QUESTION: 
We would like to know what kind of fuel was used for the Galileo space
mission, and how much was used? 

ANSWER from Todd Barber on February 20, 1996:
Thanks for the excellent question!  Galileo uses monomethylhydrazine for its 
fuel, the same fuel used to launch Titan missiles, and also to control the 
position of the space shuttle once in orbit.  However, in all these cases, and 
unlike your car on Earth, there is no free oxygen (or any other oxidizer around)
with which to react with the fuel.  So Galileo also has to carry its own 
oxidizer; in our case, we use nitrogen tetroxide.  In fact, Galileo uses about 
60% more oxidizer than fuel (by weight) for any given thruster burn!  

At launch, Galileo had 959 kg of total propellant on-board, of which about 595 
kg was nitrogen tetroxide and the remaining 364 kg was monomethylhydrazine.  As 
of this writing, we have used 393 kg of oxidizer and 243 kg of fuel, most of our
total for the mission.  By far the largest single consumer of propellant was the
Jupiter Orbit Insertion (JOI) burn on December 7th of last year; this used 
one-third of our total propellant in a 49-minute span!

There are many reasons nitrogen tetroxide and monomethylhydrazine were chosen to
propel Galileo--long history, low freezing point, good performance, space 
storability, etc.  However, one of the main reasons this combination was used 
was because these two propellants are hypergolic.  I invite the reader to 
research the meaning of the word "hypergolic" and try to understand why this is 
important for the Galileo mission, with many tens-of-thousands of thruster 
firings.