QUESTION: 
How many gallons of fuel does the Pathfinder use in order to get to Mars?

ANSWER from Chris Salvo on February 17, 1997:
The Pathfinder spacecraft itself holds about 80 kilograms of 
monopropellant.  It is called monopropellant because it acts as both a 
fuel and an oxidizer (it "burns" all by itself).  In your car, 
gasoline is the fuel and oxygen from the air is the oxidizer, and both 
are needed in order for combustion to occur.
     
Back to Pathfinder.  The density of the propellant on Pathfinder is 
about 1004 kg per cubic meter (very similar to water) and there are 
about 264 gallons in a cubic meter.  So take 80 kg and divide by 1004 
kg per cubic meter to get the volume of the propellant in cubic 
meters.  Then multiply that by 264 gallons per cubic meter to get how 
many gallons.  The answer is similar to how much gas your car holds.  
Isn't that amazing?  We go hundreds of millions of miles on a tank of 
gas!  In fact, most of the propellant will probably still be in the 
tanks when we get to Mars (we won't need to use it unless we run into 
problems.  We put lots of extra just in case).  
     
How do we manage to get all the way to Mars on just a few gallons of 
propellant?  We got a big push from our launch vehicle.  A Delta II 
rocket placed us on our trajectory to Mars back in December.  The 
propellant on the spacecraft is only needed to make slight course 
corrections along the way.  The Delta rocket weighs several hundred 
thousand pounds before it takes off and most of that weight is 
propellant.  There are several different kinds of propellant on the 
Delta so the densities are different than the propellant on our 
spacecraft.  Some of the propellant isn't even liquid, it is solid 
(like gun powder or dynamite).  Despite the differences, it is clear 
that the amount of propellant on the spacecraft is very small compared 
to the amount on the Delta launch vehicle.  So, it really took much 
more than the little bit of propellant on the spacecraft to get to Mars.