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.
- Has there ever been an instance of asteroid discovery by radar; seen first by radar rather than being observed after optical discovery?
- How does Deep Space Network's DSS-14 transmit radar and receive it at (almost) the same time? (monostatic radar)
- Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough?
- First satellite of an asteroid (or double asteroid) ever imaged by delay-Doppler radar?
And for larger objects like Mercury, Venus under its clouds, and even Saturn's rings can be imaged and studied by radar.
- Why do radar maps of the surface of Venus have missing slices?
- Farthest distance to a solar system object that's been measured by radar? (answer: Saturn's (very large) rings)
- How did Arecibo detect methane lakes on Titan, and image Saturn's rings? (answer: very carefully!)
- How did Arecibo make radar images of ice on Mercury's poles?
- Why is Saturn invisible in this radar image of its rings?
- What causes "North-South ambiguity" when doppler radar imaging a planet surface equator?
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.
- Are direct conversion optical receivers being looked at for future deep-space communication?
- How is the maximum data rate of the Psyche mission's Deep Space Optical Communications (DSOC) system expected to scale with distance?
- Quantitatively, why will optical communication be better than X-band for deep-space communications?
- Is it possible to extend high speed data transmission with lasers to the distance Earth to Mars?
- Receiver and transmitter in RF/optic satellite communication: distance vs data rate v2
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:
- this answer to Is pinging the Moon with a laser as shown on "The Big Bang Theory" possible?
- this answer to When they shoot lasers at the Moon for ranging, what is the shape of the beam?
- How to get an initial setting of the range gate for a Lunar Laser Ranging using a new Retro Reflector for the first time?
- How soon after “The Eagle” landed did they first attempt to bounce a laser off the Moon? When did it first succeed?
