Soyuz 4 was a dual mission with Soyuz 5 that conducted the first-ever docking of two crew vehicles together in orbit and the first time crew transferred from one craft to another. Soyuz 4 launched with a single person and three seats. After Soyuz 5 launched the next day, the two craft docked in orbit. It marked the first docking for the Soviet Union and the first-ever link up of two crew vehicles in orbit. Afterward, two crewmembers from Soyuz 5 spacewalked over to Soyuz 4. The crew transfer proved a cosmonaut could spacewalk between two vehicles, a vital part of the Soviet Moon landing plan. Soyuz 4 became the first human spaceflight to return to Earth with more people than when it launched.