RingWorld
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      / The desire to reach the stars can be the cradle of
        Extropian spirit, but we can't live in the cradle forever /

      / Extropians should spend their fuel launching memes /
   
		   				- Alexander Chislenko

   
   This is my belated answer to the question of whether the RingWorld
- a system of rings around the Sun to collect its energy and host
humans - is a useful and technologically sound idea.


 General design
 --------------

  I describe here my understanding of the Pokrovsky Shell suggestion.
I believe RingWorld whatever it is isn't that different.

    Let's imagine a ring construct a bit wider than the Earth's orbit,
10 km wide and thick enough to host some equipment, housing, etc.
It forms a perfect circular orbit around the Sun.
    Then we make another ring, just like this, but a few kilometers wider,
and orbiting under a slightly different angle. And another...
The rings will overlap at one point, partially overlap close to it, and
will lie at a small distance from each other in the middle.
The whole thing looks like a gigantic shell with 2 openings (good for
spaceships).
  The rings will NOT completely hide the whole star from the outside
observer, since complete covering in the 'middle' of the rings would mean
wasteful overlaps everywhere else.

  Stability and safety.
  ---------------------

  Let's consider one ring.
We can't make it perfectly round, just *close*.
Now, a piece that is a bit closer to the Sun, should move a bit faster,
and vice versa.  Otherwise, closer parts will start falling in the moment
we put the ring in orbit, and other parts will start falling out.
No way the ring can be strong enough to resist it.
The orbit can not be made perfectly round, and even if it was, there are
many enough distortions in the gravity field to ensure the ring's instability.
   So it is absolutely necessary to make rings elastic and design an
extremely sophisticated active position-control system.
So our ring will snake thru space, bending and stretching, with its engines
constantly on.
   I am not sure the captured solar energy will be sufficient for these
tasks, but I would definitely recommend not placing any valuable objects,
including other rings, any close to such unstable and mobile construct.

  Meteorite protection.

The Ringworld will need all the materials of the asteroid belt, and a good
portion of the planetary material to construct.  Asteroids can be a better
source of material since their lower gravity makes them easier to take apart.
However, there will be left trillions of small stones (with size between 0
and, say, 10 meters), that are too small to be of interest as materials.
With their speeds of tens of kilometers per second, however, they are deadly
dangerous for the rings (which do not have any atmosphere to protect them).
No material can be strong enough to withstand a hit of even a relatively
small pebble.
  The only working protection I can think of is an SDI-like system that
would constantly watch the surroundings of every ring for the tiniest stones
and immediately reduce them to dust with laser beams or whatever.
Not an easy task. Also, it may have to use a considerable portion of the
ring's energy balance (if it has any surplus left after the position-
correcting expenses).

   Good habitat?
   -------------

   Oh, just one more energy-consuming thing:
We'll need lots of rotating cameras within the ring to provide artificial
gravity for humans, animals [?] and plants.

   We should also remember that the ring will lack many of conventional
niceties of our Earth: blue skies, mountains, oceans, rivers and lakes,
clouds, rains and snows, highways and many other things that make our
lives what they are.
   However, rings will provide MORE LIVING SPACE - and this is one of the
two major purposes they are needed for.  And more living space is absolutely
necessary for the multiplying human race.
   IMHO, though, it's much easier to teach humans proper use of condoms to
limit their *thoughtless* multiplication than to colonize the solar system
and persuade the same humans to abandon their natural habitats.

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   Also, lots of human activities require spatial proximity of their
participants and lots of other objects.  This clusterization tendency by
itself will make sure that people will live in 3-D clusters, and leave
the rings for energy collection only.
Flat habitats just won't work.
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   Energy collection.
   ------------------

This is the second (after large living space) - and the last - most
important argument for ring construction.

  So far, I came to the conclusion that people will rather live in isolated
3-D constructs (and most stay on Earth and, maybe, a couple of terraformed
other planets).   Rings can be left much leaner and serve for energy
collection only.

  Still, the rings may collect a lot more energy than we can now find from
any other source, and will still be very useful after the several centuries
of extremely expensive work, provided no breakthroughs in energy consumption
or generation will be made by that time.
This by itself is a condition