The Role of Radiation Hardening in the Galileo Project
Figure 8. The Galileo spacecraft. Provided by JPL
Galileo's journey to Jupiter required that it's electronic equipment be designed to continue working after a dosage of 150 J/kg. Processing steps such as extreme cleanliness, thin oxides, and reduced processing temperatures were used in conjunction with higher voltages, synchronous circuits, and limiting leakage paths to meet the radiation-hardening needs of the project.
Although general radiation problems were there, Single Event Upsets were the primary cause for concern in Galileo's flight. Heavy ions of sulfur and oxygen are present in the space surrounding Jupiter because of volcanic action on the moon of Io. These ions combine and form a part of the Jovian magnetosphere. The ions are accelerated by the magnetic fields, and could have caused the radiation effect known as a Single Event Upset (SEU). If an ion collides with a transistor, the possibility exists that the ion will change the bit. If the wrong bit is flipped, the SEU could cause a chain reaction that would destroy the spacecraft.
The Galileo Project members spent a year trying to understand the "ion atmosphere" around Jupiter and the possible effects that it would have on Galileo's most important stage in the flight. Due to their research they decided to redesign the Attitude and Articulation Control Subsystem (AACS) and to replace key components in the Command and Data Subsystem (CDS).
With no small part of the credit going to the radiation hardening of the AACS and CDS, Galileo has just completed its two year primary mission around Jupiter and the picture below is an example of the magnificent results of space travel.
Figure 9. The turbulent region near the great red spot. This picture provided by JPL
References: See Bibliography references 10 and 11.