See Photos From the Historic Apollo-Soyuz Mission

This artist's concept illustrates the Apollo-type spacecraft about to dock with a Soviet Soyuz-type spacecraft (right).

President Richard Nixon and Soviet Premier Alexei Kosygin sign an agreement in May of 1972 that paved the way for the Apollo–Soyuz mission.

The U.S.-USSR crew for the 1975 Apollo Soyuz Test Project mission: astronaut Tom Stafford (standing on left), commander of the American crew; cosmonaut Alexei Leonov (standing on right), commander of the Soviet crew; astronaut Donald Slayton (seated on left), docking module pilot of the American crew; astronaut Vance Brand (seated in center), command module pilot of the American crew; and cosmonaut Valeriy Kubasov (seated on right), engineer on the Soviet crew.

The Soviet Soyuz launch vehicle leaves from the Baikonur Cosmodrome in Kazakhstan.

The Saturn IB launch vehicle lifts the American crew into orbit on July 15, 1975.

Activity inside Mission Control in Houston during the Apollo Soyuz Test Project docking mission.

American astronauts Tom Stafford and Deke Slayton hold tubes of vodka given to them by Russian cosmonauts during their historic rendezvous and linkup of Apollo and Soyuz spacecraft.

The American Apollo spacecraft as seen in Earth orbit from the Soviet Soyuz 19 spacecraft during the joint U.S.-USSR Apollo Soyuz Test Project (ASTP) mission.

The Soviet Soyuz spacecraft approaches the Apollo for linkup in space, the first rendezvous and docking of vessels from different nations.

Astronaut Tom Stafford (in foreground) and cosmonaut Alexei Leonov make their historic handshake in space on July 17, 1975.

President Gerald R. Ford watches crewmen Tom Stafford, Donald Slayton and Valeriy Kubasov on a television as he talks to them by phone while they orbited the Earth on July 18, 1975. The American Apollo spacecraft and Soviet Soyuz spacecraft were docked.

American astronaut Deke Slayton and Russian cosmonaut Alexei Leonov during the US-Soviet Apollo-Soyuz linkup.