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Other than propulsion that allows for spacecraft to cover vast distances in a practical amount of time, the greatest challenge for human space flight is creating and designing spacecraft that can support life over a significant period of time in the hostile environment of outer space. If you think about it, space is as deadly and antithetical to life as it can be. So scientists and researchers need to better understand what happens to the human body when it is exposed over time to the harsh conditions in space. In the early days of space exploration it was simply too dangerous to go straight to human testing. There were also the ethical and political implications to think about. That is why animals such as monkeys in space were used to test early space craft.
So why use monkeys? Biologically monkeys share common biological traits with human beings due their genetic makeup. This means that by sending monkeys into space it would be the next best thing to actually sending human beings there to test the scientific equipment and spacecraft. There is another added benefit because monkeys, on average, also have a higher intelligence than other animals which allows them to perform more complex tasks. The simulated tasks set up as part of these missions are similar to those needed to be done by actual astronauts in human missions. Each of these reasons made monkeys the test animal of choice for emerging space programs as they became more complex.
Animal testing of spacecraft had been going on in one form of another since the advent of national space programs. They started with simpler organisms such as insects and graduated to more complex animals. The earliest tests were in the 1940s. However it would not be until the 1960s that monkeys would be regularly used in missions as passengers for spacecraft. Like most early tests, many test subjects did not survive reentry. This was because the early models did not have advanced safety systems or climate control. Scientist literally learned by correction of mistakes and errors. This ended with the improvement of landing systems and climate systems. Successful animal testing with monkeys in space led to human milestones such as the first American astronaut in space and the Apollo program.
The use of animal testing in science has always been controversial because of its political connotations. However the advances in space craft design and safety make testing safer than ever for animals as well as humans. The new experiments and missions now focus on the long term effects of staying for long periods of time in space. Recently the Soviet Union has been the only program using monkeys in experiments. However this may change as space programs are now gearing towards developing technologies deep space human exploration.
Astronomy Cast has an episode on spacesuits.