During the 1950s and 1960s the USSR used dogs for sub-orbital and orbital space flights to determine whether human spaceflight was feasible.
In this period, the Soviet Union launched missions with passenger slots for at least 57 dogs. The number of dogs in space is smaller, as some dogs flew more than once. Most survived; the few that died were lost mostly through technical failures, according to the parameters of the test. A notable exception is Laika, the first dog to be sent into orbit, whose death was expected from the outset.
Dogs were the preferred animal for the experiments because scientists felt dogs were well suited to endure long periods of inactivity. As part of their training, they were confined in small boxes for 15–20 days at a time. Stray dogs, rather than animals accustomed to living in a house, were chosen because the scientists felt they would be able to tolerate the rigorous and extreme stresses of space flight better than other dogs. Female dogs were used because of their temperament and because the suit the dogs wore in order to collect urine and feces was equipped with a special device, designed to work only with females.
Their training included standing still for long periods of time, wearing space suits, being placed in simulators that acted like a rocket during launch, riding in centrifuges that simulated the high acceleration of a rocket launch and being kept in progressively smaller cages to prepare them for the confines of the space module. Dogs that flew in orbit were fed a nutritious jelly-like protein. This was highly fibrous, and assisted the dogs to excrete during long periods of time while in their small space module. More than 60% of dogs to enter space were reportedly suffering from constipation and gallstones on arrival back to base.