NASA Administrator Mike Griffin’s decision to redirect the Project Prometheus nuclear research initiative toward surface power generation is one more indication that he is putting NASA’s priorities in the correct order. His plan to defer some of the initial research activities aboard the international space station is another.

Chief among NASA’s priorities, after safely returning the space shuttle to flight, are to retire the aging orbiter fleet as soon as possible and to press ahead with the Crew Exploration Vehicle (CEV) on an accelerated timetable. Next in line is developing the hardware that, along with the CEV, will enable astronauts to return to the Moon and, once there, engage in worthwhile activities.

Because all of this will require enormous resources, and given that NASA has no plans to abandon its aeronautics and science programs, there simply is no way the agency can afford to press ahead right now with development of nuclear-thermal and nuclear-electric propulsion and power systems. Along with surface power generation, these nuclear technologies comprise the three legs of the Project Prometheus research triad.

As Mr. Griffin points out, nuclear reactors represent the most practical technology within reach for providing power to large astronaut crews on the Moon for extended periods. And if returning to the Moon is to be more than a stunt — a demonstration that NASA can repeat the feats of Apollo some 50 years later — extended stays are a must.

Nuclear-thermal propulsion has the potential for reducing the transit time to Mars, an attractive feature given the debilitating physiological effects and formidable life support needs of long-duration space travel. But human missions to the red planet are at least a generation away, perhaps two, which necessarily takes nuclear-thermal propulsion down a notch on the priority list.

Nuclear-electric propulsion and power systems would revolutionize robotic exploration of the outer planets, enabling NASA to do things it couldn’t dream of using spacecraft equipped with solar-power and chemical-propulsion systems. But the notion that NASA could afford to undertake mega-scale planetary missions while simultaneously sending humans to the Moon and preparing for missions to Mars is pure fantasy.

NASA’s Jupiter Icy Moons Orbiter mission was designed to showcase what could be accomplished with nuclear-electric power and propulsion. By the middle of last year, however, the massive scale of the mission became apparent — it would require at least two heavy-lift launches, on-orbit assembly, twice the world’s annual xenon production for fuel and an entirely new generation of instruments to take advantage of the onboard power. NASA had no choice but to cancel the mission earlier this year.

In addition, the development of nuclear-electric propulsion will be difficult and very expensive. Had the proponents of robotic versus human exploration of the solar system won the day, nuclear-electric propulsion and power would be a top priority. But since the nation’s leadership has chosen human exploration, this technology, while worthy, must take a back seat for now.

This does not mean NASA is abandoning outer planetary exploration. To the contrary, Mr. Griffin told Congress recently that NASA may attempt to revive a mission to orbit and explore one of the most scientifically intriguing bodies in the solar system — the jovian moon Europa. NASA canceled the Europa Orbiter mission in 2002, well before U.S. President George W. Bush unveiled his exploration agenda.

Mr. Griffin also has made the right choice in deferring some space station research activities, particularly those that are not directly applicable to sending astronauts to the Moon. That is part of the price NASA will have to pay to retire the shuttle orbiter fleet by 2010 and accelerate the CEV to eliminate or at least minimize the gap between the availability of those systems.

This has raised some concern among space station supporters, but as Mr. Griffin noted during a Senate Commerce science and space subcommittee hearing May 12, the space station “will be flying for many, many years.” That leaves plenty of time for NASA to undertake basic physical and biological research aboard the station — once the agency has a new means of getting there and back. Further, there is nothing in NASA’s plan that precludes its international partners from conducting basic research aboard the space station.

In his first press conference as NASA administrator, Mr. Griffin warned that the agency would have to rein in some activities to fulfill its main objectives. Between returning the shuttle to flight, developing a shuttle replacement, servicing the Hubble Space Telescope and covering the earmarks that Congress insists on inserting into the NASA budget each year, there is precious little money for research that does not offer direct payoffs in the near term.

The term “tough choices” is frequently invoked when ambition comes face to face with budgetary reality. The choices Mr. Griffin is making are indeed tough, but they also are judicious.