FIGURE 6.1 Space-physics-related Explorer launch history: number of launches and satellite mass.

Figure 6.1a shows the number of Explorer launches from 1958 to the present, along with projected launches for programs approved through 1997 (projected launch times should be considered uncertain). The data show a clear and continuing decrease in space-physics-related launches since the 1960s. However, we are not able to conclude from this result alone that the number of research opportunities has decreased. If, for example, satellites increased in size and thus carried more experiments, the number of actual research opportunities (as measured by experiments flown) may not have decreased.

Figure 6.1b shows the mass evolution of the Explorer satellites. Again, the masses shown for future launches should be considered uncertain. The Figure shows a general increase in Explorer size since 1958. Further, it appears to be possible to separate the data into two categories: small and large Explorers. (Interestingly, this implies that at a very early stage, the space physics community saw the need for both small and large missions.)

How do these factors affect the number of experiments flown? Figure 6.2 shows a repeat of the number of space-physics-related Explorer launches since 1958, along with the number of experiments flown onboard those Explorer satellites. The decrease in number of launches from the 1960s through the 1970s is compensated for by the increasing satellite sizes, giving a comparable number of experiments flown in both decades. However, the launch frequency became so



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