Why doesn't my laser pointer make a red dot on the moon?

By Viktor T. Toth


Oh, but it does. It’s just that your eye is not good enough to see it.

First of all, tight as they are, laser beams spread over distance, too.

The spread of a typical laser pointer is said to be around 3 mrad (milliradians). Multiply that by the distance to the Moon, which is about 385,000 kilometers… and you get a spot that is almost 1,200 kilometers wide, more than one third the diameter of the Moon.

So you are using the power of a handheld laser, a few milliwatts, to illuminate the area about one and a half times the size of Texas. That’s not a lot of light!

In reality, things get even worse, because the laser beam has to travel through the Earth’s atmosphere, which scatters it a fair bit, too.

But you know what? let us imagine that you have a “perfect” laser pointer: one that has zero beam spread, so the beam that arrives is as narrow when it hits the Moon as it is when it leaves the gizmo in your hand. Or better yet… let us imagine that an astronaut is standing on the Moon and does you a favor: he uses his laser pointer, just inches from the lunar surface, to illuminate a spot.

That is to say, he aims the grand total of 5 milliwatts of light at the lunar surface. A surface that has pretty low reflectivity, actually, so most of that 5 milliwatts is absorbed. But never mind that… say our astronaut found a spot that is perfect white. So now you have a red spot on the Moon, glowing with the full intensity of your laser beam: 5 milliwatts.

Will you see a 5 milliwatt light source on the Moon? Hell no. You won’t see a 5 watt light source either, or even a 5 kilowatt light source.

So how do they manage to bounce laser beams off the Moon, then?

First, they use a somewhat more powerful laser (see C Stuart Hardwick's answer).

Second, the laser beam is focused through a large telescope, to reduce its spread dramatically.

Third, and most importantly, they aim the laser at retroreflectors that have been left on the Moon by the Apollo missions. These retroreflectors are like a cat’s eye, reflecting incoming light back in the same direction that the light came from. So instead of being spread in all conceivable directions, most of the reflected light is aimed back in the direction of the transmitter.

Retroreflector left behind on the Moon by the Apollo 15 mission.

Fourth, instead of the naked eye, they use very sensitive instrumentation to detect the reflected light, capable of sensing single photons, combined with a narrowband filter that rejects light that has the wrong wavelength (color).

Put this all together and you have an experiment that is sensitive to the Earth-Moon distance with submillimeter precision, sensing ever so tiny discrepancies due to atmospheric disturbances, vertical displacement of the ground where the experiment sits, e.g., due to tides, even the tiny effect of solar pressure on lunar motion.

But sadly, no red spot visible to the naked eye.