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Comment by dr_dshiv | Hacker News Reader

After a couple thousand years of computational evolution, if you don't think there'd be a possible reason for leaving behind massive silicon crystals... Well, you haven't been reading enough science fiction.

If you could grow pure crystals more easily (thinking Diamon Age), why not have giant plates serving as a piezo generator or ambient electromagnetic waveguide harvesters?

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So a civilization thousands years more advanced than us failed to go to space (evidenced by the fact that they did not leave behind any space junk) but somehow produced energy from crystals?

FWIW, space junk wouldn't necessarily survive that long. Things on lower orbits would deorbit themselves quickly without station keeping. Things on higher orbit would get pushed away by solar radiation over long time, and degrade under high-energy radiation (such as UV rays). What remains would be hard to find, because space is big, so it would have to be fairly large and/or fairly reflective for us to notice now.

Wouldn’t any constant force tend to ellipticize the orbit and lead to a return to earth?

Would have to do the math to be sure, but doing some quick simulations in my mind - I believe you're correct, modulo the impact of the orientation of that force changing (Earth goes around the Sun). Let me illustrate with a diagram:

                      PROGRADE
                     (raise apo)
                          | 
           +--------------+--------------+
           |                             | 
           |   ->      --->---      ->   |
           +-  ->    -/       \-    ->  -+
               ->   /           \   ->  
             / ->  /    /II\     \  |   -+
       +----+  ->  |   |IIII|    |  |    +- IN SHADOW
       |     \ ->  \    \II/    /   |   -+
       |       ->   \           /   ->
       |   +-  ->    -\       /-    ->  -+
       |   |   ->      ---<---      ->   |
    RADIAL |                             |
     IN    +--------------+--------------+
  (also raise apo)        |
   ((I think...))     RETROGRADE
                     (lower peri)

This shows a body in orbit (the empty sphere) around the body (the filled sphere). The orbit goes clockwise, sun radiation comes from the left, arrows illustrate the acceleration it creates on the body. Simulating it in my head, I believe such setup would lead to the orbit getting elliptical, with the apogee getting larger and the perigee getting smaller, all the way until either the perigee hits the atmosphere or the body reaches escape velocity (thanks to small, but unbalanced contribution labeled RADIAL IN).

I'm not sure how long such a process would take for a satellite not designed to exploit solar radiation for propulsion, nor whether it wouldn't be dwarfed by other orbit-disturbing influences such as the Moon and other planets.

Also: aerobraking a highly elliptical orbit isn't going to leave you much of a satellite to find.

(Note: I studied orbital mechanics in university on Kerbin, under Jebediah Kerman, so I might be wrong here.)

It would only be eclipsed, daily, during a small part of each half-year, no? And, on alternating sides of its orbit.

Usually people blame the moon, and three-body dynamics, for disrupting other Earth orbits.

> evidenced by the fact that they did not leave behind any space junk

Whatever space junk we leave behind will be gone 100 years from now. https://www.nasa.gov/news/debris_faq.html

That’s really not what that means. The 4th satellite Vanguard 1 was put into orbit in 1964 and it’s expected to have a 240-year orbital lifetime despite getting to within 657.3 kilometers of earth. https://en.wikipedia.org/wiki/Vanguard_1

Geosynchronous satellites 35,786 km (22,236 mi) are going to be up there for a very long time.

PS: If you’re interested https://aiaa.kavi.com/apps/group_public/download.php/3172/IS...'

And anything that went to solar orbit will be the more or less for ever, subject only to the slow erosion of thermal cycles over the eons. And all Pioneers, Voyagers and New Horizons will not have to sustain even that, just comic rays.

BTW, it's a bit mind boggling how big and empty even just interplanetary space is - there could be wrecks of an unlucky discovery fleet paying a visit couple milion years ago and we might never find them unless they come really close to Earth or other sensor rich human inhabitted place or possibly still produce some active emissions (outgassing, sporadic radio signals, abnormal heat absorption/radiation, weird albedo variations, etc.).

I think that’s just for lower orbits, I was under the impression our geosynchronous satellites will be up there for millions of years (unless future humanity cleans up the geosynchronous graveyard orbit)

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