Being a NASA administrator, especially one charged with reshaping the agency to fulfill a bold presidential vision for human exploration of the solar system, inevitably means having to make some very unpopular, even agonizing decisions. Among those awaiting Mike Griffin, who only assumed the job April 14, is whether to discontinue operating a number of scientific spacecraft that continue to send back valuable data long after the end of their originally planned missions.

Extending the mission of a healthy spacecraft that has just completed its original tasks often is an easy decision — the scientific return usually more than justifies the relatively low cost. The Mars Exploration Rovers, still operating a year after completion of their initial three-month missions, are a good example.

But the decision becomes harder over time, especially as the volume of useful data diminishes. Add to that the fact that NASA these days is scrimping every last dollar to get its exploration initiative off the ground with minimal budget increases, and the pendulum swings in the other direction.

NASA thus decided late last year to shut down several aging but still-productive spacecraft, including Voyager 1 and Voyager 2, Ulysses, Wind and Polar. The news predictably triggered an uproar from the scientific community, and Mr. Griffin has since promised to review the decisions.

Budget reality means many of these spacecraft should be shut down. That will certainly seem cold or indifferent to the scientists who treasure the data these spacecraft collect, but the same is true for every NASA employee and contractor working on programs that inevitably must come to end. Consider, for example, the jobs that are likely to be lost as NASA closes out the space shuttle and other programs that do not contribute to the agency’s goal of sending humans back to the Moon and beyond. Or the economic impact to the host communities of NASA centers whose operations are likely to be scaled back.

The fact is that NASA must take these kinds of painful actions or else the United States will have to resign itself indefinitely to a space exploration status quo that dates back 30 years to the end of the Apollo program.

So if Mr. Griffin ultimately decides that the scientific benefits of continuing to operate these spacecraft do not outweigh the costs, then so be it. The astronomy community will no doubt be unhappy, but everybody must bear some of the burden of change for the greater good.

But there is one clear exception to this logic — actually two. They are the Voyager 1 and Voyager 2 interplanetary probes, both of which are on the verge of reaching interstellar space.

Launched in 1977 toward Jupiter, the two 825-kilogram spacecraft helped NASA explore every planet in the outer solar system except Pluto. Now they are far beyond the orbit of Pluto and speeding away from the solar system. As such they represent humankind’s farthest extension into the universe.

But as compelling as the emotional arguments are for keeping the probes alive, the scientific justification may well be stronger. The Voyagers have yet to reach what is known as the heliopause, the point in space where the solar wind and magnetic influence of the sun cease and interstellar space begins.

Scientists originally thought one of the Voyagers might reach the heliopause in the early 1990s, but the probes continue to send back data indicating they still feel the sun’s influence. For scientists, particularly physicists, the search for the heliopause is the search for the answer to a fundamental question of solar physics that is more than deserving of continued research funding.

The Voyager probes were launched more than a generation ago. Who knows how many more generations will pass before scientists get another chance to explore interstellar space — if only robotically?

The 2005 budget for the Voyager program is about $4 million. While every dollar counts, NASA ought to be able to find that much money to continue searching for the answer to a question so important to our understanding of the solar system.

As Mr. Griffin noted during his first press conference as NASA administrator April 18, the U.S. space agency has a lot of great missions. Nothing would be better than to be able to fund all of them. That may not be possible, but it is possible to make certain that one small program with great promise continues to get funded.