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Since the planning for Apollo missions, radiation protection for manned space missions has included the monitoring and prediction of solar particle events in the near-earth environment. Experience since the Apollo missions provides a baseline for estimating the frequency and intensity of exposure to solar particle events in during the construction of the International Space Station (ISS). ISS construction will fall across the predicted sun-spot peak of solar cycle 23, which began in late 1996 and is expected to reach maximum in 2000 or 2001. Experience from recent solar cycles indicates that the construction coincides closely with the maximum phase of the occurrence of solar particle events extending from two to three years before the sunspot maximum to three to four years after maximum. Solar cycle 23 will see the use of new sophisticated methods for solar particle event monitoring and forecasting, including the use of new monitors of solar activity and monitoring of the solar wind.

Solar Particle Events (SPE) are a rapid increase over the normal background in the near-earth space of charged particles, including protons and heavier ions. During SPE energetic particle fluxes from the Sun increase to thousands of times background in the space of a few hours. The largest SPE pose a hazard for crews of space missions, including the International Space Station, and operational responses to the events need to be developed. As a first-level strategy, measurements of SPE fluxes made in real time from satellites allow a continuous check of the particle populations and allow comparison to dosimeters on board spacecraft. The wide range of event sizes poses a problem in developing a strategy of avoiding high radiation doses during large events and frequent operational interruptions at times of small events that pose less hazard to space crews. Monitoring the Sun and Sun-earth space provides a means of forecasting the occurrence of SPE several hours in advance.

The 11-year solar cycle modulates radiation hazards throughout interplanetary space. The galactic cosmic ray (GCR) background varies approximately inversely with the 11-year cycle, as defined by the traditional Wolf sunspot number, by nearly 30 percent over the course of the cycle; the background reaches minimum around the time of sunspot maximum and maintains a broad maximum around the time of sunspot minimum. Solar particle events (SPEs) are short, intense events that typically last a few days but produce particle fluxes much larger than the GCR background for brief periods of time. SPEs may occur at any time in the 11-year solar cycle, but historically have been more frequent during the years around sunspot maximum. Protection against radiation exposure during future exploration-class space missions depends on knowledge of the form and length of the solar cycle, both of which are extremely variable.