QUESTION:
How does the Galileo spacecraft's software work? What language
was the software written in? 

ANSWER from FAQ:
Before we start to discuss software, we need to know a little about
the hardware, since there can be up to 18 microcomputers running
on Galileo at any given time. 

First off, although all eleven science instruments use microprocessors, 
only eight of the instruments are actually reprogrammable in flight.
Each of the eleven science instruments contains an RCA COSMAC
1802 microprocessor which is used to control the instrument and to
communicate with the CDS. Eight of the instruments are
reprogrammable in flight because at least part of the programs can
be run out of Random Access Memory (RAM). The other three run
entirely out of Read Only Memory (ROM), which cannot be
changed in flight.

Only two of the major engineering subsystems--the Attitude and
Articulation Control Subsystem (AACS) and Command and Data
Subsystem (CDS)--are programmable in flight. These two use very
different computer architectures. 

Each of the AACS's attitude functions requires a tremendous
amount of mathematical computation, all of which is the job of the
AACS software. 

Just like the word processor or spreadsheet on a home computer
system gets revised into newer, sometimes better versions, the
AACS software has evolved from what was originally launched
with the spacecraft in 1989--except that the AACS improvements
really are better! The software will continue to evolve for the
remainder of the mission. 

The Command and Data Subsystem (CDS) and its software
perform a number of functions for the spacecraft. For example: 

1. CDS receives and processes commands sent from Earth.
   These commands can be in the form of real-time (i.e. do
   this NOW!) commands, or they can be stored sequences
   which are long series of instructions to tell the spacecraft
   what to do for an extended period of time (as long as
   several weeks). 

2. The CDS also collects information from all of the science
   instruments and engineering subsystems and packages it
   into the data that is actually sent to the Earth. The collection
   process and CDS operations methods will change after
   Galileo arrives at Jupiter. 

3. The CDS performs a number of other functions for the
   spacecraft such as storing data on a tape recorder for later
   playback to Earth, and broadcast of the timing coordination
   signal to all spacecraft subsystems (basically, a way for all
   the subsystems to synchronize their "watches"). 

4. CDS also performs the major "fault protection" functions
   for the spacecraft. If a piece of hardware on the spacecraft
   fails, the CDS is programmed to a) figure out what is
   wrong (fault detection), and then b) isolate the problem so
   an operational spacecraft can be recovered (i.e. the rest of
   the spacecraft can continue to operate). Fault protection is
   needed because at Jupiter radio signals take about an hour
   just to reach Earth, so ground controllers cannot respond
   in real time to fix urgent problems on the spacecraft; the
   spacecraft must take care of itself.