to several minutes) for trapping of particles in the geomagnetic field. The interfering signals can have a duration ranging from a few seconds to many minutes and can also have highly variable amplitudes and rates.

The amplitude of the photon events depends simply on the location of the reactor relative to the detector, whereas the particle-induced event rates are more complicated to quantify because they also depend on the orientation and strength of geomagnetic field lines near the location where they were emitted, as well as on the time between emission and detection. All these effects have been well documented in data from the Gamma-Ray Spectrometer aboard the Solar Maximum Mission, which operated from 1980 to 1989 and which suffered from significant background of this type generated by the Russian Cosmos orbiting ~100-kW nuclear reactors.4-7 These factors suggest that unless it can be demonstrated that this type of interference will not occur, nuclear reactors on future spacecraft should only be operated well outside Earth’s magnetosphere.b

Recommendation: Determination of the cost of NEP-class missions should take into account the cost of necessary associated technologies and programs. Particular emphasis should be placed on studies of the means to maintain or, if possible, increase the fraction of launch mass allotted to science payloads above that typical for current space science missions.


1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.

2. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003.

3. G.R. Schmidt, R.L. Wiley, R.L. Richardson, and R.R. Furlong, “NASA’s Program for Radioisotope Power System Research and Development,” Space Technology and Applications International Forum—STAIF-2005, M.S. El-Genk, ed., American Institute of Physics, Melville, N.Y., 2005.

4. E. Reiger et al., “Man-Made Transients Observed by the Gamma-Ray Spectrometer on the Solar Maximum Mission Satellite,” Science 244: 441, 1989.

5. G.H. Share et al., “Geomagnetic Origin for Transient Particle Events from Nuclear Reactor-Powered Satellites,” Science 244: 444, 1989.

6. E.W. Hones and P.R. Higbie, “Distribution and Detection of Positrons from an Orbiting Nuclear Reactor,” Science 244: 448, 1989.

7. O’Neill et al., “Observations of Nuclear Reactors on Satellites with a Balloon-Borne Gamma-Ray Telescope,” Science 244: 451, 1989.


The situation for NEP and NTP systems is somewhat different and has to be studied. The low thrust of the former means that an NEP system may spend a prolonged period (months to years) in the magnetosphere as it spirals out of Earth orbit. The high thrust of the latter means that an NTP system will likely make a rapid transit (minutes) through the magnetosphere as it boosts out of Earth orbit.

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