electric power to maintain SAFIR at optimal temperature, which would mean that these cryocoolers would dominate the power budget of the observatory.
To supply just the cryocooler power budget for SAFIR at 1 AU, at least five Cassini-class RPS units (see Table 1.2) would be required. The power needs can also be met, however, by ~4 m2 of solar panels, which are substantially smaller than the ~100 m2 sunshield required for the observatory and are much less expensive than the RPSs. This sunshield would maintain an orientation perpendicular to the Sun, which is the orientation that is also most efficient for the solar panels. Thus, both the solar panels and the sunshield could be conveniently engineered together as part of an observatory structure. The advantages of solar panels relative to RPSs for SAFIR are multiplied by the more favorable power-to-mass ratio of solar panels. Although RPSs would not be as deployment-dependent as solar panels, they would add to the weight of the observatory about twice the weight that the solar panels would add.
Because of the high minimum power (~0.5 to 1 MWt) required for reactors with competitive power-to-mass ratios, fission systems appear a poor choice to power an observatory that would otherwise require only a few kWe. Dumping most of the thermal power from an onboard fission generator into free space would be difficult to do without adding large heat loads to the observatory itself. In addition, the strong gamma-ray flux from the fission system would seriously affect the performance of the observatory sensors.
Under what circumstances might nuclear power systems offer value to cryogenic infrared observatories? Although the power needs for such telescopes seem to be met economically by solar arrays at 1 AU, telescopes at larger heliocentric distances might benefit. Large heliocentric distances offer lower zodiacal background and increased operational efficiency. While both cooling power requirements (which are determined largely by solar insolation) and photovoltaic-power generation together decrease with distance, the benefit equation changes when the distance is large enough that active cooling no longer dominates the observatory power budget. In addition, communication power requirements rise even faster with distance for a given bandwidth. For such cases (at, for example, 3 to 5 AU) RPS power might be highly enabling. For the reasons noted above, however, onboard fission systems would remain problematic.
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3. Board on Physics and Astronomy–Space Studies Board, National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.