solar cells coupled with batteries yield enough power for many types of orbital and surface exploration missions, there are other experiments that require sustained power for months to years. When the solar flux is too low (e.g., beyond the asteroid belt), the Sun is not visible for long periods (e.g., during the lunar night), or solar cells are likely to deteriorate over time (e.g., from dust on Mars or radiation damage incurred close to the Sun), then experiments powered by solar power sources have limited utility. However, some experiments, such as monitoring the seismic properties of a planetary body or atmosphere/surface seasonal interactions, require the availability of power over a span of months to years. Such experiments often do not need large quantities of power, but rather need power that can be available continuously for long periods or periodically over long timescales. Advanced RPSs are a solution for such needs, providing moderate power outputs for extended periods. The Long-Lived Mars Network concept (see Box 6.2) is an example of such a power-enabled mission.
Communications to return data to Earth. High-priority investigations discussed in the SSE decadal survey will generate very large datasets that must be transmitted to Earth. The data rates required for timely transfer of these datasets can outstrip the current capabilities of the Deep Space Network (DSN) as well as of spacecraft telecommunications and power systems. All other parameters such as communication distance and receiver performance being equal, telecommunication systems’ signal strengths (and thus data rates) for two-station systems (i.e., no intervening “repeater” stations) are proportional to three parameters: the transmitted power, the area of the transmitting antenna, and the area of the receiving antenna. Practical approaches to significantly increasing data rates from a given location in the solar system must involve increases in at least one of those three parameters. In the past, spacecraft power and launch constraints limited transmitted power and transmitting antenna size, which tied the limits of data-transmission rates to fixed DSN assets. Nuclear power sources, in particular fission, promise to greatly enhance data-transmission rates via large increases in transmitted power. Some increase in the transmitting aperture might also be possible. Another possibility being explored by NASA is to migrate from radio-frequency to optical communications systems. And a fourth option should be considered: a large increase in ground-based receiving apertures, possibly involving arrays of many antennas that would offer flexibility and simultaneous servicing of multiple missions and would obviate the requirement for high power on all serviced missions.
Transfer to Earth of samples collected for study. The collection of samples from the surfaces of planetary bodies for return to Earth is an important goal of solar system exploration. The capabilities of analytic instruments available in terrestrial laboratories far exceed what can conceivably be packaged to fit on a planetary spacecraft in the foreseeable future. Ices abound in the solar system. Many of these ices are highly evolved, but some are primitive, enabling studies of material left from the early solar nebula. Although in situ laboratories are useful for the initial studies of these ices, more can be learned by analyzing them in the superior laboratories on Earth. Careful collection and preservation of samples, then, would allow for more in-depth study of the structure of the ices, which would yield information about their deposition and evolution. Holding a sample of ice at low temperatures in space is within the bounds of current technology and can be accomplished with low-power refrigeration or radiators. However, returning these samples to Earth in their ice phase is a very difficult process that will require excellent refrigeration and protection, something that may be advantageously accomplished using RPSs. Cryogenic sample return is a technology that will have to be developed for future cometary and Mars polar-sample missions. The Cryogenic Comet Sample Return mission concept (see Box 6.3) discussed in Chapter 6 is an example of a sample-return-enabled mission.
1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
2. Mark Sykes, ed., The Future of Solar System Exploration 2003–2013—Community Contributions to the NRC Solar System Exploration Decadal Survey, ASP Conference Proceedings 272, Astronomical Society of the Pacific, San Francisco, Calif., 2002.
3. S.W. Squyres et al., “The Spirit Rover’s Athena Science Investigation at Gusev Crater Planum, Mars,” Science 305: 794–799, 2004. Also see subsequent papers (pp. 800–845) in this issue of Science.
4. S.W. Squyres et al., “The Opportunity Rover’s Athena Science Investigation at Meridiani Planum, Mars,” Science 306: 1698–1703, 2004. Also see subsequent papers (pp. 1703–1756) in this issue of Science.