From: szabo@techbook.com (Nick Szabo)
Subject: Comet deflection & mining
Organization: TECHbooks of Beaverton Oregon - Public Access Unix
Date: Fri, 6 Nov 1992 22:46:06 GMT

Deflecting anything but the strongest nickle-iron asteroid
with a nuclear explosive is silly.  Many asteroids are probably 
rubble piles, not single big rocks, and comets are so fragile we've 
seen some calve off big chunks and obliterate themselves just from
internal gas pressure.  For a comet, farting can be suicide!

10^11 tonnes about equals the annual U.S. consumption of
fresh water, so P/Swift-Tuttle wouldn't make make much difference
down here even if you could get it through the atmosphere
without damaging things.  A much better use is Earth orbit,
in suitably small wrapped pieces.  If we're to have any significant 
manufacturing industry in space, we're going to need tons of volatiles.  
For example, here is the water used to make various kinds of products 
on earth:

			gallons/unit
			------------
finished steel, ton	40,000
automobiles, unit	12,000
trucks, buses, unit	20,000
ref: Mark's Standard Handbook for Mechanical Engineers, 1987

Presumably we could economize and recycle more than we do on Earth, 
but life is much nicer when there's a lot of water handy -- not to 
mention the nitrogen, methane, etc. found mainly on comets.  Also
not included in those figures is the copious amount of air for cooling 
and lubrication assumed by most industrial processes, eg milling.
Furthermore, most of the mass launched into space is propellant
for orbit-transfer and stationkeeping.  We can easily convert from comet 
ice to various kinds of propellant or use as is in thermal rockets.

P/Swift-Tuttle at 50 km/s delta-v is extremely difficult
to get to.  It doesn't make sense to try to capture it; there
are many Jupiter-family comets with periods of 3.5-6 years
and only 8-10 km/s away.  There may be ice closer still in
some of the "mini-asteroids" or larger Apollo-Amors, but that
requires more exploration to confirm or eliminate.

With automated equipment we form a reasonably pure cylindrical 
ice shape, attach a small thermal rocket, and capture over 10% of 
that ice mass into Earth orbit, or 25% into Mars orbit, while 
expelling the rest as thermal rocket exhaust.  Moving the same mass 
of material from a gravity well would require rocket with five or 
more orders of magnitude power.  Economics hinge on the mass-thruput 
ratio of the extraction equipment, which could be similar to water-wells 
and ice-makers on Earth.  At around 2,000:1 MTR it becomes economical 
to capture Jupiter-family comet ice into Earth orbit.

Back to the P/Swift-Tuttle deflection problem.

If upper-stage technology advances sufficiently over the next 30-40 
years, eg magsails powered by the solar wind + a very advanced nuclear 
electric second stage, we might be able to catch up with P/Swift-Tuttle 
at perihelion in 2057 to track it.  Alternately, we might develop very
good telescopes capable of tracking it that far out, eg huge microgravity-
based reflectors combined with optical interferometry.  Who knows what
technology we will have after 2100, but one possibility is to focus
sunlight with a large parabolic mirror over the period of several
months to change the time P/Swift-Tuttle crosses earth orbit by one day. 
Anyone want to tackle the math on how large a mirror would be needed?  
Even with this gentle method, we need to gaurd against the possibility 
of disrupting the comet rather than deflecting it.  Rendesvous with 50 km/s 
incoming will also be a challenge, perhaps several years with a tacking
magsail.


-- 
Nick Szabo					 szabo@techboook.com
Hold Your Nose: vote Republocrat  ////////  Breathe Free: vote Libertarian
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