 In 1959, the first Bulgarian book on human spaceflight, "The Human Organism and Interplanetary Flight", was published.The intensity of the Space Race increased further after Yuri Gagarin became the first human in space.In 1964, Bulgarian Air Force commander-in-chief Lt. Gen. Zahari Zahariev discussed with Soviet defence minister Rodion Malinovsky the possibility to send four Bulgarian pilots, the Stamenkov brothers, in space.