QUESTION:
     Was the Low-gain antenna 
     built so if the main antenna was built, there would be an extra?
  
ANSWER from Dr. Richard B. Pomphrey on February 7, 1996:
     Actually the low-gain antenna (LGA) was NOT built as an extra or a 
     back up to the high gain antenna (HGA).   Instead, it's purpose was to 
     enable us here on earth to get a command into the spacecraft if the 
     high gain antenna was not pointed at the earth. 
     
     BACKGROUND:
     The spacecraft has two transmission frequencies for communication with 
     the earth:
     
                X-band - primary communications frequency
                S-band - a back-up frequency or back-up communications path 
                         for the spacecraft to earth link.
     
     The high gain antenna was designed as the primary communications link 
     between the Galileo spacecraft and us here on earth.  This antenna has 
     a very narrow beamwidth.  That is, the high gain antenna has to be 
     pointed very accurately at the earth:
     
        a.  for us to be able to receive the very narrow beam of radio      
            energy transmitted from the spacecraft, or 
        b.  for the spacecraft to receive the signal which we transmit from 
            the ground to the spacecraft.  
     
     If the high gain antenna is not pointed accurately at the earth, we 
     would not be able to receive its signal, and we also would not be able 
     to command the spacecraft from the earth.
     
     [Keep in mind that the distance from the earth to the spacecraft is 
     very large.  It takes light approximately 8 minutes to travel from the 
     sun to the earth.  It takes the radio signal (traveling at the speed 
     of light) approximately 55 minutes to travel from the Galileo 
     spacecraft to earth.]
     
     If an error occurs on the spacecraft which causes the high gain 
     antenna to be "pulled off" the earth, there is an automatic recovery 
     process on-board which points the antenna at the sun.  However, when 
     the high gain antenna is pointed at the sun, 
        a. we on earth cannot intercept the narrow beam of energy           
           transmitted from the high gain antenna.
        b. the high gain antenna cannot receive commands broadcast from the 
           earth.
     
     Therefore low gain antenna was designed with a sufficiently broad 
     beamwidth that even if the high gain antenna is pointed at the sun and 
     not the earth, the low gain antenna can still receive radio signals 
     from the earth, and we can command the spacecraft through that low 
     gain antenna.
     
     It is true that we are now using the low gain antenna to receive all 
     the radio signals from the spacecraft.  However, this required an 
     in-flight redesign of how we transmit data from the spacecraft to the 
     ground.