Would an ordinary laser pointer reach the reflectors placed on the moon? I want to reflect a laser off of the moon. How powerful must my laser be?
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3$\begingroup$ Mandatory xkcd what-if reference. $\endgroup$– Andréas SundströmCommented Sep 6, 2022 at 6:50
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$\begingroup$ Further to @AndréasSundström's comment, if you bounce a laser off a target a distance $r$ away, not only does it have $r^{-2}$ attenuation at the target, but your detector of the reflection has to contend with another such factor, making $r^{-4}$ overall. $\endgroup$– J.G.Commented Sep 6, 2022 at 21:14
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2 Answers
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For sure the lasers used by the agency are strong , and what is important, the detection of the return from the reflector difficult even for the agency:
Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally occurring photons challenging.
For sure an ordinary laser cannot work.
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The first measurement of the distance to the moon by timing the round-trip flight of laser pulses was done using a 50 Joule ruby laser (avg power during each 500 microsecond pulse was 100 kW) in 1962. That was before they put retroreflectors on the lunar surface.
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#History
I'm not 100% certain, because it's hard to find pricing information about lasers in that category without speaking on the phone to a sales consultant, but I'm pretty sure that there are comparable "commercial" lasers.
Obviously, the size of the laser matters, but what also matters is the size of the telescopes that you use (one to collimate the outgoing laser beam, and one to catch the reflected light,) and also, the sensitivity of your detector.
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$\begingroup$ What was pulse repetition rate? Commercial lasers should definitely be available to hit the required specs, but definitely outside the “regular laser pointer” category. $\endgroup$ Commented Sep 6, 2022 at 17:55