Statement of Task
Background: NASA's Sun-Earth Connection (SEC) program office has asked the National Research Council to assess potential impact that energetic particle radiation caused by solar storms might have on scheduling EVAs during the construction phase of the International Space Station (ISS). This assessment bears upon determining the priority the SEC program office might assign to predicting and specifying the intensities of solar energetic particle events and the radiation belts.
The concern that solar storms might non-negligibly affect the scheduling of EVAs arises because NASA plans to place ISS in a circular orbit similar to the Russian MIR at an altitude of approximately 444 km (240 nmi) with a 51.6 degree inclination to the equatorial plane. This inclination is higher than planned prior to the decision to collaborate with Russia. It places ISS in a space environment that is vulnerable to space weather disturbances from auroral influences on the upper atmosphere, from solar and interplanetary energetic particles, and from the galactic cosmic ray flux. Concern about the potential levels of radiation exposure by Station workers and crew, especially those engaged in work outside Station modules, is heightened by a timetable for construction that is scheduled to occur during the upcoming period of maximum of solar activity. (Scientific considerations for the next solar activity maximum are addressed in the new NRC report "Readiness for the Upcoming Solar Maximum," in press.) The higher inclination orbit also increases vulnerability to "single event upsets" in solid-state devices and electrical discharges on material surfaces in space when the sun is active.
NASA plans to rely on NOAA's Space Environment Center (NOAA/SEC) in Boulder, Colorado, for space weather warnings. However, NOAA/SEC does not provide services that are tailored specifically for Space Station needs. Moreover, there is no group within NASA that is specifically supported to "translate" the more general NOAA/SEC space weather products for ISS operations use.
Plan: A study will be undertaken that will examine, on the one hand, the risk of radiation exposure to ISS and, on the other hand, NASA's plans to use SEC space weather information to manage the radiation risk. The Committee on Solar and Space Physics, working in conjunction with its federated partner, the Committee on Solar-Terrestrial Research, will take the lead in producing a report. The study will make recommendations that take advantage of current research in the Solar Connections Enterprise in OSS and the developing interagency National Space Weather Program. A report of the order of 25 pages is planned.
It is anticipated that the report would accomplish the following objectives:
Provide a preliminary assessment of the radiation exposure impact to ISS assembly activities. This aspect of the study will be executed in two parts. In the first part, the committees will simulate the radiation exposure to ISS during the coming solar maximum (1998-2002) by shifting the radiation events of the last solar cycle (cycle 22) ahead one cycle. This simulation is justified since the consensus prediction for solar cycle 23 (during which ISS will be constructed) is that it will be very similar to solar cycle 22 (for which there are data on radiation intensities from solar storms). The radiation events will be superimposed on the Space Shuttle flight and EVA (extravehicular activities) schedule for ISS assembly. For the second part of the study, the committees will estimate how often "significant" disruption to activities might occur. This first-order estimate will not be based on a detailed analysis of the radiation dose to an astronaut. Instead, the committees will first establish thresholds (based on intensity and duration) which, if exceeded, would be expected to disrupt the mission schedule. Then they will examine solar events from cycle 22 to determine how often this threshold was exceeded during scheduled EVAs. This exercise will be repeated several times with different start times for the assembly flights to obtain statistics with which to express the impact assessment.
Examine existing arrangements within NASA to manage the radiation risk problem. This fact-finding part of the study would provide background information on ongoing and planned activities at NASA centers and at headquarters. It would also examine arrangements between the NASA science and life sciences codes.
Recommend how operational radiation support to ISS might be optimally accomplished. This part of the study would focus on current capabilities and planned roles and responsibilities in NASA, NOAA, and DOD for generating, disseminating, and using operational space weather products, and it would identify gaps or overlaps and opportunities for improving support.
Meetings: The Committee on Solar and Space Physics, working in conjunction with its federated partner, the Committee on Solar-Terrestrial Research, will take the lead in this study. The committees have considerable expertise related to space weather and its potential effects on Space Station. The study is planned as a 1-year effort with three 3-day meetings for further fact finding, discussions, and drafting the report. The first meeting will be held on June 29-July 1, 1998. Subsequent meetings would be held in the fall of 1998 and late January/early February 1999. The final meeting to complete the report would likely be held at the Beckman Center. Publication of the report would occur by July 1, 1999.