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Path to Mars
 
 
 
- Phase Angle
 
 
 
 

 
     
 
PHASE ANGLE
 
 
Another aspect for the launches is the Phase Angle (Ψ) required for launching from Earth and arriving at Mars.  The Phase Angle is the angle between Earth at launch and the position of Mars at launch time.  The Phase Angle must account for the travel of Mars during the transfer so that the satellite transfer orbit will intersect the Mars orbit at the same time.  This requirement is depicted below.		  
 

Figure 5: The Phase Angle
Figure 5: The Phase Angle
  
 

The Phase Angle is different for each type of launch, based on the length of time for the transfer.  These specific Phase Angles repeat periodically for the orbits of Earth and Mars.  This repeating period is known as the Synodic Time Period, and for the Earth Mars system is equal to 777.9 days, or 2.135 years.  So if a specific transfer is desired, but the launch window is missed, the next available launch date that matches the Phase Angle requirements is 2.135 years later.  However, since each transfer has a different phase angle associated with it, it is not always necessary to wait 2.135 years.  However, changing launches changes the travel time, and can affect the budget of the launch.