The Limitations of Lidar in Deep Space

[GeeKay]

This question concerns possible future uses in space of Lidar (Laser Imaging, Detection & Ranging) with regards to the detection of incoming asteroids, comets etc. Thus what would be the limiting distance of a Lidar system of a given power output in terms of resolution? Could its laser beams in theory be able to extend from Earth orbit to as far away as (say) the Main Asteroid belt, or even further afield? Or would the beam's width or 'spot-size' by then be too distended/incoherent to be of any use? Forgive any imprecision in the use of scientific terms here.

Many thanks.

[swansont]

I’m not sure what specific advantage LIDAR would give you. A laser beam with a small divergence means you only cover a tiny solid angle, so you’d likely only use it if you already knew where to look

But the answer would depend on how much laser power you have. They use laser ranging with the retro-reflectors on the moon

At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide^[21] and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers (1.9 mi) away. The reflected light is too weak to see with the human eye. Out of 10²¹ photons aimed at the reflector, only one is received back on Earth, even under good conditions.^[22]

https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment

The intensity would drop off with distance squared (the beam gets bigger) and other objects would be less reflective than the mirror array, and the aiming problem gets worse, since you still want to hit it with the middle of the beam

[GeeKay]

Yes, I sort of guessed that beam spread etc might be too problematic beyond a certain distance. So it appears then that optical telescopes are still the only reliable means we have for detecting asteroids and so forth. All the same, spotting any incoming comets by optical means alone while they're still beyond the Jovian snowline could be quite a challenge, especially given their generally very low albedos. 

I can't (as yet) find the article which prompted this thread, but a recent one from Centauri Dreams may suffice:

https://www.centauri-dreams.org/2021/09/02/deep-space-network-a-laser-communications-future/