We asked George Schmidt, deputy manager of the Propulsion Research Center at NASA's Marshall Space Flight Center, for his thoughts on the prospects for using nuclear power to propel future spacecraft.

SPACE.com: Given that the Bush administration has put Earth-based nuclear power back on the table, do you think this stands to renew interest and support for using nuclear power in space?

SCHMIDT: The potential benefits of using nuclear fission for space power and propulsion have been recognized for some time. The United States and former Soviet Union/Russia have conducted many research and development programs focused on this technology since the late-1950s. Although the former Soviet Union/Russia has placed over 30 reactors in Earth orbit to support sophisticated high-power spacecraft, the U.S. has flown only one – SNAP-10A in 1965, which was shut down after only 43 days of operation.

The fact that the country is willing again to consider use of nuclear energy for commercial power may improve the prospects of applying this technology to space exploration. However, it is important to note that the rationale and context for space nuclear systems are very different from those for ground-based applications. For one, space fission systems do not begin operating until they are deployed safely in space. Before then, radiation levels in these devices are extremely low and are at least four orders of magnitude less than the power sources used on current deep space probes. Space nuclear systems also benefit by circumventing the issues of waste handling and disposal. NASA's current interest is in power systems to propel sophisticated scientific probes into the outer solar system and deep space. For these applications, the reactor would never return to Earth.

Admittedly, some ground-based nuclear testing will be necessary to develop these systems for flight. However, the size of these reactors is considerably smaller than most university research reactors in use today, and their power levels are roughly 10,000 times lower than commercial power reactors. In addition, much of the development testing will employ high-performance electrical heaters in the cores to simulate fission heat release, thus minimizing use of nuclear materials.

SPACE.com: Is there a misunderstanding about the safety of nuclear propulsion for space travel, and if so what would be needed to clear it up and shift political will and public opinion?

SCHMIDT: Most people associate "nuclear propulsion" with NERVA/Rover, the nuclear thermal rocket technology program conducted from 1955 to 1973. They may also be aware of the brief renewal of interest in the late-1980’s and early 1990’s for the Strategic Defense Initiative (SDI) and NASA’s Space Exploration Initiative (SEI). This propulsion concept heats hydrogen directly in a reactor core and expands it through a nozzle to produce thrust. It utilizes propellant nearly twice as effectively as chemical rockets, while delivering the large thrusts necessary for human missions to Mars and other near-Earth destinations.

The NERVA/Rover program evaluated many reactor and engine prototypes in open-air tests at the Nevada Test Site. One of the most notable demonstrations was the 12-minute test of the Phoebus-2A. It was the most powerful nuclear reactor ever built and delivered over 4,000 megawatts of thermal energy. Although NERVA/Rover showed that nuclear thermal rockets could be developed and operated safely, the cost for developing and testing such a system with today’s stricter environmental policies would be very high.

The current interest in nuclear propulsion is focused on "nuclear electric propulsion," an entirely different concept in which a small nuclear power plant produces electrical energy to power high-performance electric thrusters. The fact that these reactors are much smaller than commercial reactors and are closed to the environment makes this system much easier to test than open-cycle thermal rockets. In addition, safety features that could absolutely prevent inadvertent reactor startup in any credible launch accident would be much easier to implement with these types of reactors.

It is important that public be made aware of the tremendous benefits offered by nuclear electric propulsion and the minimal risk it poses to the environment.

SPACE.com: Has research into potential use for nuclear power in space been thwarted by political and social resistance over the past decade, or has it progressed in anticipation that it might become politically viable again?

SCHMIDT: There are many reasons why nuclear fission has not been used in space. But in general, there has never been a firmly established and approved space mission that clearly depended on this technology. While it would have been useful and even enhancing for several missions in the past, other power systems (solar cells, batteries, RTGs) and propulsive techniques (chemical rockets, gravitational swingbys) have always been able to meet the requirements of both robotic and human exploration. Research and development continued, but at a very low level in anticipation for the ambitious missions of the future (fast outer planetary missions, human missions to Mars).

We are now entering an era where the transportation requirements for scientific missions have become much more sophisticated. Nuclear electric power and propulsion offers a means of more rapidly accessing and maneuvering around deep space destinations. It also offers a power-rich environment that could support much more sophisticated scientific instruments and enhanced data communication with Earth.

SPACE.com: If nuclear power became politically and socially acceptable, what would be the single biggest advantage to space exploration, and in what time frame?

SCHMIDT: Nuclear power affords several advantages to space exploration. All of these stem from its unique ability to provide large quantities of energy from a relatively small compact source, and to do so in virtually any extraterrestrial environment. The latter feature is particularly important when we consider applications involving high power in deep space, beyond the orbit of Mars. At this location, the energy flux available for solar-based power systems is too low to support anything other than small space vehicles and low-power scientific experiments (e.g., Pathfinder). Radioisotope sources (e.g., Galileo and Cassini) are effective for power levels below 100 Watts to 1 kilowatt, but fuel cells and batteries are severely restricted in terms of the amount of energy they can store relative to their mass.

The most immediate and attractive application of nuclear power is in the area of propulsion. Power systems based on nuclear fission could provide ample power to extended operation of high-performance electric propulsion thrusters. The concept of nuclear electric propulsion has been around for some time, and the supporting technologies (e.g., small lightweight reactors, power conversion, electrostatic and electromagnetic thrusters) have matured to a point that such a system can be seriously considered for flight development. Assuming that proper resources are placed in this area, it is reasonable to assume that missions using this technology could be initiated by the latter part of this decade.