**4. Causes of death and comparative mortality for Soviet and Russian cosmonauts**

In comparison to astronaut mortality, there has been comparatively little research on cosmo‑ naut mortality. To date, only three studies have been published on this topic, all since 2014 [21–23]. From this research, we know that the patterns of mortality risk of cosmonauts are quite similar to those of astronauts in terms of how they compare to Soviet and Russian general population controls.

Cosmonauts tend to have the same major causes of death as astronauts and, like astronauts, tend to have lower mortality rates for those causes than the general population. This is perhaps unsurprising given the similar vocational backgrounds, similar (and now joint) training and physical readiness criteria required to be either a cosmonaut or astronaut, and the similar lev‑ els of biomedical monitoring of cosmonauts throughout their careers [24, 25]. That cosmonauts would be at elevated risk of death due to accidents, especially in the early years of the space programs, might also be expected, and as we shall see, this is indeed true. We refer the reader again to **Tables 1**–**3** for demographic and actuarial information about the cosmonaut corps.

For comparisons, Russian general population mortality rates were taken from the Human Mortality Database [2] and the Russian Fertility and Mortality Database from the Center for Demographic Studies at the New Economic School [26]. The Human Mortality Database sup‑ plied all-cause mortality rates for the years 1960 through 2015; the Russian Fertility and Mortality Database supplied cause-specific rates for the years 1960 to 2014. The latest rates available (2014 or 2015) were used for comparison with cosmonaut data from 2015, 2016, and 2017 as needed.

### **4.1. Numbers and causes of cosmonaut deaths**

**Figure 6** shows the causes of death for cosmonauts through 31 October 2017. What should be immediately apparent in **Figure 6** is the relatively high number of deaths due to unknown causes. There were 24 completely unknown causes of death, and a single unknown external cause, for a total of 25 causes of death with some degree of uncertainty as to their causes. The 24 totally unknown causes of death represent 25% of the 96 total cosmonaut deaths. Unknown causes do not pose a problem for an analysis of all-cause mortality. However, depending on which causes these unknown might actually represent, this may substantially alter cause-spe‑ cific mortality rates and SMRs. We will address these issues further in the sections that follow.

The first study of cosmonaut mortality, published in 2014, studied the entire cohort of people who were not only selected for but also completed cosmonaut training [21]. The study reported a dramatic (and statistically significant) reduction in risk of death from all causes. However, SMRs reported in this study were incorrect, as SMRs were computed using probabilities of death for the astronaut cohort rather than mortality rates. While the difference between agespecific probabilities of death and mortality rates may be negligible at young ages, as age increases mortality rates become substantially higher than probabilities of death. This made SMRs and the corresponding bounds on their 95% confidence intervals too low. Because of this, no conclusions can be drawn from the all-cause SMR results from that study alone.

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**Figure 6.** Causes of death for Soviet and Russian cosmonauts, 1960–2017.

Two additional studies on cosmonaut mortality have been published after the 2014 Reynolds et al. study. These two studies are highly similar to each other in that they use only cosmo‑ nauts who went to space. The first, published in 2016 in the Russian journal *Air-Ecosystem and Environmental Medicine*, studied cosmonaut mortality within the cohort of 114 cosmo‑ nauts who flew to space at least once [22]. The authors compared the mortality experience of these cosmonauts to that of the general populations of the Moscow region and the Russian Federation as a whole, with follow-up through the end of 2013. Unfortunately, the study

Similar to the astronaut cohort, the most common causes of death for cosmonauts were CVD (25 deaths) and cancer (20 deaths), and accidents (19 deaths), with a small number of deaths by other natural causes (7 deaths). However, external causes of death have accounted for a smaller share of cosmonaut deaths (21%), compared to the astronaut cohort (42%).

#### **4.2. All-cause mortality**

Much like United States astronauts, cosmonauts have been at decreased risk of all-cause mortal‑ ity when compared to the general population [21–23]. This has been shown to be true for the cos‑ monaut cohort as a whole [21], as well as the subset of cosmonauts who went to space [22, 23].

**Figure 6.** Causes of death for Soviet and Russian cosmonauts, 1960–2017.

**4. Causes of death and comparative mortality for Soviet and Russian** 

In comparison to astronaut mortality, there has been comparatively little research on cosmo‑ naut mortality. To date, only three studies have been published on this topic, all since 2014 [21–23]. From this research, we know that the patterns of mortality risk of cosmonauts are quite similar to those of astronauts in terms of how they compare to Soviet and Russian general

Cosmonauts tend to have the same major causes of death as astronauts and, like astronauts, tend to have lower mortality rates for those causes than the general population. This is perhaps unsurprising given the similar vocational backgrounds, similar (and now joint) training and physical readiness criteria required to be either a cosmonaut or astronaut, and the similar lev‑ els of biomedical monitoring of cosmonauts throughout their careers [24, 25]. That cosmonauts would be at elevated risk of death due to accidents, especially in the early years of the space programs, might also be expected, and as we shall see, this is indeed true. We refer the reader again to **Tables 1**–**3** for demographic and actuarial information about the cosmonaut corps.

For comparisons, Russian general population mortality rates were taken from the Human Mortality Database [2] and the Russian Fertility and Mortality Database from the Center for Demographic Studies at the New Economic School [26]. The Human Mortality Database sup‑ plied all-cause mortality rates for the years 1960 through 2015; the Russian Fertility and Mortality Database supplied cause-specific rates for the years 1960 to 2014. The latest rates available (2014 or 2015) were used for comparison with cosmonaut data from 2015, 2016, and 2017 as needed.

**Figure 6** shows the causes of death for cosmonauts through 31 October 2017. What should be immediately apparent in **Figure 6** is the relatively high number of deaths due to unknown causes. There were 24 completely unknown causes of death, and a single unknown external cause, for a total of 25 causes of death with some degree of uncertainty as to their causes. The 24 totally unknown causes of death represent 25% of the 96 total cosmonaut deaths. Unknown causes do not pose a problem for an analysis of all-cause mortality. However, depending on which causes these unknown might actually represent, this may substantially alter cause-spe‑ cific mortality rates and SMRs. We will address these issues further in the sections that follow. Similar to the astronaut cohort, the most common causes of death for cosmonauts were CVD (25 deaths) and cancer (20 deaths), and accidents (19 deaths), with a small number of deaths by other natural causes (7 deaths). However, external causes of death have accounted for a

smaller share of cosmonaut deaths (21%), compared to the astronaut cohort (42%).

Much like United States astronauts, cosmonauts have been at decreased risk of all-cause mortal‑ ity when compared to the general population [21–23]. This has been shown to be true for the cos‑ monaut cohort as a whole [21], as well as the subset of cosmonauts who went to space [22, 23].

**cosmonauts**

population controls.

**4.1. Numbers and causes of cosmonaut deaths**

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**4.2. All-cause mortality**

The first study of cosmonaut mortality, published in 2014, studied the entire cohort of people who were not only selected for but also completed cosmonaut training [21]. The study reported a dramatic (and statistically significant) reduction in risk of death from all causes. However, SMRs reported in this study were incorrect, as SMRs were computed using probabilities of death for the astronaut cohort rather than mortality rates. While the difference between agespecific probabilities of death and mortality rates may be negligible at young ages, as age increases mortality rates become substantially higher than probabilities of death. This made SMRs and the corresponding bounds on their 95% confidence intervals too low. Because of this, no conclusions can be drawn from the all-cause SMR results from that study alone.

Two additional studies on cosmonaut mortality have been published after the 2014 Reynolds et al. study. These two studies are highly similar to each other in that they use only cosmo‑ nauts who went to space. The first, published in 2016 in the Russian journal *Air-Ecosystem and Environmental Medicine*, studied cosmonaut mortality within the cohort of 114 cosmo‑ nauts who flew to space at least once [22]. The authors compared the mortality experience of these cosmonauts to that of the general populations of the Moscow region and the Russian Federation as a whole, with follow-up through the end of 2013. Unfortunately, the study counted observation time as starting with enrollment in the cosmonaut corps, meaning SMRs were incorrect (biased downward) due to an *immortal time bias* [27]. In this instance, the time between selection and the first space mission is guaranteed survival time, as the cosmonauts who may have died before their first space flight would not be included in the analysis. The effect of this is to systematically bias SMRs downward. Once again, this error renders the study inconclusive on the mortality experience of cosmonauts.

Though mortality for cosmonauts in the 1960s was 179% of that expected, the SMR was not statistically significant. Cosmonauts experienced 87% of the mortality expected in the 1970s, also not significant. From the 1980s onward, the mortality risk for cosmonauts has been statis‑ tically significantly *reduced* in comparison to the general population, with SMRs ranging from a low of 36 to a high of 59, and an overall SMR for the 1960–2017 period of 51 (95% CI = 41–63). These long-term and correctly-computed findings solidify the result that cosmonauts are at an overall lower risk of mortality than the Russian general population. They also largely agree with the prior results from the 2016 study by Ushakov et al. [22] even with slightly different cohort definitions. From this, we can conclude that, overall, cosmonauts have been at a lower risk of death from all causes, between 1960 and 2017, than the year-, age- and sex-matched

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As noted above, (known) external causes of death represent just over one-fifth of cosmonaut deaths. Yet, the only previously reported mortality measure in relation to external causes came from the 2016 study, which, as previously mentioned, was biased [22]. Nevertheless, the authors reported that, for the period 1960–2013, cosmonauts who had flown on at least one mission to space had SMR for external causes of death of 42 (95% CI = 16–107) in comparison

**Figure 8** shows updated SMRs for all external and accidental causes of death for all trained cosmonauts by decade between 1960 and 2017. The 1960s saw three times the number of deaths from external causes as expected, but this quickly tapered off to cosmonauts having

Results were generally not statistically significant by decade, but over the entire period from 1960 to 2017, cosmonauts were at significantly lower mortality than the general population, with SMR of 47 (95% CI = 29–72). This overall SMR is quite close to that published by Ushakov et al. [22], in spite of the bias in that article and the differing cohort definitions. Overall, the evidence shows that cosmonauts are at lower risk of death from external causes than is the

Similar to astronauts, the majority of deaths due to external causes (19 of 20 for which causes were known) for cosmonauts have been accidental. Given this, it is not surprising that the pattern of SMRs for accidental deaths (**Figure 8**) is very similar to that for all external causes

SMR of 62 (95% CI = 38–98) for the entire 1960–2017 period is close to the SMR of 52 (95% CI = 19–139) reported by Ushakov et al. [23] The wider confidence interval on the Ushakov SMR is a direct result of the more limited data used in that study (only male cosmonauts who

general population of Russia.

*4.3.1. All external causes of death*

fewer deaths than expected.

Russian general population.

*4.3.2. Accidental causes of death*

combined.

to the general population of the Russian Federation [22].

had been to space, with follow-up to the end of 2014).

**4.3. External causes**

A second study published in late 2017 added an additional year of follow-up to the 2016 study and corrected the immortal time bias. This study confirmed that cosmonauts are at reduced risk of death from all causes, reporting SMR of 40 (95% CI = 27–61) for death by all causes in comparison to the Russian Federation [23].

A reanalysis (with correction) of the Reynolds et al. data [21] is presented in **Figure 7**, updated to 31 October 2017. This shows the trend in SMRs for all-cause mortality for cosmonauts over time.

**Figure 7.** SMRs for all causes of death for Soviet and Russian cosmonauts, 1960–2017.

Though mortality for cosmonauts in the 1960s was 179% of that expected, the SMR was not statistically significant. Cosmonauts experienced 87% of the mortality expected in the 1970s, also not significant. From the 1980s onward, the mortality risk for cosmonauts has been statis‑ tically significantly *reduced* in comparison to the general population, with SMRs ranging from a low of 36 to a high of 59, and an overall SMR for the 1960–2017 period of 51 (95% CI = 41–63). These long-term and correctly-computed findings solidify the result that cosmonauts are at an overall lower risk of mortality than the Russian general population. They also largely agree with the prior results from the 2016 study by Ushakov et al. [22] even with slightly different cohort definitions. From this, we can conclude that, overall, cosmonauts have been at a lower risk of death from all causes, between 1960 and 2017, than the year-, age- and sex-matched general population of Russia.

### **4.3. External causes**

counted observation time as starting with enrollment in the cosmonaut corps, meaning SMRs were incorrect (biased downward) due to an *immortal time bias* [27]. In this instance, the time between selection and the first space mission is guaranteed survival time, as the cosmonauts who may have died before their first space flight would not be included in the analysis. The effect of this is to systematically bias SMRs downward. Once again, this error renders the

A second study published in late 2017 added an additional year of follow-up to the 2016 study and corrected the immortal time bias. This study confirmed that cosmonauts are at reduced risk of death from all causes, reporting SMR of 40 (95% CI = 27–61) for death by all causes in

A reanalysis (with correction) of the Reynolds et al. data [21] is presented in **Figure 7**, updated to 31 October 2017. This shows the trend in SMRs for all-cause mortality for cosmonauts over time.

study inconclusive on the mortality experience of cosmonauts.

**Figure 7.** SMRs for all causes of death for Soviet and Russian cosmonauts, 1960–2017.

comparison to the Russian Federation [23].

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#### *4.3.1. All external causes of death*

As noted above, (known) external causes of death represent just over one-fifth of cosmonaut deaths. Yet, the only previously reported mortality measure in relation to external causes came from the 2016 study, which, as previously mentioned, was biased [22]. Nevertheless, the authors reported that, for the period 1960–2013, cosmonauts who had flown on at least one mission to space had SMR for external causes of death of 42 (95% CI = 16–107) in comparison to the general population of the Russian Federation [22].

**Figure 8** shows updated SMRs for all external and accidental causes of death for all trained cosmonauts by decade between 1960 and 2017. The 1960s saw three times the number of deaths from external causes as expected, but this quickly tapered off to cosmonauts having fewer deaths than expected.

Results were generally not statistically significant by decade, but over the entire period from 1960 to 2017, cosmonauts were at significantly lower mortality than the general population, with SMR of 47 (95% CI = 29–72). This overall SMR is quite close to that published by Ushakov et al. [22], in spite of the bias in that article and the differing cohort definitions. Overall, the evidence shows that cosmonauts are at lower risk of death from external causes than is the Russian general population.

#### *4.3.2. Accidental causes of death*

Similar to astronauts, the majority of deaths due to external causes (19 of 20 for which causes were known) for cosmonauts have been accidental. Given this, it is not surprising that the pattern of SMRs for accidental deaths (**Figure 8**) is very similar to that for all external causes combined.

SMR of 62 (95% CI = 38–98) for the entire 1960–2017 period is close to the SMR of 52 (95% CI = 19–139) reported by Ushakov et al. [23] The wider confidence interval on the Ushakov SMR is a direct result of the more limited data used in that study (only male cosmonauts who had been to space, with follow-up to the end of 2014).

**Figure 8.** SMRs for all external causes and accidental causes of death for Soviet and Russian cosmonauts, 1960–2017.

#### **4.4. Natural causes**

Natural causes of death are responsible for more than 50 cosmonaut deaths to date (**Figure 6**). Primary among them have been cancer and cardiovascular disease, with a small number of assorted other natural causes.

**4.5. Cardiovascular disease**

One of the major components of death by natural causes, CVD has been responsible for 25 out of 52 (48%) of the known natural-cause deaths among cosmonauts. Prior research reported that cosmonauts who have been to space at least once had between 35% and 40% risk of death

**Figure 9.** SMRs for all natural-cause, CVD, and cancer mortality among Soviet and Russian cosmonauts, 1960–2017.

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Looking to **Figure 9** once again, we can see SMRs for the trained cosmonaut cohort (orange). Interestingly, there were no deaths from CVD in either the 1960s (not shown) or 1970s. From 1970 to 2017, SMRs show decreased risk of death from CVD, with SMRs ranging from 0 to 62.

The trend here may be due in some part to missing information on CVD deaths in the decades since 1990. In those decades, there have been 25 deaths from unknown causes. Depending on how many of those deaths were due to CVD, this could be enough to push some of these

by CVD compared to the general population through the end of 2014 [22, 23].

Overall, the SMR from 1960 to 2017 is 28 (95% CI = 18–41).

No prior research has reported measures of mortality for all natural-cause deaths for cos‑ monauts. **Figure 9** shows SMR for natural causes (darker blue), CVD (orange), and cancer (lighter blue) for 1960 to 2017.

SMRs for all natural causes of death in all decades (darker blue) were below 100, though they only reached statistical significance in the 1980–1989 period and remained significant thereafter. SMRs ranged between 0 and 76, with SMR of 33 (95% CI = 25–43) for the entire 1960–2017 period, indicating that cosmonauts were at one-third the risk of death by natural causes as the general population of Russia.

**Figure 9.** SMRs for all natural-cause, CVD, and cancer mortality among Soviet and Russian cosmonauts, 1960–2017.

#### **4.5. Cardiovascular disease**

**4.4. Natural causes**

assorted other natural causes.

(lighter blue) for 1960 to 2017.

causes as the general population of Russia.

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Natural causes of death are responsible for more than 50 cosmonaut deaths to date (**Figure 6**). Primary among them have been cancer and cardiovascular disease, with a small number of

**Figure 8.** SMRs for all external causes and accidental causes of death for Soviet and Russian cosmonauts, 1960–2017.

No prior research has reported measures of mortality for all natural-cause deaths for cos‑ monauts. **Figure 9** shows SMR for natural causes (darker blue), CVD (orange), and cancer

SMRs for all natural causes of death in all decades (darker blue) were below 100, though they only reached statistical significance in the 1980–1989 period and remained significant thereafter. SMRs ranged between 0 and 76, with SMR of 33 (95% CI = 25–43) for the entire 1960–2017 period, indicating that cosmonauts were at one-third the risk of death by natural One of the major components of death by natural causes, CVD has been responsible for 25 out of 52 (48%) of the known natural-cause deaths among cosmonauts. Prior research reported that cosmonauts who have been to space at least once had between 35% and 40% risk of death by CVD compared to the general population through the end of 2014 [22, 23].

Looking to **Figure 9** once again, we can see SMRs for the trained cosmonaut cohort (orange). Interestingly, there were no deaths from CVD in either the 1960s (not shown) or 1970s. From 1970 to 2017, SMRs show decreased risk of death from CVD, with SMRs ranging from 0 to 62. Overall, the SMR from 1960 to 2017 is 28 (95% CI = 18–41).

The trend here may be due in some part to missing information on CVD deaths in the decades since 1990. In those decades, there have been 25 deaths from unknown causes. Depending on how many of those deaths were due to CVD, this could be enough to push some of these SMRs to reflect significantly increased risk for cosmonauts. (See Section 4.7 below for further discussion and analysis of this point.) Nevertheless, based on **Figure 9** and the prior research, it appears that cosmonauts are at decreased risk of CVD mortality compared to the Russian general population.

#### **4.6. Cancer**

The other major component of natural causes of death for cosmonauts is cancers of various types. Previous studies of cosmonaut mortality reported cosmonauts who had been to space as having an SMR of approximately 75 for death by cancer [22, 23].

**Figure 9** also shows SMRs for cancer in the wider cosmonaut cohort (lighter blue points and lines). While most SMRs over time are not statistically significant, the SMR of 60 for the entire period from 1960 to 2017 did reach significance (95% CI 36–92). As with natural causes and CVD, SMRs for cancer from 1990 onward could be influenced by the number of unknown causes of death that might rightly be attributed to cancer. Section 4.7 explores this possibility.

#### **4.7. The effects of unknown causes**

One potential limitation of the cause-specific analyses presented for cosmonauts is the 24 deaths due to unknown causes. Depending on the distribution of the true causes of these deaths, they could dramatically alter the cause-specific SMRs reported here. In order to explore this possibility, we recomputed SMRs under various assumptions about the distribu‑ tion of the unknown causes of death.

We assumed for these analysis that the unknown causes were cancer deaths, CVD deaths, or deaths due to other natural causes. This allows us to more deeply explore the question of whether space travel is shortening the longevity of space explorers through increased rates of death by chronic disease, a question of primary concern (that accidents related to space travel will shorten the lives of some astronauts is accepted).

In **Figure 11**, the orange points and lines represent observed SMRs and 95% confidence intervals for natural causes, CVD and cancer (exactly as in **Figure 9**), omitting from the analyses all unknown causes. The blue points and lines in **Figure 11** are SMRs and 95% confidence intervals calculated by counting all deaths due to unknown causes as deaths due to the respective cause (natural causes overall, CVD or cancer). As in **Figure 6**, the light blue rectangular zone surrounding these sets of SMRs shows the total range of possibility for true SMR (with 95% CI) by cause. As in the prior SMR figures, the red dashed line is drawn across 100, the point of parity with the general population.

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Observed SMRs for death by all natural causes combined in **Figure 11** show significantly reduced risk for cosmonauts in comparison to the general population, even with the inclusion of the unknown causes within this causal category. The light blue interval for the entire study period 1960 to 2017 shows that the true value of SMR for the entire study period of 1960 to 2017 is likely to be between 25 and 60, whatever the truth about the unknown causes of death may be. In total, we conclude that, though the value of SMRs reported in Section 4.4 above may be too low due to misclassification of some deaths, the overall conclusion remains the

The implications of categorizing all unknown causes of death as CVD deaths have similar implications to the overall natural-cause death analysis, although in this case SMRs for two of

*4.7.1. Cosmonaut natural-cause SMRs revisited*

**Figure 11.** Alternative SMRs for Soviet and Russian cosmonauts, 1990–2017.

*4.7.2. Cosmonaut CVD SMRs revisited*

same: cosmonauts are at reduced risk of death from natural causes.

As reflected in **Figure 10**, the 1990–1999 period had 3 unknown deaths, the 2000–2009 period had 5 deaths with unknown causes, and the 2010–2017 period had 16 deaths with unknown causes.

To see the effect these deaths may have on natural-cause SMRs, we recomputed SMRs assum‑ ing that all deaths due to unknown causes in a period were due to each respective cause in turn (natural causes overall, CVD or cancer). The result is **Figure 11**.

**Figure 10.** Distribution over time of deaths by unknown causes for Soviet and Russian cosmonauts.

**Figure 11.** Alternative SMRs for Soviet and Russian cosmonauts, 1990–2017.

In **Figure 11**, the orange points and lines represent observed SMRs and 95% confidence intervals for natural causes, CVD and cancer (exactly as in **Figure 9**), omitting from the analyses all unknown causes. The blue points and lines in **Figure 11** are SMRs and 95% confidence intervals calculated by counting all deaths due to unknown causes as deaths due to the respective cause (natural causes overall, CVD or cancer). As in **Figure 6**, the light blue rectangular zone surrounding these sets of SMRs shows the total range of possibility for true SMR (with 95% CI) by cause. As in the prior SMR figures, the red dashed line is drawn across 100, the point of parity with the general population.

#### *4.7.1. Cosmonaut natural-cause SMRs revisited*

SMRs to reflect significantly increased risk for cosmonauts. (See Section 4.7 below for further discussion and analysis of this point.) Nevertheless, based on **Figure 9** and the prior research, it appears that cosmonauts are at decreased risk of CVD mortality compared to the Russian

The other major component of natural causes of death for cosmonauts is cancers of various types. Previous studies of cosmonaut mortality reported cosmonauts who had been to space

**Figure 9** also shows SMRs for cancer in the wider cosmonaut cohort (lighter blue points and lines). While most SMRs over time are not statistically significant, the SMR of 60 for the entire period from 1960 to 2017 did reach significance (95% CI 36–92). As with natural causes and CVD, SMRs for cancer from 1990 onward could be influenced by the number of unknown causes of death that might rightly be attributed to cancer. Section 4.7 explores this possibility.

One potential limitation of the cause-specific analyses presented for cosmonauts is the 24 deaths due to unknown causes. Depending on the distribution of the true causes of these deaths, they could dramatically alter the cause-specific SMRs reported here. In order to explore this possibility, we recomputed SMRs under various assumptions about the distribu‑

We assumed for these analysis that the unknown causes were cancer deaths, CVD deaths, or deaths due to other natural causes. This allows us to more deeply explore the question of whether space travel is shortening the longevity of space explorers through increased rates of death by chronic disease, a question of primary concern (that accidents related to space travel

As reflected in **Figure 10**, the 1990–1999 period had 3 unknown deaths, the 2000–2009 period had 5 deaths with unknown causes, and the 2010–2017 period had 16 deaths with unknown causes.

To see the effect these deaths may have on natural-cause SMRs, we recomputed SMRs assum‑ ing that all deaths due to unknown causes in a period were due to each respective cause in

as having an SMR of approximately 75 for death by cancer [22, 23].

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general population.

**4.7. The effects of unknown causes**

tion of the unknown causes of death.

will shorten the lives of some astronauts is accepted).

turn (natural causes overall, CVD or cancer). The result is **Figure 11**.

**Figure 10.** Distribution over time of deaths by unknown causes for Soviet and Russian cosmonauts.

**4.6. Cancer**

Observed SMRs for death by all natural causes combined in **Figure 11** show significantly reduced risk for cosmonauts in comparison to the general population, even with the inclusion of the unknown causes within this causal category. The light blue interval for the entire study period 1960 to 2017 shows that the true value of SMR for the entire study period of 1960 to 2017 is likely to be between 25 and 60, whatever the truth about the unknown causes of death may be. In total, we conclude that, though the value of SMRs reported in Section 4.4 above may be too low due to misclassification of some deaths, the overall conclusion remains the same: cosmonauts are at reduced risk of death from natural causes.

### *4.7.2. Cosmonaut CVD SMRs revisited*

The implications of categorizing all unknown causes of death as CVD deaths have similar implications to the overall natural-cause death analysis, although in this case SMRs for two of the decades would no longer be statistically significant (**Figure 11**). The overall conclusion for the period from 1960 to 2017 also does not change: cosmonauts are at significantly lower risk of CVD mortality than the Russian general population.

#### *4.7.3. Cosmonaut cancer SMRs revisited*

**Figure 11** demonstrates that the impact of reassigning deaths with unknown causes has the most dramatic effect when all are counted as cancer deaths. In this case, all SMR point esti‑ mates that were below 100 are now above 100, though in most cases the confidence intervals continue to include 100 (thus failing to reach the level of statistical significance).

The results of these exploratory analyses for cancer suggest that cosmonauts likely have little difference in cancer-specific mortality rates compared to those of the Russian general popula‑ tion between 1960 and 2017.

### **5. Astronauts vs. cosmonauts**

Having examined the mortality experience of astronauts and cosmonauts separately, we can conclude that both groups have similar patterns of mortality in comparison to general popu‑ lation control groups: lower mortality rates overall, with higher rates of accidental deaths (more so for astronauts) and much lower rates of death from chronic diseases. We now turn our attention to how astronauts and cosmonauts compare directly to one another. Given the similarity in selection criteria, background, training, and career duties, we might expect to find similar mortality rates for astronauts and cosmonauts over the last 60 years. However, given that mortality rates for the Russian general population are known to be higher than those in the United States, we may find some differences.

To explore these possibilities, we computed SMRs using the observed age-, sex-, and periodspecific mortality rates among United States astronauts to generate expected numbers of deaths for cosmonauts based on their corresponding age-, sex-, and period-specific exposure times. The ratios of observed cosmonaut deaths to expected deaths determined in this way thus provided SMRs for cosmonauts compared to astronauts.

**5.2. External causes**

1960–2017.

*5.2.1. All external causes*

for all external causes here for the first time.

No measures of mortality have been previously reported comparing rates of death from all external causes between cosmonauts and astronauts. We report cosmonaut to astronaut SMRs

**Figure 12.** SMRs for all causes of death among Soviet and Russian cosmonauts compared to United States astronauts,

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In **Figure 13**, we see that the external-cause SMRs for periods before the year 2000 are similar to those for death by all causes, which might be expected given that astronauts and cosmonauts were relatively young in those years and most deaths observed were externally caused deaths. In the case of the 1960s, SMR for all causes and external causes is identical, as the only causes of death to both astronauts and cosmonauts in the 1960s were accidental, a subset of external causes.

#### **5.1. All-cause mortality**

**Figure 12** displays SMRs for cosmonauts in comparison to astronauts for death by all causes. In all but two decades, cosmonauts were at significantly greater risk of death than astronauts; only in the 1960s were cosmonauts at reduced risk of death, and only in the 1980s was there no significant difference between the two groups of space explorers.

What is perhaps most striking about **Figure 12** is that from 1990 to 2017 cosmonauts were more than twice as likely to die as astronauts. Confidence intervals for individual decades are wide, but over the entire period from 1960 to 2017, cosmonauts experienced a nearly doubling of risk compared to astronauts (SMR = 186, 95% CI = 150–228).

**Figure 12.** SMRs for all causes of death among Soviet and Russian cosmonauts compared to United States astronauts, 1960–2017.

#### **5.2. External causes**

the decades would no longer be statistically significant (**Figure 11**). The overall conclusion for the period from 1960 to 2017 also does not change: cosmonauts are at significantly lower risk

**Figure 11** demonstrates that the impact of reassigning deaths with unknown causes has the most dramatic effect when all are counted as cancer deaths. In this case, all SMR point esti‑ mates that were below 100 are now above 100, though in most cases the confidence intervals

The results of these exploratory analyses for cancer suggest that cosmonauts likely have little difference in cancer-specific mortality rates compared to those of the Russian general popula‑

Having examined the mortality experience of astronauts and cosmonauts separately, we can conclude that both groups have similar patterns of mortality in comparison to general popu‑ lation control groups: lower mortality rates overall, with higher rates of accidental deaths (more so for astronauts) and much lower rates of death from chronic diseases. We now turn our attention to how astronauts and cosmonauts compare directly to one another. Given the similarity in selection criteria, background, training, and career duties, we might expect to find similar mortality rates for astronauts and cosmonauts over the last 60 years. However, given that mortality rates for the Russian general population are known to be higher than

To explore these possibilities, we computed SMRs using the observed age-, sex-, and periodspecific mortality rates among United States astronauts to generate expected numbers of deaths for cosmonauts based on their corresponding age-, sex-, and period-specific exposure times. The ratios of observed cosmonaut deaths to expected deaths determined in this way

**Figure 12** displays SMRs for cosmonauts in comparison to astronauts for death by all causes. In all but two decades, cosmonauts were at significantly greater risk of death than astronauts; only in the 1960s were cosmonauts at reduced risk of death, and only in the 1980s was there

What is perhaps most striking about **Figure 12** is that from 1990 to 2017 cosmonauts were more than twice as likely to die as astronauts. Confidence intervals for individual decades are wide, but over the entire period from 1960 to 2017, cosmonauts experienced a nearly doubling

continue to include 100 (thus failing to reach the level of statistical significance).

of CVD mortality than the Russian general population.

276 Into Space - A Journey of How Humans Adapt and Live in Microgravity

those in the United States, we may find some differences.

thus provided SMRs for cosmonauts compared to astronauts.

no significant difference between the two groups of space explorers.

of risk compared to astronauts (SMR = 186, 95% CI = 150–228).

*4.7.3. Cosmonaut cancer SMRs revisited*

tion between 1960 and 2017.

**5.1. All-cause mortality**

**5. Astronauts vs. cosmonauts**

#### *5.2.1. All external causes*

No measures of mortality have been previously reported comparing rates of death from all external causes between cosmonauts and astronauts. We report cosmonaut to astronaut SMRs for all external causes here for the first time.

In **Figure 13**, we see that the external-cause SMRs for periods before the year 2000 are similar to those for death by all causes, which might be expected given that astronauts and cosmonauts were relatively young in those years and most deaths observed were externally caused deaths. In the case of the 1960s, SMR for all causes and external causes is identical, as the only causes of death to both astronauts and cosmonauts in the 1960s were accidental, a subset of external causes.

Our updated analysis comparing rates of accidental deaths in the two groups decade-by-decade is also given in **Figure 13**. SMRs show us that cosmonauts were at particularly lower risk of acci‑ dental death in the 1960s (owing to several astronaut plane crash deaths and three astronaut deaths in the Apollo 1 fire), and then again in the two decades that experienced space shuttle disasters (1980–1989 and 2000–2009). As with all external causes, the results here do not reach a level of statistical significance (aside from 1960 to 1969, which barely reaches significance).

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Most noticeable in **Figure 13** is the fact that SMRs for all external causes are largely identical to those for accidental causes, since most of the deaths by external causes in both cohorts are accidental in nature. Only in the 1990s and the 2000s are SMRs for the two different, and even

From this, we might conclude that the occupation of cosmonauts and astronauts demands of them that they lead comparably risky lives and that the estimates of relative risk for them within particular periods have more to do with chance timing than systematic differences in

**Figure 14** displays cosmonaut/astronaut SMRs for natural causes of death (darker blue), CVD mortality (orange), and cancer mortality (lighter blue). Since there were no deaths to astronauts or cosmonauts by natural causes in the 1960s, it is impossible to define an SMR for that period. Few natural-cause deaths in the 1970s and 1980s result in wide confidence intervals for those periods, with no significant evidence of excess mortality for either cosmonauts or astronauts. In general, cosmonauts have been at higher risk of death by natural causes after 1989. The exception is the 2010–2017 period, when there was essentially no difference between the two cohorts. However, this result is highly suspect, as this period contains 16 deaths in the cos‑ monaut cohort that are of unknown cause. If we examine the possible range of SMRs as we did in Section 4.7, adding as few as 9 deaths to the cosmonaut death count would render sig‑ nificant SMR of 175 (95% CI = 104–276). If all 16 deaths from unknown causes were actually deaths from natural causes, the SMR would be 243 (95% CI = 157–358). Either value certainly seems plausible, and the net effect is that we should not rule out the possibility of a significant

There were cosmonaut deaths from unknown causes in the 1990–1999 and 2000–2009 periods as well. However, since the observed SMRs in **Figure 14** already show statistically significant increases in mortality risk for cosmonauts, adding more deaths from natural causes would only further increase SMR values. For example, assuming all three deaths from unknown causes in the 1990s were truly from natural causes would raise SMR to 229 (95% CI = 138–357) up from the current 193 (95% CI = 110–313). Assuming all five deaths from unknown causes in

Given the SMRs (and hypothetical SMRs from assumptions about the distribution of causes of death among unknowns), we conclude that cosmonauts are at higher risk of death by natural

2000–2009 were from natural causes would raise SMR to 453 (95% CI = 296–664).

then only slightly.

risk of accidents.

**5.3. Natural causes**

*5.3.1. All natural causes*

increase in mortality for cosmonauts in this period.

causes than are astronauts from 1990 through 2017.

**Figure 13.** SMRs for all external and accidental causes of death among Soviet and Russian cosmonauts compared to United States astronauts, 1960–2017.

From 2000 onward, the all-cause and external-cause SMRs diverge, as the cohorts' age and other causes of death are observed more frequently among astronauts and cosmonauts. None of the decade-specific results are statistically significant, nor is the overall SMR of 81 (sug‑ gesting a somewhat lower risk of externally caused deaths for cosmonauts, but with 95% CI = 49–126).

#### *5.2.2. Accidents*

SMRs comparing rates of accidental death among cosmonauts to those of astronauts were reported in the first published study on cosmonaut mortality [21]. SMRs showed an insig‑ nificant reduction in risk in the Soviet era, an insignificant increase in risk in the Roscosmos era, and an insignificant reduction in risk for the overall 1960–2013 period (SMR = 88; 95% CI = 54–136) [21].

Our updated analysis comparing rates of accidental deaths in the two groups decade-by-decade is also given in **Figure 13**. SMRs show us that cosmonauts were at particularly lower risk of acci‑ dental death in the 1960s (owing to several astronaut plane crash deaths and three astronaut deaths in the Apollo 1 fire), and then again in the two decades that experienced space shuttle disasters (1980–1989 and 2000–2009). As with all external causes, the results here do not reach a level of statistical significance (aside from 1960 to 1969, which barely reaches significance).

Most noticeable in **Figure 13** is the fact that SMRs for all external causes are largely identical to those for accidental causes, since most of the deaths by external causes in both cohorts are accidental in nature. Only in the 1990s and the 2000s are SMRs for the two different, and even then only slightly.

From this, we might conclude that the occupation of cosmonauts and astronauts demands of them that they lead comparably risky lives and that the estimates of relative risk for them within particular periods have more to do with chance timing than systematic differences in risk of accidents.

### **5.3. Natural causes**

From 2000 onward, the all-cause and external-cause SMRs diverge, as the cohorts' age and other causes of death are observed more frequently among astronauts and cosmonauts. None of the decade-specific results are statistically significant, nor is the overall SMR of 81 (sug‑ gesting a somewhat lower risk of externally caused deaths for cosmonauts, but with 95%

**Figure 13.** SMRs for all external and accidental causes of death among Soviet and Russian cosmonauts compared to

SMRs comparing rates of accidental death among cosmonauts to those of astronauts were reported in the first published study on cosmonaut mortality [21]. SMRs showed an insig‑ nificant reduction in risk in the Soviet era, an insignificant increase in risk in the Roscosmos era, and an insignificant reduction in risk for the overall 1960–2013 period (SMR = 88; 95%

CI = 49–126).

United States astronauts, 1960–2017.

278 Into Space - A Journey of How Humans Adapt and Live in Microgravity

*5.2.2. Accidents*

CI = 54–136) [21].

### *5.3.1. All natural causes*

**Figure 14** displays cosmonaut/astronaut SMRs for natural causes of death (darker blue), CVD mortality (orange), and cancer mortality (lighter blue). Since there were no deaths to astronauts or cosmonauts by natural causes in the 1960s, it is impossible to define an SMR for that period. Few natural-cause deaths in the 1970s and 1980s result in wide confidence intervals for those periods, with no significant evidence of excess mortality for either cosmonauts or astronauts.

In general, cosmonauts have been at higher risk of death by natural causes after 1989. The exception is the 2010–2017 period, when there was essentially no difference between the two cohorts. However, this result is highly suspect, as this period contains 16 deaths in the cos‑ monaut cohort that are of unknown cause. If we examine the possible range of SMRs as we did in Section 4.7, adding as few as 9 deaths to the cosmonaut death count would render sig‑ nificant SMR of 175 (95% CI = 104–276). If all 16 deaths from unknown causes were actually deaths from natural causes, the SMR would be 243 (95% CI = 157–358). Either value certainly seems plausible, and the net effect is that we should not rule out the possibility of a significant increase in mortality for cosmonauts in this period.

There were cosmonaut deaths from unknown causes in the 1990–1999 and 2000–2009 periods as well. However, since the observed SMRs in **Figure 14** already show statistically significant increases in mortality risk for cosmonauts, adding more deaths from natural causes would only further increase SMR values. For example, assuming all three deaths from unknown causes in the 1990s were truly from natural causes would raise SMR to 229 (95% CI = 138–357) up from the current 193 (95% CI = 110–313). Assuming all five deaths from unknown causes in 2000–2009 were from natural causes would raise SMR to 453 (95% CI = 296–664).

Given the SMRs (and hypothetical SMRs from assumptions about the distribution of causes of death among unknowns), we conclude that cosmonauts are at higher risk of death by natural causes than are astronauts from 1990 through 2017.

Three out of the four decade-specific SMRs for CVD in **Figure 14** are not statistically significant. Only the SMR for 2000–2009 was large and statistically significant (SMR 1206, 95% CI = 642– 2062). The large confidence interval on this estimated SMR is evidence of the small number of deaths in the astronaut cohort (from which the comparison mortality rates were derived). This small number of deaths led to low mortality rates and thus a low expected number of deaths. This extreme SMR would grow larger if any of the unknown causes of deaths among cosmonauts in that period were in fact deaths due to CVD. However, the very low number of observed deaths among astronauts in this period makes the estimate somewhat unstable.

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Like in the prior study [21], overall SMR is statistically significant at 332 (95% CI = 215–491), heavily influenced by the 2000–2009 period. As noted in prior sections, the number of deaths from unknown causes could change the results of recent SMRs. If even one additional death were added to the tally for the 1990–1999 period, SMR would be 243 (95% CI = 105–478), a statistically significant result. Similarly, a reassignment of some unknown causes of death in 2000–2009 and in 2010–2017 could easily raise these to a level of statistically significant elevated risk of CVD mortality for cosmonauts. From the observed data and hypothetical SMRs under various assumptions about unknown causes of death, we can conclude that cos‑

Cosmonauts have previously been reported to be at elevated risk of cancer mortality in comparison to United States astronauts [21]. Though SMRs for the Soviet and Russian peri‑ ods were not significant separately, the overall 1960–2013 SMR was significant at 177 (95%

Updated SMRs for cancer mortality for Soviet and Russian cosmonauts in comparison to United States astronauts are shown as light blue points and lines in **Figure 14**. Periods for which SMR could not be calculated include 1960–1969 or 1970–1979; there were no astronaut

SMRs for cancer show that in comparison to United States astronauts, Soviet and Russian cosmonauts have largely been at increased risk of death from cancer, though most SMRs are not statistically significant. The exceptions are 1990–1999 and the overall 1970–2017, both of which demonstrate a statistically significant increased risk for cosmonauts. SMR of 67 for 2010–2017 (95% CI = 8–241) is the only SMR that shows a reduction in risk, and not coinciden‑ tally, this is in the period in which there are comparatively many observed cosmonaut deaths

If we were to add three more deaths to the count of cancer deaths for cosmonauts in the 1990–1999 period, SMR would rise to 356 (95% CI = 178–638). As few as three extra deaths in the 2000–2009 period would drive SMR to significance at 224 (95% CI = 102–425), and six additional deaths in the 2010–2017 period would yield significantly increased SMR of 267 (95% CI = 115–527). Finally, assuming all 24 deaths from unknown causes were due to cancers would increase the overall 1980 to 2017 SMR to 392 (95% CI = 284–529). Even without these potential extra deaths, SMR seen here is highly similar to that reported previously, at 173 (95% CI = 104–271) vs. 177 (95% CI = 108–274) [21]. As with mortality due to CVD and natural causes, we conclude that cosmo‑ nauts are at increased risk of death due to cancer in comparison to United States astronauts.

monauts have been at greater risk of dying from CVD since the 1990s.

*5.3.3. Cancers*

CI = 108–274) [21].

due to unknown causes.

deaths due to cancer in either of those periods.

**Figure 14.** SMRs for all natural-cause, CVD, and cancer mortality among Soviet and Russian cosmonauts compared to United States astronauts, 1960–2017.

#### *5.3.2. Cardiovascular disease*

Cardiovascular disease is a cause of death for which both astronauts and cosmonauts have greatly reduced mortality risk in comparison to the general populations of the United States and Russia, respectively. The evidence for this relative to cosmonauts may not be as con‑ vincing given the high numbers of deaths of unknown causes, however (see Section 4.7 above). In the prior cosmonauts-to-astronauts comparison, cosmonauts were noted to have a significant increase in mortality due to CVD between 1960 and 2013 (SMR = 364, 95% CI = 225–557) [21].

**Figure 14** only includes SMRs for CVD starting with the 1980–1989 period since there were no astronaut deaths from CVD between 1960 and 1979, again making SMRs impossible to define for those two decades (orange points and lines). Cosmonauts experienced no deaths from CVD in the 1960s, but did experience one CVD death in the 1970s.

Three out of the four decade-specific SMRs for CVD in **Figure 14** are not statistically significant. Only the SMR for 2000–2009 was large and statistically significant (SMR 1206, 95% CI = 642– 2062). The large confidence interval on this estimated SMR is evidence of the small number of deaths in the astronaut cohort (from which the comparison mortality rates were derived). This small number of deaths led to low mortality rates and thus a low expected number of deaths. This extreme SMR would grow larger if any of the unknown causes of deaths among cosmonauts in that period were in fact deaths due to CVD. However, the very low number of observed deaths among astronauts in this period makes the estimate somewhat unstable.

Like in the prior study [21], overall SMR is statistically significant at 332 (95% CI = 215–491), heavily influenced by the 2000–2009 period. As noted in prior sections, the number of deaths from unknown causes could change the results of recent SMRs. If even one additional death were added to the tally for the 1990–1999 period, SMR would be 243 (95% CI = 105–478), a statistically significant result. Similarly, a reassignment of some unknown causes of death in 2000–2009 and in 2010–2017 could easily raise these to a level of statistically significant elevated risk of CVD mortality for cosmonauts. From the observed data and hypothetical SMRs under various assumptions about unknown causes of death, we can conclude that cos‑ monauts have been at greater risk of dying from CVD since the 1990s.

### *5.3.3. Cancers*

**Figure 14.** SMRs for all natural-cause, CVD, and cancer mortality among Soviet and Russian cosmonauts compared to

Cardiovascular disease is a cause of death for which both astronauts and cosmonauts have greatly reduced mortality risk in comparison to the general populations of the United States and Russia, respectively. The evidence for this relative to cosmonauts may not be as con‑ vincing given the high numbers of deaths of unknown causes, however (see Section 4.7 above). In the prior cosmonauts-to-astronauts comparison, cosmonauts were noted to have a significant increase in mortality due to CVD between 1960 and 2013 (SMR = 364, 95%

**Figure 14** only includes SMRs for CVD starting with the 1980–1989 period since there were no astronaut deaths from CVD between 1960 and 1979, again making SMRs impossible to define for those two decades (orange points and lines). Cosmonauts experienced no deaths from

CVD in the 1960s, but did experience one CVD death in the 1970s.

United States astronauts, 1960–2017.

280 Into Space - A Journey of How Humans Adapt and Live in Microgravity

*5.3.2. Cardiovascular disease*

CI = 225–557) [21].

Cosmonauts have previously been reported to be at elevated risk of cancer mortality in comparison to United States astronauts [21]. Though SMRs for the Soviet and Russian peri‑ ods were not significant separately, the overall 1960–2013 SMR was significant at 177 (95% CI = 108–274) [21].

Updated SMRs for cancer mortality for Soviet and Russian cosmonauts in comparison to United States astronauts are shown as light blue points and lines in **Figure 14**. Periods for which SMR could not be calculated include 1960–1969 or 1970–1979; there were no astronaut deaths due to cancer in either of those periods.

SMRs for cancer show that in comparison to United States astronauts, Soviet and Russian cosmonauts have largely been at increased risk of death from cancer, though most SMRs are not statistically significant. The exceptions are 1990–1999 and the overall 1970–2017, both of which demonstrate a statistically significant increased risk for cosmonauts. SMR of 67 for 2010–2017 (95% CI = 8–241) is the only SMR that shows a reduction in risk, and not coinciden‑ tally, this is in the period in which there are comparatively many observed cosmonaut deaths due to unknown causes.

If we were to add three more deaths to the count of cancer deaths for cosmonauts in the 1990–1999 period, SMR would rise to 356 (95% CI = 178–638). As few as three extra deaths in the 2000–2009 period would drive SMR to significance at 224 (95% CI = 102–425), and six additional deaths in the 2010–2017 period would yield significantly increased SMR of 267 (95% CI = 115–527). Finally, assuming all 24 deaths from unknown causes were due to cancers would increase the overall 1980 to 2017 SMR to 392 (95% CI = 284–529). Even without these potential extra deaths, SMR seen here is highly similar to that reported previously, at 173 (95% CI = 104–271) vs. 177 (95% CI = 108–274) [21]. As with mortality due to CVD and natural causes, we conclude that cosmo‑ nauts are at increased risk of death due to cancer in comparison to United States astronauts.

### **6. Chapter summary**

In this chapter, we have examined the mortality of astronauts and cosmonauts in comparison with the general populations of their respective nations, with specially selected controls, and in comparison with one another. The results of prior research and the new analyses presented here indicate that both astronauts and cosmonauts have much lower rates of death by chronic disease (such as CVD and cancers) than do their respective general populations. However, the mortality rate from plane crashes and spacecraft accidents over the years has made both groups of space explorers more likely to die from external causes in general and accidental death in particular, than is expected in the general population. The net effect is that all-cause mortality risk for space explorers is still lower than that of the general population.

partnership over the last 17 years collaborating on the International Space Station, the overall

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Finally, the quality of medical care could be an explanation for the differential mortality rate due to chronic diseases. Even among equal rates of incidence, differences in the accessibility or effectiveness of treatment for CVD and cancers could lead to a higher case-fatality rate in cosmonauts compared to astronauts. This could result in cosmonauts dying younger of the

The balance of evidence accumulated to date regarding mortality of space explorers sug‑ gests that they are, overall, at less risk of death on an age- and gender-matched basis than the baseline risks in their respective countries of origin. As humans continue to explore space, and in particular as they engage in longer trips deeper into space, it may be inevitable that the unique exposures they will face will ultimately lead to some increased risk of mortality due to at least some particular causes (tragic accidents and radiation‑related cancers perhaps chief among them). However, it may be worth bearing in mind that as a profession, being an astronaut or cosmonaut is not a terribly risky business. Based on the research reviewed here, as well as the original research presented for the first time, the job of space explorer should

[1] Nicogossian AE, Williams RS, Huntoon CL, Doarn CR. Living and working in space: An overview of physiological adaptation, performance, and health risks. In: Nicogossian AE, Williams RS, Huntoon CL, Doarn CR, Polk JD, Schneider VS, editors. Space Physiology

[2] Human Mortality Database. University of California, Berkeley (USA), and Max Planck Institute for Demographic Research (Germany). Available from: www.mortality.org or

[3] Centers for Disease Control and Prevention, National Center for Health Statistics. Compressed Mortality File 1968-1978. CDC WONDER Online Database, compiled

[4] Centers for Disease Control and Prevention, National Center for Health Statistics. Compressed Mortality File 1979-1998. CDC WONDER On-line Database, compiled from Compressed Mortality File CMF 1968-1988, Series 20, No. 2A, 2000 and CMF 1989-1998,

from Compressed Mortality File CMF 1968-1988, Series 20, No. 2A, 2000

dose per person-day in space in recent years is likely equivalent.

same diseases afflicting astronauts, driving up SMRs.

not make any top 10 lists of the world's deadliest jobs.

\*Address all correspondence to: rreynolds@mortalityresearch.com

and Medicine. 4th ed. New York: Springer; 2016. p. 95-134

www.humanmortality.de [Accessed: Oct 27, 2017]

Robert J. Reynolds\* and Steven M. Day

Series 20, No. 2E, 2003

Mortality Research and Consulting, Inc., USA

**Author details**

**References**

Careful interpretation is needed for the reduced risk of chronic disease among astronauts and cosmonauts. Space agencies to date have intentionally tried to limit the potential harmful exposures from space radiation. This has included both projecting and measuring the lifetime dose of radiation for individual astronauts [28]. The evidence gathered thus far seems to indi‑ cate that few to no astronauts or cosmonauts have received detrimental doses of space radia‑ tion and that we will only begin to understand the mortality risks space radiation can bring once humans start performing longer missions, such as to Mars or beyond.

When comparing cosmonauts to astronauts, we see that astronauts tend to have a slightly higher risk of accidental death compared to cosmonauts, but a significant reduction in the risk of CVD and cancer. The net difference places cosmonauts at an overall greater all-cause mortality risk than United States astronauts.

The lower death rate among cosmonauts from accidental causes is due to fewer plane crashes among cosmonauts, fewer spacecraft accidents, as well as fewer deaths per spacecraft acci‑ dent (owing to the smaller, 3-person crews in Soviet spacecraft as compared to 7-person crews on United States space shuttles). The reduction in risk due to accidental causes was most pro‑ nounced in the 1960s and 1970s, periods of relatively many accidental deaths for both nations. The combined loss of 11 United States astronauts in space shuttles in 1986 and 2003 coincided with no Russian spacecraft accidents in the same period. From this perspective, we could rate the Soviet and Russian space programs as "safer" than the United States program.

The reason behind the greater rate of death by CVD and cancer among cosmonauts is unclear. The most obvious explanation would be lifestyle differences between the United States and Russia, as reflected in the greater mortality risk for these diseases in the general population death rates between Russia and the United States. This could be most salient after retirement from active duty of astronauts and cosmonauts, as the mortality rate from CVD and cancers begin to climb steeply after age 50, in both the United States and Russia [4, 26]. Differences in diet and greater alcohol consumption and tobacco use in Russia/Ukraine than in the United States may explain the risk dif‑ ferences between the groups, especially if those habits were consistent over a period of years [29].

Still another possibility, though less likely, is differing occupational exposures between the cohorts, particularly radiation dose while in space. We are aware of no published work to date that has examined the relationship between time in space or radiation dose and cos‑ monaut death rates from cancers or cardiovascular disease. However, given the similarity of the Soviet/Russian and the United States space programs over the years, and their explicit partnership over the last 17 years collaborating on the International Space Station, the overall dose per person-day in space in recent years is likely equivalent.

Finally, the quality of medical care could be an explanation for the differential mortality rate due to chronic diseases. Even among equal rates of incidence, differences in the accessibility or effectiveness of treatment for CVD and cancers could lead to a higher case-fatality rate in cosmonauts compared to astronauts. This could result in cosmonauts dying younger of the same diseases afflicting astronauts, driving up SMRs.

The balance of evidence accumulated to date regarding mortality of space explorers sug‑ gests that they are, overall, at less risk of death on an age- and gender-matched basis than the baseline risks in their respective countries of origin. As humans continue to explore space, and in particular as they engage in longer trips deeper into space, it may be inevitable that the unique exposures they will face will ultimately lead to some increased risk of mortality due to at least some particular causes (tragic accidents and radiation‑related cancers perhaps chief among them). However, it may be worth bearing in mind that as a profession, being an astronaut or cosmonaut is not a terribly risky business. Based on the research reviewed here, as well as the original research presented for the first time, the job of space explorer should not make any top 10 lists of the world's deadliest jobs.

### **Author details**

**6. Chapter summary**

282 Into Space - A Journey of How Humans Adapt and Live in Microgravity

In this chapter, we have examined the mortality of astronauts and cosmonauts in comparison with the general populations of their respective nations, with specially selected controls, and in comparison with one another. The results of prior research and the new analyses presented here indicate that both astronauts and cosmonauts have much lower rates of death by chronic disease (such as CVD and cancers) than do their respective general populations. However, the mortality rate from plane crashes and spacecraft accidents over the years has made both groups of space explorers more likely to die from external causes in general and accidental death in particular, than is expected in the general population. The net effect is that all-cause

Careful interpretation is needed for the reduced risk of chronic disease among astronauts and cosmonauts. Space agencies to date have intentionally tried to limit the potential harmful exposures from space radiation. This has included both projecting and measuring the lifetime dose of radiation for individual astronauts [28]. The evidence gathered thus far seems to indi‑ cate that few to no astronauts or cosmonauts have received detrimental doses of space radia‑ tion and that we will only begin to understand the mortality risks space radiation can bring

When comparing cosmonauts to astronauts, we see that astronauts tend to have a slightly higher risk of accidental death compared to cosmonauts, but a significant reduction in the risk of CVD and cancer. The net difference places cosmonauts at an overall greater all-cause

The lower death rate among cosmonauts from accidental causes is due to fewer plane crashes among cosmonauts, fewer spacecraft accidents, as well as fewer deaths per spacecraft acci‑ dent (owing to the smaller, 3-person crews in Soviet spacecraft as compared to 7-person crews on United States space shuttles). The reduction in risk due to accidental causes was most pro‑ nounced in the 1960s and 1970s, periods of relatively many accidental deaths for both nations. The combined loss of 11 United States astronauts in space shuttles in 1986 and 2003 coincided with no Russian spacecraft accidents in the same period. From this perspective, we could rate

The reason behind the greater rate of death by CVD and cancer among cosmonauts is unclear. The most obvious explanation would be lifestyle differences between the United States and Russia, as reflected in the greater mortality risk for these diseases in the general population death rates between Russia and the United States. This could be most salient after retirement from active duty of astronauts and cosmonauts, as the mortality rate from CVD and cancers begin to climb steeply after age 50, in both the United States and Russia [4, 26]. Differences in diet and greater alcohol consumption and tobacco use in Russia/Ukraine than in the United States may explain the risk dif‑ ferences between the groups, especially if those habits were consistent over a period of years [29]. Still another possibility, though less likely, is differing occupational exposures between the cohorts, particularly radiation dose while in space. We are aware of no published work to date that has examined the relationship between time in space or radiation dose and cos‑ monaut death rates from cancers or cardiovascular disease. However, given the similarity of the Soviet/Russian and the United States space programs over the years, and their explicit

the Soviet and Russian space programs as "safer" than the United States program.

mortality risk for space explorers is still lower than that of the general population.

once humans start performing longer missions, such as to Mars or beyond.

mortality risk than United States astronauts.

Robert J. Reynolds\* and Steven M. Day \*Address all correspondence to: rreynolds@mortalityresearch.com Mortality Research and Consulting, Inc., USA

### **References**


[5] Centers for Disease Control and Prevention, National Center for Health Statistics. Compressed Mortality File 1999-2015 on CDC WONDER Online Database, released December 2016. Data are from the Compressed Mortality File 1999-2015 Series 20 No. 2U, 2016, as compiled from data provided by the 57 vital statistics jurisdictions through the Vital Statistics Cooperative Program

[17] Reynolds RJ, Day SM. Mortality due to cardiovascular disease among Apollo lunar astronauts. Aerospace Medicine and Human Performance. 2017;**88**(5):492-496

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[18] Bacal K, Romano J. Radiation health and protection. In: Nicogossian AE, Williams RS, Huntoon CL, Doarn CR, Polk JD, Schneider VS, editors. Space Physiology and Medicine.

[19] Chancellor J, Scott G, Sutton J. Space radiation: The number one risk to astronaut health

[20] National Aeronautics and Space Administration. Risk of Cardiac Rhythm Problems During Spaceflight. Available from: https://humanresearchroadmap.nasa.gov/evidence/

[21] Reynolds RJ, Day SM, Nurgalieva ZZ. Mortality among Soviet and Russian cosmonauts: 1960-2013. Aviation, Space, and Environmental Medicine. 2014;**85**(7):750-754

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