SAN FRANCISCO (Reuters) - The head of NASA said on Tuesday that the U.S. space agency has had to scale back its plans even since he took the job in April because of "daunting fiscal realities." In early 2004, U.S. President George W. Bush outlined a vision of returning humans back to the moon by 2020 and eventually to Mars. Some had hoped such a program would reignite public interest in the space program that has waned since the 1960s and early 1970s. "We must also acknowledge the plain fact that we cannot do everything that was on our plate...