On 18 March 1965, Alexei Leonov left Voskhod 2 for the first spacewalk. Leaving a spacecraft introduced a new operational problem: a suit had to function as a small life-support system while the astronaut worked outside. It earns a place because extravehicular activity later enabled tasks such as assembly and maintenance in orbit. The first demonstration also exposed difficulties, including re-entry into the airlock, rather than proving the operation routine.
Source: 1965: Alexei Leonov’s first spacewalk.
Image credit: Sergeev L.V. · CC BY-SA 3.0. Resized and JPEG compressed.
