The Titan Express/Interstellar Pioneer concept (see Box 6.4) is an example of a mission in which an elaborate, secondary payload addressing space physics goals is piggybacked onto what is primarily a solar system exploration mission.

  • Diverse locations must be accessed. Space physics instruments have to be delivered to widely separated and diverse locations, including the corona within a few solar radii of the Sun’s surface, high heliographic latitudes, planetary magnetospheres, regions of interaction of the solar-wind plasma with planets and their satellites, regions of interplanetary space far removed from any planet, and the distant boundary of the heliosphere and beyond. Nuclear propulsion systems clearly have an important role to play in enabling access to observing locations that may be too difficult, if not impossible, to access using more conventional propulsion systems. The Interstellar Observatory (see Box 4.1), the Solar Coronal Cluster (see Box 4.2) and the Solar System Disk Explorer (see Box 4.3) are examples of missions using nuclear-electric propulsion systems to place scientific payloads in locations—e.g., the outermost and innermost regions of the heliosphere—that might be inaccessible otherwise.

  • Simultaneous multipoint measurements are advantageous. It is commonly the case that simultaneous measurements at different locations are needed to resolve basic physical processes. Using similar instrumentation on multiple spacecraft to conduct simultaneous observations is enabling because it allows for the resolution of space-time ambiguities that are characteristic of single-point measurements, and it provides stereoscopic viewing for understanding inherently three-dimensional structure and dynamics. The results from the Cluster 2 mission indicate that the use of four probes can provide fundamentally new information through global views of large dynamical systems. For example, to resolve microphysical processes such as turbulence and reconnection requires measurements from multiple spacecraft separated by short distances in what typically are relatively compact three-dimensional regions. The Jupiter Magnetosphere Multiprobe Mission (see Box 4.4) is an example of a concept that combines the propulsive capacities of nuclear-electric propulsion with the capabilities of RPSs to enable multipoint observations of the global structure of the jovian magnetosphere.

  • Observing timescales are extensive. Solar and space physics missions address phenomena that vary on a wide range of timescales from sub-millisecond plasma processes through decade-long variations in solar, heliospheric, and magnetospheric environments driven by the Sun’s 22-year magnetic cycle. In general, a statistically significant description of such phenomena requires long-term observations with relatively few data gaps. These observations have most often been achieved using dedicated space physics missions—e.g., Interplanetary Monitoring Platform 8—and by cruise-phase operation of space physics payloads on planetary spacecraft such as Voyager 1 and 2. The demonstrated multidecade lifetimes of RPSs and the potentially long life of reactor-based power systems are ideally suited to support extended-duration observations.

  • Environmental factors must be controlled. In situ measurements generally depend on sensitive instruments requiring careful control of interference and background. This has important implications for spacecraft design in areas such as control of spacecraft charging and protection from stray magnetic fields, electromagnetic emissions, and ionizing radiation. A nuclear-powered spacecraft may have requirements for long booms, shielding, or other measures to mitigate the potentially detrimental effects of the spacecraft’s power source and other systems on its payload.

In summary, space physics observations are typically made using small, focused payloads that require limited spacecraft resources. However, experience indicates that the science return could benefit significantly from spacecraft with more robust mass, power, volume, telemetry, and propulsion capabilities enabled by nuclear power and propulsion systems. Solar and space physics priorities will also advance through joint initiatives between solar and space physicists and astronomers, astrophysicists, and planetary scientists.


1. 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.

2. National Research Council, Exploration of the Outer Heliosphere and the Local Interstellar Medium, The National Academies Press, Washington, D.C., 2004.

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