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If you point a laser pointer at the moon — will the pointer be visible on it?

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If you point a laser pointer at the moon — will the pointer be visible on it?

That's exactly what the question that one of our subscribers sent me in the mail sounded like. Let's look into this issue together. The first difficulty that a person who decides to direct a laser to the moon will face is that it is not so easy to get into it with a beam, pointing at the eye, a person can miss hundreds or thousands of kilometers. But let's say some enthusiast used special equipment and was able to point the pointer exactly at the moon.

lazer beam

The beam of the laser pointer, at the exit from it, is a focused, thin beam of photons. When passing through the atmosphere, the photons will meet on their way a huge number of atoms, molecules, microscopic water droplets, and dust particles, on which the beam will be scattered. Also, scattering will occur outside the atmosphere, on rare particles located between the Earth and the Moon.

Источник: xkcd.com

In addition, it is worth considering that the laser is not designed for long distances: its power is low, and the focusing of the beam is not close to being compared with professional astronomical lasers. As a result, most of the photons emitted by the pointer will reach the surface of the moon, but the beam will be highly scattered: it will "illuminate" hundreds of square kilometers.

It will be impossible to see the light of this beam with the naked eye, and it will be possible to fix it only with the help of a telescope, and comparable in size to the largest telescopes on Earth.

Bem om the moon

Physicists have been doing something like this regularly for several decades. During the flights to the moon of Soviet lunar Rovers and during the flights of the Apollo missions, corner reflectors were placed on the moon, which returns light to the same point from which it came. They are used to clarify the distance between the Earth and the Moon. The coordinates of these reflectors are precisely known, they are guided by powerful, well-focused laser beams and with the help of large telescopes fix the reflected beam, after which it is not difficult to calculate the distance to the moon. This method is called the laser location of the moon.

One of the corner reflectors placed by the Apollo 11 mission. Source: nasa.gov

One of the corner reflectors placed by the Apollo 11 mission. Source: nasa.gov

In order for a telescope with a mirror diameter of 2 meters to be able to record a beam returning from the moon, the beam output from the laser emitting it on Earth must be at least 1-2 kilowatts, exactly the value depends on the design features of the telescope. You can compare this with the power of laser pointers: the most common red pointers have power in the region of 1-20 milliwatts, green-2 watts, blue-5 watts.

By the time it returns to Earth, the reflected beam will be scattered over an area of hundreds or thousands of square kilometers, but a large and sensitive telescope will be able to capture enough photons to detect the fact that the beam returns to Earth.


Related tags:

moon  earth  laser  laser beam  experiment


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