The competition began on August 2, 1955, when the Soviet Union responded to the US announcement four days earlier of intent to launch artificial satellites during the "International Geophysical Year" (July 1, 1957 to December 31, 1958), by declaring they would also launch a satellite "in the near future". The Soviet Union achieved the first successful artificial satellite launch on October 4, 1957 of Sputnik 1, and sent the first human to space with the orbital flight of Yuri Gagarin on April 12, 1961. The USSR demonstrated an early lead in the race with these and other early firsts over the next few years, including flight durations measured in days instead of hours, the first multi-person crewed spaceflight, and the first spacewalk.