Also, in space there is tons more damaging radiation. The very thing that makes the newer processors faster (ever smaller circuits) is a liability in space. Stray high energy radiation is more likely to damage smaller circuits than larger ones, therefore NASA uses "obsolete" off-the-shelf circuit components mainly because they are more robust than the latest, greatest.
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Also, in space there is tons more damaging radiation. The very thing that makes the newer processors faster (ever smaller circuits) is a liability in space. Stray high energy radiation is more likely to damage smaller circuits than larger ones, therefore NASA uses "obsolete" off-the-shelf circuit components mainly because they are more robust than the latest, greatest.