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Radiation and astronaut mortality, Part 2: Gathering the measurements

In order to estimate radiation doses in space, I had to make some basic assumptions. First, I assumed that radiation dose levels would vary by some measurable factors: altitude, orbital inclination, and the type of spacecraft. If this were true, then the dose received would depend on how much time was spent at any given location defined by these parameters. While these assumptions do not account for all the factors according to which radiation dose rates might vary (e.g., solar particle events and variation in the solar cycle may have an impact), they are a reasonable starting point.

To estimate average equivalent radiation doses for various locations in outer space, I would need individual measurements of radiation exposure for those locations. Such measurements, I reasoned, could be taken from results of various experiments in outer space. To ensure I got a comprehensive view of the literature on this subject, I used a systematic search of peer-reviewed journal articles.

I conducted the literature search using two search engines: MEDLINE and Google scholar. I chose MEDLINE to provide a comprehensive search of the medical literature, including aviation medicine and health physics journals. I chose Google scholar as a complementary search engine which could provide coverage of engineering and physics journals not indexed by MEDLINE.

Using these two search engines I conducted three searches in total: one MEDLINE search and two searches using Google Scholar. In the MEDLINE search I used two groups of subject headings. The first was related to exposure and the second related to locations. The search would return any article that had at least one exposure keyword coupled with at least one location keyword. The table below shows these keyword groups by search engine.

Search Engine

Exposure terms

Location terms

1

MEDLINE

radiation; radiation, ionizing; radiation monitoring; cosmic radiation

space; extraterrestrial environment; Earth (planet); Moon

2

Google Scholar

radiation; ionizing; galactic; cosmic; dosimetry

moon, space, LEO*, low-earth, orbit, extraterrestrial

3

Google Scholar

dosimetry

Apollo, Gemini, Skylab, ASTP**

*Low Earth Orbit

**Apollo-Soyuz Test Project

The Google Scholar searches were conducted in a similar fashion, with one group of exposure keywords and one group of location keywords. In the first Google Scholar search keywords were chosen to return results from any outer space location and any mission. In the second search the keywords were targeted to the earliest of NASA’s manned space missions to try to ensure adequate sampling from them. For all searches the date range was restricted to 1957 to 2012 and articles in English or Russian. This focused search strategy was only effective in the Google scholar search because of the different ways the two engines search the literature: MEDLINE uses subject headings, rather than keywords. Any relevant articles indexed in MEDLINE would have been detectable in the headings search, and therefore targeted searches for particular missions were not possible.

Together, the three searches returned a total of almost 3,800 documents. Screening of titles and abstracts, primarily to ensure that the studies involved appropriate measures of radiation, selected 819 for more in-depth review. This set contained many duplicate articles, non-peer-reviewed documents such as technical reports, and articles that only reported absorbed dose.  After further selecting articles, 38 unique peer-reviewed articles were retained which contained equivalent dose measurements. (For more details on the screening process, please see my full dissertation).  These were then subjected to a quality review by a radiation safety expert. All 38 were found to be of good quality and were retained in the final article set.

The 38 articles provided 333 potentially useable measurements of equivalent radiation dose for locations in outer space. The data represented 5 different space craft types as well as measurements made during extravehicular activity, or “spacewalks”. With these data I was able to create a dose estimation tool that allows estimation of equivalent radiation doses received in space for all astronauts. Part 3  of this series will discuss the construction of that tool.

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