As NASA begins to sketch plans for sending humans to the Moon and on to Mars, the U.S. space agency is confronting important questions about the role that nuclear systems should play in its space exploration strategy.

“Contrary to what we would all like to think, the purpose is not to fly a reactor in space. It is to do the science and accomplish the missions NASA wants to accomplish,” said Michael Wahlman of KAPL Inc., a Schenectady, N.Y.-based Lockheed Martin company that operates the Knolls Atomic Power Laboratory for the U.S. Department of Energy.

Advocates say nuclear reactors have the potential to transform the way NASA explores space, powering highly efficient in-space propulsion systems and providing kilowatts of electricity for spacecraft electronics and surface operations on the Moon and Mars. But developing these complex systems is neither cheap nor easy and is fraught with both technical and political risks.

NASA began what is now known as the Prometheus Nuclear Systems and Technology program in 2002 to overcome the power and propulsion limitations holding back a more robust effort to explore the solar system’s five outermost planets with robotic spacecraft. NASA put more money behind the program in 2003 and focused the effort on an ambitious mission to orbit and explore three of Jupiter’s icy moons, one after the other.

NASA postponed the Jupiter Icy Moons Orbiter (JIMO) mission late last year after spending nearly two years and more than $200 million discovering just how challenging and expensive the 15-year expedition would be.

The nuclear propulsion capabilities that were the driving force behind JIMO are still considered essential to NASA’s more ambitious plans for exploring the outer planets with increasingly capable robotic spacecraft. Less clear is what role, if any, nuclear systems will play in getting astronauts and their equipment to the Moon and Mars.

Officially, NASA is still interested in doing the JIMO mission, but not before proving the worth of nuclear technology with a less-challenging flight demonstration. NASA is expected to announce this summer an in-space nuclear power and propulsion demonstration that can launch by 2014.

The changes to the Prometheus program are unfolding just as NASA gets busy laying out a set of strategic roadmaps meant to help the agency plot a viable route back to the Moon and on to Mars.

NASA’s lunar and Mars exploration roadmap committees are finding that most of what the agency thinks it might want to accomplish on the Moon and Mars can be done without nuclear propulsion systems. But when it comes to generating power for human and robotic surface operations on the Moon and Mars, experts say nuclear reactors have a lot to offer.

“I am convinced at this stage that there are multiple missions for surface nuclear power and so are the people who are doing the lunar, Mars, and [in situ resource utilization] roadmaps,” David Bantine, chief technologist at NASA’s Kennedy Space Center , Fla., told members of the agency’s Nuclear Strategic Roadmap Committee during a meeting April 5-6 at a hotel outside Washington. The federal advisory committee’s job is to help NASA make sure that its nuclear ambitions align with the agency’s broader vision for space exploration. Its final report is due in September.

The two-day public meeting included updates from NASA’s exploration roadmap teams, briefings on changes under way in the Prometheus program, and presentations on safety, risk management, mission assurance, and lessons learned from SP-100, NASA’s last space reactor development program . That program, intended to culminate in a flight demonstration of a 100-kilowatt nuclear-electric reactor, consumed $468 million between 1986 and 1994 and never got close to the launch pad.

The uncertainty surrounding Prometheus and the part it will play in helping NASA get back to the Moon and eventually beyond drew unfavorable comparisons to SP-100.

“Sitting here listening to the discussion it’s like I’m back in the 80s. You have a technology looking for a mission,” said Earl Wahlquist, associate director for space and defense power systems at the U.S. Department of Energy, which footed most of the SP-100 bill.

The initial push behind SP-100, Wahlquist said, came from the Pentagon, which saw a role for in-space reactors in former President Ronald Reagan’s Strategic Defense Initiative, the forerunner to today’s missile defense program. But by the late 1980s, the envisioned missile defense role had evaporated, the Pentagon’s funding for SP-100 started to dry up, and NASA’s steady contributions were not enough to keep the project going in the absence of a clear cut need for the capability.

Wahlquist, a nuclear space veteran who invested many years in SP-100 before the government pulled the plug on the project, said Prometheus could be headed for a similar fate unless NASA establishes a clear need for the reactor technology and propulsion capability the program aims to demonstrate.

NASA’s plans for returning to the Moon are only starting to take shape and human trips to Mars are still decades away . But the tussle for NASA’s limited resources is in full swing, and some constituencies are questioning whether Prometheus is worth its likely cost.

For example, the Mars Strategic Roadmap Committee, which last met March 30, identified the cost of developing nuclear propulsion capabilities as a budgetary barrier to human expeditions to the red planet, according to Paul Mahaffy, a scientist with NASA’s Goddard Space Flight Center in Greenbelt, Md., and a member of the mars panel.

Prior to NASA’s decision to postpone JIMO, the Congressional Budget Office estimated that the mission would cost at least $10 billion. Although NASA’s 2006 budget request reflects scaled-back funding plans for Prometheus, the agency still anticipates spending more than $3 billion on the effort through 2010.

NASA has not released Prometheus budget projections beyond 2010. But John Casani, the veteran manager at the Jet Propulsion Laboratory in Pasadena, Calif., who is in charge of developing the Prometheus flight demonstration , said candidly that NASA’s current program funding profile is not “ever going to support anything with a reactor in it.”

Ray Taylor, the NASA Headquarters official in charge of Prometheus, said the program’s budget is a work in progress. “We are working to understand what the requirements would be for a 2014 launch,” he told the committee.

For the last several months, the Jet Propulsion Laboratory and its partners in industry and at the Department of Energy have been evaluating alternatives to JIMO that could launch in 2014 to demonstrate the first nuclear-electric propulsion system for deep space robotic exploration.

The lab is due to deliver its final report to NASA Headquarters here April 15 and brief NASA’s associate administrator for exploration systems, Craig Steidle, the following week.

The report will not recommend a specific mission, but instead will present a half dozen candidates that can be ready to launch by 2014 aboard a single Delta 4 or Atlas 5 rocket and complete its main objectives within three years, according to Casani .

The so-called Analysis of Alternatives involved more than a dozen mission concepts, including trips to the Moon, Mars and Venus, as well as forays out to the Lagrange points and extremely close encounters with comets and asteroids.

In an interview after the meeting , Taylor said that while Prometheus remains oriented toward flying a nuclear-electric propulsion system, the fission-reactor and other technologies needed for the mission have a variety of potential exploration applications . Prometheus, he said, is the logical pathway to mission-ready nuclear capabilities, whether for spacecraft power and propulsion or for providing heat and electricity to human outposts on the Moon and Mars. “I firmly believe that because the basic Prometheus technology is the only proven high-energy-density and high-power-density power source and given the fact that space exploration … requires power,” he said. “All the work we have done can be used in whatever may turn out to be the new priority.”

Brian Berger is editor in chief of and the SpaceNews magazine. He joined in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...