With the Mars Reconnaissance Orbiter’s arrival at the red planet, the Cassini spacecraft’s discovery of an icy geyser erupting on Saturn’s moon Enceladus, and the findings of the Stardust comet return mission challenging the conventional wisdom about comet formation, NASA once again has broadened the potential for discovery beyond the bounds of Earth.

These scientific advances are the result of NASA’s long-term commitment to expanding the frontiers of knowledge and exploration with a balanced and robust program of science and human spaceflight activities. While this commitment is unwavering, there are times when the need to address pressing issues — such as completing the international space station and replacing the aging space shuttle fleet — requires setting strategic budget priorities. While NASA’s 2007 budget request of $16.8 billion increases a healthy 3.2 percent over last year’s appropriation, the agency needs to slow the growth of NASA’s Earth and space science portfolio to 1.5-percent growth next year, and 1-percent growth the following four years.

I understand the concerns of many scientists about the impact this reduction in growth will have on future missions. But any objective assessment must conclude that, with proposed funding of $5.3 billion in the 2007 budget, science is healthy at NASA and will remain so.

It is important to put this matter in proper perspective. Over the last 15 years, the various space science communities have come to expect 5 percent to 7 percent yearly growth in funding for NASA’s Science Mission Directorate. This has led to a real growth of science within the agency, from 24 percent of our portfolio in 1992 to 32 percent in 2006, growth which occurred at the expense of other mission areas. But we now foresee no more than inflationary growth of 2.4 percent in the NASA top line over the next several years.

That, coupled with our need to ensure a balanced portfolio of work in aeronautics, science and human exploration, have rendered unrealistic the rate of growth which the science community had come to expect.

There are several strategic imperatives leading to this conclusion. Foremost among them is that our nation will keep its commitment to our international partners to complete the international space station. To replace the space shuttle in the period following its retirement in 2010, we need to bring the Crew Exploration Vehicle (CEV) online in a timely manner, so as not to suffer an extended gap in our nation’s ability to launch our own people into space. To further delay the CEV would be more damaging to the American space program than the delays being incurred by some of our scientific missions, because such delays would cause our space industrial base to wither, vital expertise to be lost and would put at risk American leadership in space exploration.

In this time of constrained resources, we must set priorities and work within realistic budget assumptions for all of our programs. NASA is committed to working with the science community through the National Academies’ decadal survey process and the NASA advisory committee apparatus to make certain that we have a well-planned science program moving forward.

This planning effort will allow us to build on our ongoing science work, which includes 54 satellites and payloads currently operating in concert with the science community and our international partners. And in the future, our program to expand human horizons to the Moon, Mars and beyond will open up dramatic new opportunities for scientific advancement. So even in this difficult time, science is beyond healthy at NASA, and will remain so.

Michael D. Griffin is NASA Administrator.