Why did Galileo only visit Io once during the primary mission?
It was not feasible for the spacecraft to return to Io during Galileo’s primary mission, which ran through to the end of 1997. However, the follow-on mission which was called the Galileo Europa Mission, or GEM, started immediately after the end of the primary mission in December 1997 and continued until January 2000. Galileo intensively studied Europa, conducted an in-situ study of the Io Plasma Torus, and ended with a spectacular close encounter with Io, flying just 300 KM above its surface! Given NASA’s limited resources, funding for GEM was by no means assured! The key resources that Galileo needed to accomplish the GEM were money, propellant, radiation tolerance and general good health, and power, more or less in that order. Galileo was healthy, and GEM was funded considering the prospect of there being enough propellant to navigate the craft; don’t forget that gravity assists provided the majority of the trajectory changes required! There was also enough power left in the spacecra
