Consequently, for a given distance to the Sun d and concentration ratio C, the equation (10) could be used to compute the enlarged solid angle Qc. Then the energy flux can be determined by means of any of the two procedures we proposed. Let us shortly analyze the error of the less accurate procedure, which considers the Sun as a source of isotropic radiation. We define this error as follows: Figure 2 shows the dependence of the error on the distance to the Sun and angle 0 in case of unconcentrated solar radiation. The error diminishes by increasing the distance to the Sun and decreasing the zenith angle 0. At normal incidence the first procedure is an excellent approximation. However, in case of concentrated radiation the error generally increases (figure 3). Normal incident radiation continues to be evaluated very accurately. Note that, for a given zenith angle 0 there is a minimum distance to the Sun for which the procedure applies. This is a consequence of equation (9), where, of course, in case of concentrated solar radiation 5 must be replaced by an equivalent value bc corresponding to the enlarged solid angle Qc subtended by the Sun. For given distance to the Sun and concentration ratio the value 6C can be determined by using equations (11) and (4). For better accuracy the results that follow were obtained by considering the Sun as a source of non-isotropic radiation. Figures 4 a and b show the solar energy flux in interplanetary space as a function of the distance to the Sun, for different values of the zenith angle 0. The dependence of the energy flux on 0 is significant, whatever the distance to the Sun is. We conclude that, apart from some terrestrial applications, the utilization of solar energy in space generally requires an accurate orientation of the receiver. The following question arises naturally: Is the solar energy flux big enough to be used in the space missions of the near future? We note that a significant number of advanced space missions have been already identified. These missions include: manned planetary outposts and bases essentially requiring hundreds of kilowatts to megawatt power levels for sustained operation and interplanetary electric propulsion cargo vehicles with requirements in the 2 to 5 megawatt power range [6]. From the view-point of the above question the solar system could be shortly divided into two parts: the inner planets including Mars and, perhaps, the asteroids and the outer planets. In the first case the amount of solar energy is big enough to allow its use as a main energy source in space missions, even if the present technology is considered. On the other hand, the large scale utilization of solar energy for space missions in the region of the outer planets seems to be a question of the far future. Indeed, because of the reduced value of
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