QUESTION:
I was wondering; how will or did the radiation belt of Jupiter
affect the orbiter and the probe?

ANSWER from Steven Tyler on February 20, 1996:
Thank you for your question.  When Galileo first became a project,
nearly 20 years ago, many of us wondered what would happen in the
Jupiter radiation belts.  We did not have much information.  Pioneer
had shown that there might be some intense radiation, and Voyager had
not yet reached Jupiter.  However, in 1979 both Voyagers reached
Jupiter and we found out a little more.

What we discovered were three threats to our craft.  First, there was
a "total dose" threat of low-energy particles.  This was a lot like
the threat to a person.  A big dose would be bad...a bigger dose even
worse.  Different parts would react differently to large radiation
doses, and would begin to fail.  Galileo was planning to travel to
within 3 Jupiter radii of the surface, and the doses we were seeing
indicated that after a couple of orbits, we would be in serious
trouble.  We decided to solve this problem by raising our minimum
distance from Jupiter after the first orbit.  We'd come no closer than
seven to ten Jupiter radii from the cloud tops on the later orbits,
and greatly increase the lifetime of the Orbiter.

The second threat was from higher energy particles.  These were often
highly charged nuclei of Oxygen and Sulfur.  These often moved with
just the right speed to go straight through the wall of our craft and
deposit plenty of energy in one of Galileo's memory chips or
registers.  The problem was that Galileo's computers run on
electricity, and a single ion would supply enough electricity to
change a setting in the computer.  We began to worry about the craft
mysteriously following some unintended command after being hit by a
stray ion.  We did not have enough information about the Jupiter
environment to be sure how safe we would be.  We finally decided to
avoid unnecessary risk, so we shielded the most sensitive computer
parts.  It was expensive, but it provided some critical protection.

The third threat was that the radiation belt particles would charge up
our craft so much that electric arcs (like little lightning bolts)
would begin to appear inside the craft, shorting out some of the
equipment or possibly causing unintended firing of bolts or squibs.
This would wreak havoc on Galileo, so we avoided it by shielding key
circuits.

When Galileo finally got to Jupiter, many of us wondered whether or
not one or more stray ions would get through to the on-board
computers.  As it turned out, we had no problems; none of the bits
were changed by the high-energy ions.  Nor did we have any serious
problems from short circuits.  But how much total radiation did we
absorb?  We're still not sure.  As we approached Io, we expected to be
hit by quite a few particles (mostly electrons).  But we did not get
hit by anywhere near as many as we feared.  After we reached Io, the
total dose seems to have gone way up.  When we added it all up, it
seemed that we came close to getting the dose we expected.  Our
maneuver to raise the perijove (the minimum distance we are from
Jupiter in our orbit) is scheduled for March 14, and after this we'll
be much safer.  Without that maneuver, it looks like one more orbit
would give us a fatal radiation dose.

The good news is that the Orbiter does not seem to be suffering from
any obvious symptoms of radiation poisoning.  It's still possible that
our craft may last longer than we originally expected in this
radiation environment.

The Probe spent a shorter time in the radiation environment and made
some measurements of it.  The Probe lasted only about an hour after
entering Jupiter's atmosphere, and I don't know of any signs of
radiation damage to it.  Heat and pressure and battery lifetime should
fully explain the loss of the Probe's signal (the Probe went to twice
the pressure it was required to reach before the signal stopped).
We're still looking at Probe data, and some radiation effect may turn up.