system
Given the following post, respond as if you're commenting on Reddit:
human
If I point a laser to a planet, does it reach its surface?

I've been reading an article about a laser that was pointed to a mirror on Moon's surface and reflected back, calculating the exact distance between the Earth and Moon.

That came with the question in my mind, if I take a laser pen, point it directly and correctly to a planet, it will reach its surface, or would it be disturbed by the atmosphere and/or other factor? What would be the effects suffered by the laser out of the atmosphere and outer space? The laser have light speed? How much time would it take to reach Mars, by example?

Sorry for any grammar errors.
gpt
It depends on what you mean by "reach its surface."  If you're just asking if any photons from the laser beam reach the other planet, the answer is yes.  And they will go there at light speed.  But then, your question is no different from asking if light from a light bulb on Earth reaches another planet, or asking if light reflected from the other planet reaches us.

If you're asking whether or not the laser beam reaches the other planet's surface like a coherent beam of light, the question gets more interesting.  

* While a laser beam may look perfectly collimated at short distances, diffraction from the aperture of the laser actually causes the beam to spread out a bit as it travels.  When you shine a normal, handheld laser at the moon, for example, you illuminate a giant patch of the moon many square kilometers in size.  Also, since the power/number of photons is constant as you go along the beam, if the beam spreads out, the intensity will drop.  (The light would be too weak to see with the naked eye.  Moon laser ranging experiments use very high-powered laser pulses in order to get a signal, which is still very weak, back from the reflectors.)

* You also have to take into account the dispersive and refractive effects of our own atmosphere.  The molecules in the air will attenuate the beam, making it much weaker by the time it gets to space.

* You also have to take into account the dispersive and refractive effects of the other planet's atmosphere, if it has a significant one.

All in all, your beam will be extremely weak, extremely spread out, and pretty much useless (assuming our current level of technology).  But theoretically, some small fraction of light from your laser will reach the surface.