envisaged, this is likely to entail a programmatic cost. Given the budgetary pressures facing the solar and space physics community, advanced nuclear systems are likely to enhance research capabilities only if they do not displace the small and medium-sized missions that are so important both to the health and the vitality of the community and to the implementation of the exploration initiative.
A variety of implementation features common to many solar and space physics missions are rather different from those typical of astronomical or planetary missions. The characteristics and requirements detailed below influence the application of fission reactors and RPSs to space physics missions as well as to many other aspects of mission design. Mission concepts highlighting some of these characteristics are described in Chapter 4. Features important for the implementation of solar and space physics goals include the following:
In situ observations are important. Space physics has traditionally been dominated by in situ observation of solar, heliospheric, and magnetospheric phenomena. As the discipline has matured, the ability to access new environments near the Sun and in the solar system and beyond will become increasingly important. Nuclear propulsion systems clearly have a role to play in enabling access to observing locations not easily accessible using existing chemical propulsion systems. Another trend seen in recent years has been the development and increasing importance of remote-sensing techniques—for example, energetic neutral atom (ENA) imaging—as opposed to the in situ techniques that have traditionally dominated space physics. Space physics instruments do not, in general, require fine pointing accuracy or knowledge; generally a pointing knowledge of ~0.1° to 1° will suffice. In situ and remote-sensing instruments are prominent features of most, if not all, of the space physics mission concepts described in Chapter 4.
Mass, power, and data rates are modest. There is considerable flexibility in the design of in situ sensors, and in many circumstances considerable progress can be made using instruments with modest mass and power requirements—generally less than ~10 kg and 10W, depending on the instrument—delivered to suitable orbits in the heliosphere. Although it is true that some investigations of microphysics occurring on short timescales demand high instantaneous data rates, the use of onboard data storage means that only low-to-moderate average data rates—say 10 kilobits per second (kbps)—have traditionally been used. On the other hand, processes such as cosmic ray modulation can be studied using data averaged over periods of days or longer, and require modest telemetry rates. Abundant electrical power from nuclear systems potentially opens up the possibility of using new types of instruments, such as those that actively probe extraterrestrial particle and field environments by the use of such techniques as incoherent scatter radar. The use of these and related approaches has, to date, generally been limited to terrestrial applications because the relevant instruments have very large power requirements. The availability of abundant power can also enable larger datasets to be returned to Earth than have typically been used in space physics experiments. One possible benefit of such improved data transmission rates is the ability to routinely sample all three electric and magnetic components of plasma waves, together with three-dimensional plasma distribution functions, rather than limited data subsets as is now typical. This complete set of measurements allows for direct determination of the source and the nature of wave-particle interactions such as sources of free energy for the waves or pitch-angle scattering of the charged particles by the waves. The ability to return complete datasets of this type and/or the inclusion of active instruments would likely be features of concepts such as the Neptune-Triton System Explorer mission (see Box 6.5).
Space physics experiments as secondary payloads. The modest power, mass, and data rate requirements of most space physics instruments make them ideally suited for inclusion as secondary payloads on solar system exploration missions. Because many space physics investigations address interplanetary phenomena, the ability to conduct cruise-phase investigations on planetary missions has been particularly important for the development of this discipline. The ability of nuclear propulsion systems to transport comprehensive scientific payloads to diverse planetary environments (see Chapter 6) will necessarily increase the available opportunities to include secondary space physics instruments. Moreover, the potential development of a new generation of small RPSs (see Table 1.3) creates additional opportunities since they could, for example, be used to power small, deployable subsatellites.7