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ProfRob's answer to Would it be practical to map out the Kuiper Belt and Oort Cloud via Radar? gives us a "sobriety check" on the idea. Citing an example of a 1 million watt transmitter at the late Arecibo dish with a beam of 2 arcminutes, a 140 m asteroid might be detected with an S/N ratio of five out as far as 0.18 AU only.

It goes on to remind us that:

  • The Kuiper Belt is circa 30-50 AU
  • he whole sky comprises of more than 10 million sky patches of this size

But astronomical applications of radar are many - it gives us not only precise distance and velocity measurements of near-earth asteroids but exploiting doppler shift and asteroid rotations, actual reconstructed images of their surface and identification of "moons" of asteroids.

And for larger objects like Mercury, Venus under its clouds, and even Saturn's rings can be imaged and studied by radar.

Alas, Arecibo is no longer with us.

On the other hand, with it's 4-5 orders of magnitude shorter wavelengths, only a few meter aperture can give us beams adjustable from arcminutes to sub-arcseconds wide (if wavefront correction is used). And while there aren't a lot of easy-to-run CW lasers at 1 MW, pulsed lasers can produce pretty hefty pulses at kHz rates and 100's of watts average power.

In the past, S/N ratio for optical communications reception has been limited by photodetectors - the double conversion of light to electron-hole pairs followed by their collection and conversion to a current means reception S/N already scaled as $1/r^4$ in addition to the $1/r^2$ for transmission. For LIDAR that would be $1/r6$ overall.

Now however, fast, cryogenic bolometric detectors that convert a thermal pulse to a charge pulse directly have made $1/r^4$ LIDAR at astronomical distances possible.

We already bounce light pulses off of tiny retroreflector cubes on the Moon and recieve photons for ranging purposes, even during the day (using filters, gated electronics and photomultiplier tubes - I'm still looking for the answer post here or in Space SE that gives those details...)

And there are and have been LIDAR topography mapping systems in orbits around the Earth, Moon, Mars, and several asteroids.

So I wonder: What are the technologies needed to make deep space LIDAR competitive with RADAR? Are there any plans for tests, prototypes or pathfinders for this kind of observatory?


further reading:

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