From 2002 until this year, NASA’s mission statement, prominently featured in its budget and planning documents, read: “To understand and protect our home planet; to explore the universe and search for life; to inspire the next generation of explorers ... as only NASA can.”

In early February, the statement was quietly altered, with the phrase “to understand and protect our home planet” deleted. In this year’s budget and planning documents, the agency’s mission is “to pioneer the future in space exploration, scientific discovery and aeronautics research.”

David E. Steitz, a spokesman for the National Aeronautics and Space Administration, said the aim was to square the statement with President Bush’s goal of pursuing human spaceflight to the Moon and Mars.

But the change comes as an unwelcome surprise to many NASA scientists, who say the “understand and protect” phrase was not merely window dressing but actively influenced the shaping and execution of research priorities. Without it, these scientists say, there will be far less incentive to pursue projects to improve understanding of terrestrial problems like climate change caused by greenhouse gas emissions.