User:Jssteil/sandbox/Review of space flight issues

In considering radiation risks for astronauts, it is useful to consider the historical recommendations that NASA has received from external advisory committees. These have formed the basis for the dose limits and risk projection models. Early radiation effects usually are related to a significant fraction of cell loss that exceeds the threshold for impairment of function in a tissue. These "deterministic" effects are so called because the statistical fluctuations in the number of affected cells are very small compared to the number of cells that are required to reach the threshold. Maintaining dose limits can ensure that no early effects occur these are expected to be accurately understood. As late effects can result from changes in a very small number of cells, statistical fluctuations can be large and some level of risk is incurred even at low doses. Referring to them as "stochastic" effects recognizes the predominance of statistical effecs in their manifestation.

In recommendations by the NAS in 1967, it was noted that radiation protection in human space flight is phiilosphically distinct from the protection practices of terrestrial workers because of the high-risk nature of space missions. This report y the NAS from 1967 did not recommend "permissible doses" for space operations, noting the possibility that such limits may jeopardize the mission, but instead estimated what the likely effects would be for a given dose of radiation.

In 1970, the NAS Space Science Board recommended guidelines for career doses to be used by NASA for long-term mission design and human operations. At that time, NASA employed onle male astronauts and they typical age of astronauts was 30 to 40 years. A "primary" reference risk" was proposed that was equal to the natural probability of cancer over a period of 20 years following the radiation exposure (using the period from 35 to 55 years of age); this was essentially a doubling dose. The estimated doubling dose of 382 rem (3.82 Sv), which ignored dose-rate reduction factor, was rounded to 400 rem (4 Sv).  The NAS panel noted that its recommendation were not risk limits, but, rather, a reference risk, and that a higher risk could be considered for planetary missions or a lower level of risk for a possible space station mission.  Ancillary reference risks were described to consider monthly, annual, and career exposure patterns.  However, the NAS recommendations were implemented by NASA as dose limits that were used operationally for all missions until 1989.