From: 71450.1773@compuserve.com (Steven B. Harris)
Newsgroups: sci.nanotech
Subject: Teraforming Venus/ Throwing Snowballs
Date: 23 Jul 92 01:05:42 GMT


   Jon Leech (leech@cs.unc.edu) says (17 July) about teraforming
Venus:

   >>You need to get rid of most of the atmosphere, which can be
done in two obvious ways: (1) Eject it to space. Unfortunately
this requires an energy input equivalent to thousands of years of
sunlight falling on Venus.  (2) Chemically remove most of the
carbon in solid form. Some possibilities for nanotech here,
perhaps. How much energy is needed to convert 4*10^23 grams of
CO2 into diamond and oxygen?  Once you've done that, you need to
import fairly large quantities of hydrogen from gas giant
satellites or rings to make water, and you probably want to
shield the planet somehow to keep the temperature down.<< 

   My comment: Jon is basically correct, and I agree with his
numbers.  Rather than convert the CO2 to diamond, it would be
much more energy efficient to convert it to carbonates.  Earth's
outer crust probably has as much carbon as Venus, but on Earth
it's all locked up as carbonates in rock, whereas on Venus there
is an absence of liquid water (and the hydrogen to make it) to
dissolve the CO2.  Venus' atmosphere has only .001% of the water
of Earth's oceans.  So a LOT of hydrogen has to be imported (1.5*
10^23 grams if you want an ocean like Earth's).  But hydrogen is
energy-expensive in the solar system: Getting mass out of
Saturn's F ring (for example) is five times more difficult than
getting mass out of Venus' atmosphere, so it looks like if we
import enough hydrogen to give Venus an Earth-type ocean it will
cost us about as much energy as simply removing the Venusian
atmosphere (and we have to get that energy out where there is
little sunlight, too).  

   Things will be different, of course, if we can find some big
iceballs orbiting in the outer solar system beyond Pluto (but
well inside the Oort cloud).  Such things would have tremendous
gravitational potential energy with regard to the Sun, and that
energy might be usefully employed to remove the Venusian atmos-
phere.  To employ this mechanism, you search the outer solar
system until you find a slowly orbiting iceball maybe 1000 km in
diameter (this is the same mass as the Venus atmosphere and the
same order of magnitude as the total water in Earth's oceans),
and then you stop its orbital motion and let it FALL on Venus. 
Such a thing would look like a giant comet coming in, of course,
and we can call the whole thing Operation Velikovski (TM).

   You get a lot of energy dropping things from the outer solar
system: every kilo gets the equivalent of several times Venus
escape energy, and Venus' orbital velocity and gravity adds to
that so that in the end every kilo of ice dropped on Venus from
the outer solar system has enough energy to blow itself and 3.5
kilos of atmosphere off into space.  Thus, if you drop an iceball
with the mass of the Venusian atmosphere onto Venus, you're not
guaranteed that you'll blow ALL the atmosphere off into space,
but since you've got several times the energy you need, surely
much of it will go, and what's left will be likely have enough
water for both shallow oceans and CO2 removal. (And yeah, think
of the environmental impact statement...)

   So: the good news is that you can save a lot of energy going
to the very outer solar system for your Venusian teraforming ice
(assuming you can find it there), but the bad news is that if you
want to do things in any kind of reasonable time, the energy
savings isn't much more than an order of magnitude.  

   In-fall time to the Sun, for something falling freely in a
line, is the classic circular orbital period divided by 2pi. 
Thus, at the orbit of Neptune (165 years), in-fall time (or high-
eccentricity orbit half-time) is about 26 years.  For an in-fall
time of 100 years (our arbitrary time limit for this project),
the distance corresponds to about 63 A.U. (twice the distance to
Pluto).  "Retro-rocket" energies for stopping things in orbit at
63 A.U. are still high enough that our project energy for a
4*10^23 g snowball is (at least) 3*10^27 Joules.   This energy,
although less than 1/7 what is necessary to remove the Venusian
atmosphere directly, is still 750 billion megatons, which sounds
like more than could be easily done, even with fusion devices
(one can imagine moving a giant iceball/asteroid with bombs,
Heinlein style, but what kind of thing would a billion megaton
bomb be??).   

   Since the in-fall time scales as the -2/3 power of the neces-
sary comet retro-bomb energy (in Keplerian fashion), you don't
get energy savings very fast as you permit larger and larger in-
fall times and distances: for instance a 200 year in-fall time
still requires 63% of the retro energy.  Moreover, as it happens,
giving asteroids a ~10^4 m/sec push so that they reach the inner
solar system fast enough to make a difference on this time scale
(i.e., ~100 years) requires enough energy that we again loose all
savings that we gain by going farther out.  We can also save
energy by putting things in Hohman transfer orbits to Venus, but
again we lose in time.

   So, in short, it looks like we're stuck on the issue of
teraforming Venus, unless we're willing to wait much longer than
several hundred years, or unless we find some way to spend energy
on Venus at a far higher rate than the rate Venus itself receives
energy from the Sun.  Perhaps putting up REALLY large numbers of
mirrors to broil Venus is the answer, as suggested by Drexler and
others (I believe).  The pesky planet's horrible climate combined
with its near proximity does seem an affront to good taste, and I
do agree that we ought to do something about it, if only for the
sake of aesthetics.



                                     Steve Harris