Date:  Sun, 13 Jan 91 08:47 CDT
From: Bill Higgins-- Beam Jockey <HIGGINS%FNAL.BITNET@UICVM.uic.edu>
Subject:  Radiation belts and moons [Long](was Re: Humankind's Second
	Off-world Colony)

One point has been overlooked in the discussion of colonies on Ganymede.
The radiation problem there-- even within the Jovian radiation belts--
may not be so bad.  The reason is that a satellite orbiting within a
Van Allen belt leaves a "hole" in its wake.

      (( o ))
         ^  ^
         |  |
Jupiter---  -----Radiation Belts

The radiation belts consist of charged particles (like protons and
electrons) trapped in the planet's dipole magnetic field.  Consider
one such particle. It is constantly moving back and forth along lines
roughly corresponding to the parentheses above.  It spirals around
such a "magnetic line of force," heading for the north pole.  Close to
the pole it encounters a high density of field lines, slows down, and
"bounces," moving southward along the line until it gets near the
south magnetic pole, where it rebounds again.   So it oscillates from
pole to pole.  (It may eventually lose enough energy to get tired and
leave the trap.  But another particle from the Sun or a cosmic ray
will soon come in to replace it.)

Now imagine that the space around Jupiter is filled with such
particles (it is, I assure you) zipping back and forth, roughly
north-to-south, throughout a doughnut-shaped volume.

Let's introduce a moon orbiting within the Van Allen belt, in the
plane of the planet's equator.  What happens to the radiation
particles?

The ones heading northward plow into the southern hemisphere of the
satellite.  The ones heading southward are stopped by the northern
hemisphere.  As the satellite moves along in its orbit, it absorbs
radiation.  Just behind it, then, is a zone where there are no trapped
particles.  Eventually, other particles will diffuse into the "vacuum"
and fill up the void.  But there is a region for some distance behind
the moon that is swept clean of radiation.  Your Geiger counter will
click very slowly there, compared to nearby regions of Jovian space
(say, just inside the moon's orbit, or just outside it).

If the time it takes to replenish the radiation is longer than the
moon's orbital period, a toroid of low-radiation space exists all
along its orbital path.  Just behind the satellite, space will be very
clean;   further behind, say halfway around the orbit, there will be
somewhat more radiation, since there has been time to "fill in" the
void.  Send a flyby spacecraft past the planet and its radiation
counter will produce a graph something like this:


          |
          |         -----   -----       -----   -----
Radiation |        -     \ /     \     /     | |     -
Intensity |       -       v       |   |      | |      -
          |     -                  | |       \ /        -
          |   -                    | |        V           -
          |                        | |
          |_________________________________________________
                          ^         ^         ^
                          |         |         |
Where the moon was        |      Where the    |    Where the moon
half an orbit ago   -------      planet is    ----- is now

In fact, on its Saturn flyby in 1979, Pioneer 11's radiation counters
actually discovered a satellite this way.  Furthermore, according to
my trusty copy of NASA TM-82501, "Space and Planetary Environment
Criteria Guidelines for Use in Space Vehicle Development, 1982
Revision," Volume II, Saturn's rings kill radiation counts
dramatically (at least a factor of 100), there is a definite depressed
region of proton flux due to absorption by Dione, Tethys, Enceladus,
and Mimas with about 100 times less flux between about 3 and 6 Saturn
radii, and particle flux at Jupiter has similar troughs due to
Amalthea, Io, and smaller satellites 1979J1, 1979J2, and 1971J3.

For most satellites of Jupiter and Saturn, only a modest dip in the
flux seems to occur. I presume this is because the void behind the
satellite fills up pretty fast with particles.  Perhaps a
space-physics guru could comment further.  What factors govern this
filling time? (Dale Greer?  Pat Reiff?  You reading this?)

The ultimate question is this: For a chosen satellite, does the
absorption effect reduce Van Allen radiation to safe levels for
operating a habitat there?

If the moon is tide-locked, there will be a trailing hemisphere which
will see a minimum of radiation, and would be a good site for a base.
On the other hand, I would expect that prudent construction would
include using native regolith for shielding the base, just as lunar
and Martian bases would do.  On the other other hand, there is the
problem-- already discussed-- of combining cryogenic solids with
relatively hot imported equipment.  Not impossible, but certainly
eased if you need less shielding in the first place.

I still wouldn't want to operate a base deep in Jupiter's or Saturn's
radiation belts.  I'd have to overcome the radiation hazards to
spacecraft traveling to and from the base.  Bad for people and robots
alike.  And I'd be inside some mighty deep gravity wells, so delta-vee
costs would be high.  Seems to me that bases on an outer moon would be
preferred if you had to go there at all.

                        ///        Bill Higgins
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