On April 19, 1971, the Soviet Union launched the world's first space station, Salyut 1. This space station was a modified version of the Soviet Union's Almaz space station, which was part of a highly classified military program and was still under development at the time. After NASA managed to put astronauts on the moon, the Soviet Union decided that its next big feat in the Space Race would be to put a crewed space station in orbit. The first crew to visit Salyut 1 in orbit launched just four days after the space station did. However, that crew had some technical problems while trying to dock with the space station in their Soyuz spacecraft, so they went back home without ever actually entering the station. Another crew launched two months later, and after a successful docking, they spent 23 days aboard the station.