Today we take international cooperation pretty much for granted when it comes to spaceflight—last year an Israeli moon probe hitched a ride aboard an American SpaceX rocket, to name just one example, and of course the ISS continues to fly with the participation of eighteen different countries. This is the obvious way of doing things. Why invest in all the expensive infrastructure for a space program, when you can share the costs? But it wasn’t always so. Manned space travel remained the exclusive domain of the two superpowers for much of the Cold War, until 1978, when the Soviet Interkosmos program opened the door for even small countries to send people into space.

Interkosmos began in 1967 for unmanned flights, while the first non-Soviet cosmonaut, Vladimir Remek of Czechoslovakia, launched aboard Soyuz 28 in 1978. The program continued all the way until the demise of the Soviet Union, at which point it transitioned into the Shuttle-Mir program and ultimately the ISS. Most of the participating countries were fellow socialist nations—there were cosmonauts from East Germany, Cuba, Vietnam, and Mongolia, among many others—but Britain, France, Austria, and India also sent their people into space atop Soviet rockets, in a feat of international cooperation that put the Cold War on pause for just a moment.
Every Interkosmos mission visited one of the Soviet Union’s orbiting outposts. There were nine flights to Salyut 6, two to Salyut 7, and seven to Mir. Salyut 6 and 7 were single-module craft, the last of the monolithic space stations; Mir, however, was a technological leap forward, comprising a grand total of seven modules docked together, and its design paved the way for the International Space Station a few years later.

Most of the Interkosmos cosmonauts were involved in their home countries’ air forces, naturally enough. For instance, Arnaldo Tamayo Méndez, the first Cuban and first person of African descent to enter space, was a military pilot who flew MiG-15s, and the first Mongolian, Jügderdemidiin Gürragchaa, later became an air defense minister. For countries without dedicated astronaut programs, it made sense to pick the next best thing and send up pilots.
As for what the cosmonauts did, beyond simply being there for PR stunts, they conducted a variety of scientific experiments. Let’s look at Arnaldo Tamayo Méndez’s flight for an example of a typical Interkosmos mission:

Méndez took off from Baikonur Cosmodrome on September 18, 1980, aboard Soyuz 38. The craft docked with Salyut 6 shortly afterwards. While there, Méndez and his Soviet colleague, Yuri Romanenko, conducted experiments on space adaptation syndrome, trying to learn more about the causes of sickness in microgravity. To this end they wore “special adjustable shoes that placed a load on the arch of the foot for six hours a day”(Wikipedia); I’m not certain how that helps to investigate space sickness, but I can’t find any more sources about the experiment, so we’ll just take Wikipedia’s word for it. Méndez also studied the zero-g crystallization of sucrose, hoping to benefit the Cuban sugar industry.
After only a week in space, he and Romanenko returned to Earth, making a nighttime landing in Kazakhstan. For his efforts he was made a Hero of the Republic of Cuba, as well as a Hero of the Soviet Union.

Interkosmos cemented the Soviet Union’s relations with its allies, while also reaching across the Iron Curtain to promote peaceful relations with its adversaries. While no American astronauts flew on Interkosmos missions, the program did lead directly into Shuttle-Mir, and from there into the ISS—a true triumph of international cooperation. As such I believe it is an important part of space history, worth remembering.
Before I go, here’s some lovely Soviet cosmonaut music, which really evokes the optimism and spirit of friendship that characterized the Soviet space program. One of these days I’m going to dedicate an entire blog post to catchy communist space tunes.
Sources:
https://en.wikipedia.org/wiki/International_Space_Station#International_co-operation