EVA start time and duration,
The status of outer zone electron belts,
Interplanetary particle flux,
Geomagnetic field conditions, and
The phase of the solar cycle.
The last four factors are the province of solar and space physics scientists (NRC, 2000).
SRAG maintains an extensive set of tools for estimating the exposure received by the crews of manned missions in LEO. This suite of tools includes time-resolved models of Earth’s magnetic field, maps of the radiation fluxes trapped in the geomagnetosphere, and trajectory translator/propagator algorithms. Space environment conditions (interplanetary proton flux, status of the electron belts, geomagnetic field conditions) from the Space Environment Center (SEC) of the National Oceanic and Atmospheric Administration (NOAA) are integrated with mission parameters (altitude and inclination of the spacecraft, location and timing of EVA) in order to project crew exposures.
Astronauts in LEO are exposed to radiation trapped in Earth’s magnetosphere and to radiation from the Sun (solar energetic particles [SEPs]) and beyond (galactic cosmic radiation [GCR]). The trapped radiation is most intense in a region off the coast of South America (the South Atlantic Anomaly [SAA]), owing to a slight offset between the magnetic dipole of Earth and Earth’s axis of rotation. The SEPs and GCR are most intense near Earth’s poles, where there are “holes” in the approximately dipolar magnetosphere. The extent of exposure in the polar regions fluctuates during periods of geomagnetic storms. Figures 3.1 and 3.2 demonstrate the radiation environments that the International Space Station is exposed to during its orbit of Earth.
Low-inclination, high-altitude flights during solar minimum produce higher dose rates than those with high-inclination, low-altitude flights during solar maximum. At higher altitudes, the area of the SAA is larger and the flux of protons is higher. Although trajectories of high-inclination flights pass through the regions of maximum intensities within the SAA, less time is spent there than during low-inclination flights, and crews on high-inclination flights typically receive less net exposure to trapped radiation for the same altitude.
During solar maximum, increases in the Sun’s activity expand the atmosphere; this expansion causes losses of some of the protons in the radiation belts owing to interactions with atmospheric gases. Therefore, trapped radiation doses decrease during solar maximum and increase during solar minimum. The impact of GCR is also lower during solar maximum, because the increased speed and density of the solar wind intensifies the interplanetary magnetic field generated by the Sun, making it more difficult for GCR to penetrate the inner solar system.
The radiation consoles in the Mission Control Center at Johnson Space Center (JSC) are staffed 4 hours daily during nominal space weather conditions and continuously during EVAs and significant space weather activity. SRAG receives data and alerts NOAA’s Space Environment Center in Boulder, Colorado. NOAA continuously monitors data received from its space weather satellites and ground stations to provide current information and forecasts about the space environment. SEC forecasters provide around-the-clock support, providing alerts and warnings about space weather conditions by telephone and pager and by displaying real-time operational space weather data via the Internet.