)
Best answer: Not exactly what you're asking, but Randall Munroe answered a similar question in an xkcd What If?
posted by lharmon at 11:45 AM on May 12, 2021 [9 favorites]

Response by poster: @lharmon, Wow that was pretty exhaustive, I guess that's about as close an answer as I can hope for. Thanks!
posted by gzimmer at 12:17 PM on May 12, 2021

If I had to guess, a far more effective plan would be to fly to the moon and carve a Taco Bell logo into the regolith. We're probably getting close to the point where that's within the advertising budget of a large multinational such as Taco Bell. Just land a few solar powered RC cars up there and let them drive around for a while.
posted by booooooze at 12:22 PM on May 12, 2021 [1 favorite]

Best answer: Some rough calculations to back up the conclusion that "no, this is not even close to practically feasible":

The biggest difficulty with xkcd's example (shining lots of laser pointers at the moon) is that he's making the assumption that you have to illuminate the entire visible surface of the moon. Let's dial it back a bit, and say that our minimal definition of an "image" is a barely-visible point of light, equivalent to maybe a fourth-magnitude star, against the background of the darkened face of the moon. Assume we're willing to spend whatever it takes on lenses and mirrors in order to focus an LED light source down to that point.

An apparent magnitude of 4 corresponds to a light source of about 0.00000005 lux (direct sunlight is about 100000 lux). To be visible with that brightness from the distance of the moon, the amount of light *reflected* from the moon would have to be about 8 billion candelas. If we assume that the moon's surface is a diffuse reflector with an albedo of 0.12, that means you would have to illuminate the moon with 200 billion lumens in a focused beam.

So you're still talking about somewhere in the ballpark of a gigawatt of power in order to produce one tiny pinpoint of light. That's probably within the realm of imagination, if you're willing to throw billions of dollars of engineering at the problem, but an actual image would be orders of magnitude more difficult.
posted by teraflop at 12:26 PM on May 12, 2021

Best answer: You'll want to consult one of Robert Heinlein's characters -- it was D.D. Harriman in The Man Who Sold the Moon, I believe -- who was working on a plan to make the moon a bill board.

He determined that projecting light all that way was impractical; instead, his plan was to scatter black powder in the shape of his client's logo, large enough to be visible from the surface of Earth.
posted by wenestvedt at 12:32 PM on May 12, 2021 [5 favorites]

If I was at Taco Bell marketing, I would pitch an augmented reality branding of the moon. Pokemon Go style - just view the moon through an app to see the ad. That idea is free, hire me if you want other terrible ideas.
posted by Gor-ella at 9:42 AM on May 14, 2021

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