The head of NASA said 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, President 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 when I assumed office," NASA Administrator Michael Griffin said in prepared remarks to a conference of the American Geophysical Union in San Francisco. Griffin did not say specifically which programs might be cut or delayed by the budget constraints.
"All of you know many reasons why this is so. NASA can only move forward on our fundamental missions of exploration, science and aeronautics at the pace that available resources will allow."
Last month Griffin said the agency faced a $3 billion to $5 billion shortfall in its space shuttle program alone over the next five years.
On Tuesday, he said the agency's ambitious plans for new missions would not shortchange science, which critics say has taken a back seat amid efforts to keep the international space station in orbit.
"I am aware that many in the science community have questioned NASA's commitment to science, and believe their own work to be gravely threatened by the vision for space exploration," he said.
"I have frequently stated my belief that exploration will be a boon for science in the long term. I have also said on many occasions that it is not our desire to sacrifice present-day scientific efforts for the sake of future benefits to be derived from exploration," he said.
"We who run NASA today are doing our very best to preserve these efforts in the face of, frankly, some daunting fiscal realities. But we also must avoid setting unrealistic expectations."
NASA's plans to return to the moon envision a capsule sitting atop a rocket, similar to that used in the Apollo program, and a separate heavy-lift vehicle to take cargo into orbit. Humans last walked on the moon in 1972.