beowulf in space

Robert G. Brown rgb at phy.duke.edu
Thu Apr 17 16:54:17 EDT 2003


On Fri, 18 Apr 2003, Mof wrote:

> Ok excuse my ignorance, but what is involved in rad harding hardware ?
> Is the cost really necessary, in that couldn't you put the unprotected 
> hardware into some sort of shielded container ?
>
> Or am I just being silly ? :-)

Not really silly, but IIRC shielding is both difficult and expensive and
sometimes actively counterproductive in space.  I'm sure the NASA guys
will have even more detail, but:

  a) Difficult, because there is a very wide range of KINDS and ENERGIES
of radiation out there.  Some are easy to stop, but some (like massive,
very high energy nucleii or very high energy gamma rays) are not.

  b) Expensive, because to stop radiation you basically have to
interpolate matter in sufficient density to absorb and disperse the
energy via single and multiple scattering events.  Some radiation has a
relatively high cross section with matter and low energy and is easily
stopped, but the most destructive sort requires quite a lot of
shielding, which is dense and thick.  This means heavy and occupying
lots of volume, which means expensive in terms of lifting it out of the
gravity well.  I don't know what it costs to lift a kilogram of mass to
geosynchronous orbit, but I'll bet it is a LOT.

  c) Counterproductive, because SOME of the kinds of radiation present
are by themselves not horribly dangerous -- they have a lot of energy
but are relatively unlikely to hit anything.  So when they hit they kill
a cell or a chromosome or a bit or something, but in a fairly localized
way.  However, when they hit the right densities of matter in shielding
they can produce a literal shower or shotgun blast of secondary
particles that ARE the right particles at the right energies to do a lot
of damage (to humans or hardware).  So either you need enough shielding
to stop these particles and all their secondary byproducts, or you can
be better off just letting those particles (probably) pass right on
through, hopefully without hitting anything.

Basically, we are pretty fortunate to live way down here at the bottom
of several miles of atmosphere, where most of the dangerous crap hits
and showers its secondary stuff miles overhead and is absorbed before it
becomes a hazard.  Our computer hardware is similarly fortunate.  Even a
mile up the radiation levels are significantly higher -- even growing up
in subtropical India I was NEVER sunburned as badly as I was in a mere
two hours of late afternoon exposure in Taxco, Mexico, just one mile up.
A single six hour cross-country plane ride exposes you to 1/8 of the
rems you'd receive, on average, in an entire year spent at ground
level.  God only knows what astronauts get.  Maybe they bank gametes
before leaving, dunno...

So definitely not silly, but things are more complex than they might
seem.  I'm sure that if a cost-effective solution were as easy as just
"more shielding" the rocket scientists (literally:-) at NASA would have
already thunk it.

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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