>you only need to worry about the Earth's atmosphere I assume you're meaning this is the main source of attenuation (signal loss) if we assume the space betwee

>you only need to worry about the Earth's atmosphere I assume you're meaning this is the main source of attenuation (signal loss) if we assume the space between Earth and Mars is an empty vacuum. This (technically called scattering) is not the main source of loss for such a system. Coherent just means the light is all in phase, it doesn't mean it doesn't spread out as it travels. If it didn't spread out at all, your receiver on earth would be just as wide as the laser transmitter (about the size of a coin), and you'd have to be locked on the entire time... not very practical. It looks like NASA has intentionally focussed their LASER into a wide cone for precisely this reason. The distance from Mars would cause the beam (cone) to spread out too much, thereby reducing the power received on earth. You would have to increase your laser power on Mars to compensate for this or make the beam more directional with some focusing optics.