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Radiation and the International Space Station: Recommendations to Reduce Risk
action. But an overly restrictive flight rule and the lack of operationally calibrated models bar the path between the flight director and such sources of information. The problematic (albeit unofficial) flight rule is the "real-time, on-site data" rule, which says that changes in flight plans in response to a radiation situation must be based on real-time, on-site data only. The first recommendation of CSSP/CSTR addresses this flight rule.
Recommendation 1: Because it denies access to valid information and thus unnecessarily restrains flight-director options, flight directors should not adhere rigidly to the (unofficial) real-time, on-site data rule.
As mentioned, the second obstacle in the path between the flight director and data sources is the lack of operationally calibrated models. In important cases, however, the state of radiation modeling is advanced enough, or with directed effort could quickly become advanced enough, to justify a flight rule that allows use of validated procedures to infer and, in some cases, to predict on-site radiation conditions from off-site data. The report cites such cases.
CSSP/CSTR notes that Russians performing EVAs will be directed out of the Russian mission control center in Moscow. Further, it is likely that U.S. and international crew members on ISS will also participate in EVAs directed out of mission control-Moscow. However, flight rules at mission control-Moscow pertaining to radiation may differ from those at NASA's mission control center. Although this report is directed at NASA, CSSP/CSTR believes that some of its recommendations could also be implemented by mission control-Moscow.
SOLAR PARTICLE EVENTS AND THE INTERNATIONAL SPACE STATION
Based on the assumption—the best now available—that the radiation characteristics of the current solar cycle will resemble those of the last cycle, there is nearly a 100 percent chance that at least 2 out of 43 planned ISS construction flights will overlap a significant solar particle event (SPE) and a 50 percent chance that at least 5 flights will overlap such an event. Moreover, the high-latitude zones to which solar energetic particles have access show a marked tendency to widen over the polar latitudes reached by the ISS orbit when SPEs are in progress, a tendency that becomes more pronounced as SPEs intensify. Two storms during 1989, near the maximum of the last solar cycle, illustrate the point. The areas around the poles accessible to SPE particles enlarged until they engulfed more than a quarter of the ISS orbit, and the flux of particles was high enough to have pushed an astronaut over the short-term limit for irradiation of skin and eyes during a single ill-timed 6-hour EVA. These results would seem to call for an aggressive program aimed at reducing solar radiation risk to astronauts during ISS construction. Recommendation 2 addresses means of implementing the elements of such a program.
Recommendation 2: For real-time SPE risk management, carry out the steps needed to make usable by SEC and the Space Radiation Analysis Group (SRAG) at Johnson Space Center (JSC) models that use real-time data to specify the intensity of SPE particles and the geographical size and shape of the zones accessible to them.
NASA, NOAA, the U.S. Air Force (USAF), and the distributed space physics community have the capability for implementing this recommendation. The project implied in this recommendation is one of the important projects that could be implemented early enough to have an impact on SPE radiation risk management during ISS construction. It should receive high priority for early implementation. (Appendix A discusses a suite of models for this application.)