area where high dose rates combine with wide SPE zones to pose a hazard to astronauts in ISS orbit who are shielded only by a space suit. The nine points inside the danger area come from two storms during 1989. If ISS were inside the danger area while it was in the part of its diurnal cycle when it most deeply penetrates the SPE zones, it would spend a quarter to a third of each orbit exposed to SPE particles. During this time, an astronaut on a 6-hour EVA could receive a radiation dose between 10 and 100 percent of the short-term limits for eyes and skin.

2.4 SUMMARY AND RECOMMENDATION

CSSP/CSTR finds that a concentrated effort aimed at reducing SPE radiation risk to astronauts during ISS construction is needed. 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 SPE and a 50 percent chance that at least 5 flights will overlap such an event. Moreover, whenever SPEs are in progress, the SPE zones show a marked tendency to widen over the polar latitudes reached by the ISS orbit, a tendency that becomes stronger with SPE severity. Two storms during 1989, near the maximum of the last solar cycle, illustrate the point. The SPE zones widened until they engulfed more than a quarter of the planned ISS orbit, while the radiation intensified enough to have pushed an astronaut over the short-term limit for skin and eyes during a single, ill-timed 6-hour EVA. CSSP/CSTR therefore recommends the rapid implementation of the following scientific elements of a program to reduce SPE radiation risk:

Recommendation 2: For real-time SPE risk management, carry out the steps needed to make usable by SEC and SRAG the 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.

This recommendation could be implemented early enough to have an impact on SPE radiation risk management during ISS construction; CSSP/CSTR views it as a high-priority item for action by NASA, NOAA, and the USAF. Chapter 4 and Appendix A of this report document other existing and potential resources that could contribute to implementing recommendation 2. Appendix A also addresses institutional issues related to developing the requisite modeling tools. Finally, chapter 5 includes a discussion of unresolved issues surrounding the ''transitioning" of research models to operational use.

Recommendation 1 in Chapter 1 is relevant to Recommendation 2, set forth above. For these recommendations to succeed, flight directors must become involved in assessing the effectiveness of the new or improved tools for SPE risk management that could allow liberalizing the current (unofficial) flight rule on radiation that says changes in flight plans can be based only on real-time, on-site data.

2.5 NOTES AND REFERENCES

1.  

For fluence to dose conversion, see A.C. Tribble, The Space Environment, Princeton University Press, 1995, Figure 5.2.

2.  

D.K. Bailey, Planet. Space Sci., 1964, p. 485.

3.  

R.E. Turner and J.E. Baker, "Solar particle events and the International Space Station," Acta Astronautica, 42, 1998, pp. 107-114.

4.  

J.A. Joselyn, J.B. Anderson, H. Coffey, K. Harvey, D. Hathaway, G. Heckman, E. Hildner, W. Mende, K. Schatten, R. Thompson, A.W.P. Thomson, and O.R. White, "Panel achieves consensus prediction of Solar Cycle 23," EOS, Trans. Amer. Geophys. Union, 78, 1997, pp. 205, 211-212.

5.  

R.E. Turner and C. Kemere, "Solar particle events and International Space Station," Report submitted to Committee on Solar and Space Physics. This material, which was presented to CSSP in August 1998, is available for viewing in the National Research Council's Public Access Records Office.

6.  

D.V. Reames, "Energetic particles and the structure of coronal mass ejections," in Coronal Mass Ejections, Geophys. Monogr. Ser., 99, N.U. Crooker, J.A. Joselyn, and J. Feynman, eds., American Geophysical Union, Washington, D.C., 1997, pp. 217-226.

7.  

H.V. Cane, D.V. Reames, and T.T. von Rosenvinge, "The role of interplanetary shocks in the longitude distribution of solar energetic particles," J. Geophys. Res., 93, 1998, pp. 9555-9567.



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