belts as observed by the Van Allen Probes mission."/>
Figure 1.7 Temporal correspondence between star tracker anomalies onboard a geostationary orbit spacecraft (vertical arrows) and enhancements in E ~ 3 MeV electron fluxes measured by sensors on the same spacecraft.
(Source: Baker, 1987.)
Figure 1.8 Induced attenuation of fiber optic cabling signals due to accumulated proton dose for two different types (S1500 and Z‐fiber®) of material
(Source: Alam, https://www.spiedigitallibrary.org/conference‐proceedings‐of‐spie/10567/105672M/Performance‐of‐optical‐fibers‐in‐space‐radiation‐environment/10.1117/12.2308184.full?SSO=1. Licensed under CC BY SA 4.0)
Figure 1.8 shows induced attenuation growth versus accumulated proton dose plots for the two fibers. S1550‐HTA fiber with the higher NA (0.16) performs fairly well in comparison to Z‐fiber®, which has relatively smaller NA (0.12). For both fibers, attenuation growth data can be represented well by the three‐term “saturating exponentials” model (Alam et al., 2006). Solid lines in the figure represent best fit to the data. S1550‐HTA shows a trend towards saturation, while Z‐fiber® does not indicate any such behavior. Using the data acquired, radiation‐induced attenuation of S1550‐HTA fiber at room temperature and to a total accumulated dose of 50 kRad was predicted to be 1.33 dB/km and 0.76 dB/km in γ‐ and proton radiation environments, respectively.
1.4. DISCUSSION AND CONCLUSIONS
This chapter has reviewed evidence that galactic cosmic rays, solar energetic particles, trapped high energy particles, and magnetospheric electrons of moderate energies constitute a significant problem for the operation of spacecraft. These particle populations occur with different frequencies, and the nature of their damage mechanisms are also quite different. Certain types of spacecraft anomalies have been closely linked with environmental factors, while other classes of disruptions may or may not be related to space environmental conditions. More work needs to be done to clarify where the environment is (or is not) implicated. Also, more work is required to understand the physical nature of some disruption mechanisms in spacecraft systems.
There is an increasing physical understanding of the near‐Earth space environment. This understanding is sufficient that there is even a significant predictive capability. The first line of defense for space environmental problems is good engineering design. However, systems often degrade over time, and sometimes unexpected sensitivities develop in space components. In such cases, a simple forecast of space environmental conditions may allow operators to change operational procedures, or at least be ready to recover quickly from operational problems should they occur. In yet other circumstances, it may be possible to turn off susceptible subsystems during periods of strong space environmental disturbance. Thus, we assert that accurate and reliable forecasts have great potential benefit to the operational community. In this way, space weather predictions and forecasts are a next logical step in our space environmental research efforts.
We have also shown in this chapter that long‐term cumulative effects from space radiation can have a variety of severe effects. Total dose and displacement damage in electronic components and in solar power systems often spell the ultimate demise of space systems. This can be a cost driver over the entire course of a mission. For example, the possibility (or likelihood) of long‐term degradation of solar array power output often forces designers to oversize the arrays. Similarly, degradation of thermal control surfaces often forces designers to oversize thermal radiator panels. This is done to be confident of end‐of‐life heat rejection, but then designers may have to add extra heaters which, in turn, require more solar array power. These compounding design aspects can drive the cost and complexity of space systems to extraordinary levels.
We also note that the degradation of semiconductor part performance often limits the effective operating lifetime of satellite subsystems in space. Having better knowledge of regional radiation environmental conditions would make a huge difference in accurately forecasting mission life and consequent program costs. Presumed knowledge of average space environmental conditions determine the operational regimes for spacecraft. These considerations dictate that certain orbits are not possible or cost effective to fly because of the severity of the radiation environments. We have shown in this chapter that the Van Allen Probes mission has revealed that for present, and recent, solar activity conditions, some parts of the near‐Earth environment have been surprisingly benign and nonthreatening. Hence, getting the space system design right requires that we get the space environments right. This ultimately means that designers must be able to forecast with confidence what the expected fluences are for various particle populations over the course of the designed mission. Of course, there needs to be a reasonable sense of margins on these numbers because of the inherent variability of the solar‐terrestrial system. This is where design and space weather climatology meet.
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