QUESTION:
On the picture of the satellite, there was an ion detector and a 
dust detector.  Do these simply detect the trace amounts of dust in 
the vicinity of the planet?  Or, will it send information when it 
lands forever on the surface of Jupiter?

ANSWER from Claudia Alexander on January 19, 1996:
The various experiments on Galileo collect information in different ways.
Some, such as the imaging camera, use a Charge Coupled Devise (called a
CCD camera) which collects information in an array of photoelectric sensors.
This means that photons hit the sensors and "darken" them, and that is
how a visible image is obtained.  Then this sensor sends information
on how well it was "darkened" to the onboard computer in the form of bits,
the same as computer bits that your home computer uses, and those bits
are encoded into a radio signal which is radioed to the earth.

Other instruments take spcetra, which means that they separate the
light signal recieved, by means of a mirror or several mirrors, into
it's component wavelengths.  The amplitude of the various wavelengths are
then registered with a reciever, and the information is sent to the
onboard computer in the form of bits, and those bits are encoded into a
radio signal which is radioed to the earth.

Other instruments sort of do the same thing but instead of recieving
light signals, they recieve radio signals.  The radio signals are
convolved into a form that can be sent to the onboard computer in bit
form, and those bits are encoded into a radio signal which is radioed
to the earth.

Still other instruments are "in-situ" instruments, which means they
obtain data by means of having something hit the instruments.  (In-situ
actually means "in place", which means that the instruments sit in a
place and things hit them).  These instruments spin around in 4-pi
steradians, which means they spin and collect data in 360 degrees in
the horizontal direction, and yet again for 360 in the vertical
direction.  This way they try to "see" in all directions at once.
Of course, they can't really do that, so they have a cycle, say 4
minutes long, in which it takes to spin all around and sample all
the detectors, so what is supposed to be an instantaneous look is
really a 4 minute look.  Particles hit the detectors of these instruments
and the instruments register how many things hit them.  In their
"instantaneous look" they may see particles streaming towards them from
a particular direction.  This is among the things they hope to measure.
So after they complete the "instantaneous" look, the  detectors encode
the information on how many hits they received into bit form, and those
bits are encoded into a radio signal which is radioed to the earth.

The Deep Space Network (DSN) then receives the signals from the
spacecraft and deconvolves them back into bit form, which the instrument
teams at Universities, and JPL, and so forth, receive and turn into data.
And that's how it works.

In the picture of the spacecraft (there should be 3 ion detectors - I think)
and a dust detector.  These instruments are all on the Orbiter, and not
the Probe.  The Probe was dropped into Jupiter's atmosphere and is now
lost forever.  The Orbiter, however, will remain in orbit for 2 years,
maybe more we hope, and so these instruments will perform their
primary function which is to measure the amounts of dust and ions being
produced not only near Juptier, but also by each satellite.  These
particles float around in the magnetosphere in fancy dynamic patterns,
and they have an effect on the general flow within the magnetosphere.
So Galileo hopes to measure where these particles are coming from
(what's the source) as well as where they are going, and also what
sorts of effects they are having on other objects within the 
Jupiter system.