candidate: Cosmonaut Valery Ryumin , wrote in his journal during a particularly difficult period onboard the Salyut 6 space station : “ All the conditions necessary for murder are met if you shut two men in a cabin measuring 18 feet by 20 and leave them together for two months . ” NASA 's interest in psychological stress caused by space travel , initially studied when their manned missions began , was rekindled when astronauts joined cosmonauts on the Russian space station Mir .