During the 1950s and 1960s the Soviet space program 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 animal to be sent into orbit, whose death during the 3 November 1957 Sputnik 2 mission was expected from its outset. Training 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 high in fiber and assisted the dogs to defecate 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. Sub-orbital flights Dogs were flown to an altitude of on board 15 scientific flights on R-1 rockets (itself a copy of the German V-2) from 1951 to 1956. The dogs wore pressure suits with acrylic glass bubble helmets. From 1957 to 1960, 11 flights with dogs were made on the R-2A series (developed from the R-1 missile, and once again, itself a copy of the German V-2) which flew to about . Three flights were made to an altitude of about on R-5A rockets