The present state of the art allows transistors to operate up to 1 year for a space probe. This is for thin-base germanium devices. Other devices, such as thick-base and silicon, are restricted to shorter periods. For devices orbiting in the center of the inner Van Allen belt, the useful life of thin-base transistors is approximately 3 days. These lifetimes may be increased by suitable shielding. Latest transistor development, such as very-high-frequency germanium devices, should increase the lifetimes by an order of magnitude.