“Terrascope”: The possibility of using the Earth as an atmospheric lens (2019)

That's brilliant, thanks! I never would have guessed. It goes completely against my intuition.

Does that mean you could use light from Mercury to start a fire?

It's a great explanation, but a "spherical chickens in a vacuum" textbook explanation. Add in realworld atmospheric effects (diffusion) and non-lens things like the internalized reflections of fiber optics and the infinite reversability of optical systems breaks down. Then look to how easy it is to start a "fire" in some substances and the moonlight-to-fire concept becomes less difficult. Greenhouse effects also throw a wrench at reversability.

I think this what-if has serious issues in its argumentation (though I agree with the conclusion):

The argument about reversibility is kind of a straw man: it answers "why can't you concentrate all light on a single point" while the real question would be "why can't you concentrate light on a smaller surface" (which you can do actually).

Similarly for the conservation of étendue: maybe you can't "swoosh the light rays closer together" but that also doesn't say you can't concentrate beams on a small surface, which might be sufficient to start a fire.

So really it all comes down to the thermodynamic argument, which has its own problem: it only works if you assume that moonlight has the same temperature as the moon. There's nothing in the article that mentions or justifies this assumption. And obviously a mirror can reflect light that is much warmer than itself, so you definitely have to explain why that's not the case with the Moon (e.g. its albedo and heat dissipation are too low).

(However I love the drawing of the encircling sun, it's great to make the point that no matter how much light you concentrate, it won't heat the body warmer than the light's temperature).

But isn't the actual source the sun (6000K) and not the moon as it is acting only as a mirror?

With a mirror you can also burn things if you focus sunlight, but the surface of the mirror stays cool.

a 5cm lens from a dollar store can burn things in the focal point

85% is the minimum distance, but lens quality improves the further out you go because the lensing region moves up into the stratosphere so you get less distortion from clouds.

Apparently at the distance of the Hill Sphere of the Earth (about 4 times the Moon's orbital distance) the effect would still work.

The moon's gravity is in play so the orbit might not be so simple, but there are all sorts of "halo orbits", orbits around the L4 and L5 points, etc.

It seems to me though that the Earth's atmosphere is always going to be "lit up" one way or another by some phenomenon like the Aurora Borealis, scattering and various sorts of skyglow, so there will be some background, but maybe it is not that bad.

The math is easy, the delta-V is hard.

There is a precise point the craft would need to stay at relative to the earth and the target. There aren't any stable orbits that keep a craft at precisely that position, which means you are firing engines for 20 hours straight.

Remember, you don't stay in space unless you are either in a stable orbit, moving at many km/s, or you're firing engines to resist gravity's pull.

A spacecraft to stay at this point for 20 hours would be massive, and 99.9% fuel tanks.

They wouldn't though.

The Lagrange points are stable positions relative to two bodies, like the earth and the moon or the earth and the sun. But those things are moving, orbiting. The points move with them. The line from the earth to any Lagrange point won't stay stably pointing at any star in the sky.

And even then, the focal point of this atmospheric lens is at a specific distance that is not, afaik, the distance to any Lagrange point.

Unfortunately, the focal point is in interstellar space, far past the orbit of Pluto.

So even if we'll live to see this project funded, started and launched, there's pretty much no way we'll still be alive once the sensor probe reaches it's duty station.

Also, the telescope will be pretty much purpose build to look at one thing. Swiveling the telescope to look at something different or to track a moving object involves moving the sensor probe millions of kilometers across space.

Not quite, if I understand it correctly.

This is like chapter ~one of book one (if you skip over the tangential Chinese politics stuff that I didn't understand on my second read either), so not really a spoiler. In the book it's some magic amplification that happens inside the sun, not based on a physical effect we've actually observed in real life as far as I know.

In this case, it's about gravitational lensing (when using the sun) and additionally atmospheric refraction (when using the earth; terrascope).

I'm also not sure we could use this, as in 3BP, to send anything. What this does is collect light rays coming in essentially parallel and focus it on a line along which we can place a detector. If we emit from somewhere along that line, it would be scattered as a ring the size of the lens (the earth or sun, in the examples, plus whatever altitude we use to avoid clouds or corona) because the rays are basically parallel from there. At least, that's my guess. I didn't understand even what my high school teacher tried to explain about lenses so this is speculation based on a drawing I saw in the terrascope video linked elsewhere in the thread (https://www.youtube.com/watch?v=jgOTZe07eHA).