The robots are starting to replace pilots in the U.S. Air Force. The first to go are those who fly the U-2 reconnaissance aircraft. All 33 U-2s will be retired over the next five years. They will be replaced by Global Hawk UAVs. Both aircraft are similar in size and carry similar sensor packages, but the lack of a pilot enables the Global Hawk to stay in the air twice as long (24 hours.) The air force is buying over 40 Global Hawks to replace the U-2, and several other recon aircraft. The U-2 has been in service for half a century, while the Global Hawk is just entering regular service, after nearly a decade of development.