The process and factors that contribute to how NASA chooses its astronauts are public knowledge. Qualifications include U.S. citizenship, at least a bachelor’s degree in the fields of engineering, science, or mathematics, and of course a certain level of physical ability.

But other demographic factors — and perhaps biases — that could be playing into NASA’s choices are far more mysterious, such as gender, race, and age. A new study, “Analysis of age as a factor in NASA astronaut selection and career landmarks,” published this month the in the journal PLoS ONE, by Gregory T. A. Kovacs of Stanford University and Mark Shadden of Elite Research has shed some light on at least one trend among successful applicants: they all seem to be around 40 years old.

"Deep Space Homer"
Our favorite fictional 40-year-old NASA astronaut.

The researchers used data obtained through the Freedom of Information Act about NASA astronaut applicants in 2009 and 2013. Using “a series of multinomial and logistic regressions,” the data was then analyzed to determine whether age affected the likelihood of acceptance into the program. The researches also combed through publicly available data on every astronaut selected from 1959 to 2013 to record what age they were selected at, as well as their age during other career milestones.

“To the best of our knowledge, this is the first effort to investigate this opaque selection process for bias,” the authors write.

While the pool of applicants to enter the final frontier starts out pretty big (3,566 in 2009), the group that makes it through each interview round to be eventually selected for the space program is quite small (eight in the same year).

It’s tough competition: an illustration of 2009 selection data that was obtained  via FOIA request shows how many people actually make it into the space program.

Based on the research, the probability of receiving an interview invitation and the probability of finally being selected into the astronaut program appears to increase as applicants approach their late thirties. After 40, these favorable odds begin to decline.

What’s more, the average age of selectees has been steadily climbing since 1959, reaching an average age of 36.6 years old by 2013. Despite this, the average retirement age has remained close to the same: approximately 48 years old. It should be noted that there is no official age cap for applying to be an astronaut.

“Despite a valid applicant age range typically spanning nearly five decades, selectees over many selections (1959 to 2013) have ended up within a very narrow age range,” the study notes.

And apologies to Gen-Xer’s, the findings also imply that there’s a very narrow chance of being accepted to astronaut school if you’ve already passed the average retirement age of 48.

Another interesting finding within the data: applicants with prior military service had were eight to ten times more likely to be selected than those without.

Abstract

NASA’s periodic selection of astronauts is a highly selective process accepting applications from the general population, wherein the mechanics of selection are not made public. This research was an effort to determine if biases (specifically age) exist in the process and, if so, at which points they might manifest. Two sets of analyses were conducted. The first utilized data requested via the Freedom of Information Act (FOIA) on NASA astronaut applicants for the 2009 and 2013 selection years. Using a series of multinomial and logistic regressions, the data were analyzed to uncover whether age of the applicants linearly or nonlinearly affected their likelihood of receiving an invitation, as well as their likelihood of being selected into the astronaut program. The second used public data on age at selection and age at other career milestones for every astronaut selected from 1959 to 2013 to analyze trends in age over time using ordinary least-squares (OLS) regression and Pearson’s correlation. The results for the FOIA data revealed a nonlinear relationship between age and receiving an interview, as well as age and selection into the astronaut program, but the most striking observation was the loss of age diversity at each stage of selection. Applicants younger or older than approximately 40 years were significantly less likely to receive invitations for interviews and were significantly less likely to be selected as an astronaut. Analysis of the public-source data for all selections since the beginning of the astronaut program revealed significant age trends over time including a gradual increase in selectee age and decreased tenure at NASA after last flight, with average age at retirement steady over the entire history of the astronaut program at approximately 48 years.

Must Read: The 10 Worst Astronauts of All Time


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