Things weren’t going badly for the Soviet Union in 1975. They and the United States had decided to fly a Soyuz and Apollo to rendezvous in July. Their Salyut 4 space station, launched in late 1974, was doing fine and ready to accept a new crew after hosting one for a month long stay back in January 1975. And neither their government nor the US government was talking about blowing each up, which pleased everyone else worldwide as well.
But while the United States was closing their Apollo program in favor of their new Space Shuttle project (the Apollo-Soyuz Test Project would, in fact, fly the very last Saturn rocket and the very last Apollo spacecraft), the Soviets had no plans to take it easy.
So the next Salyut 4 crew, Oleg Makarov and Vasili Lazarev, launched on a Soyuz 7-TK spacecraft, a pretty stripped-down thing with just enough batteries for station docking and re-entry, on April 5, 1975, on what was supposed to be known as Soyuz 18.
All looked good until the second stage decided it really loved sticking around for the mission. Not all the connectors between the second and third stages released when the third stage engine ignited, blowing the clinging second stage away, but also causing the whole rocket to careen off-course, somewhat downward to earth.
The Soyuz spacecraft, still attached to the third stage, gyrated wildly from the stress of the disconnection. Finally the Soyuz’s automatic abort system separated the spacecraft from the third stage and then jettisoned the orbital and equipment modules for an emergency re-entry. The launch escape tower was already jettisoned.
It’s bad enough to have to abort. But now Makarov and Lazarev’s module came in very steeply, suffering an organ-crushing 20Gs of deceleration.
And things weren’t yet bad enough. The module landed on a snow-covered slope and began to roll. Their roll was stopped when the module’s parachutes snagged on some brush–moments before the module would’ve fallen over a sheer 500-ft cliff.
Lazarev, seriously injured by the high-Gs, and Makarov didn’t know where they landed. Fearing they landed in China–a country whose relations with the Soviets at that time were anything but nice–they started burning sensitive documents related to planned military-related experiments, donned their cold-weather gear, and set out to find someone friendly.
Thankfully, the crew did land close to the outskirts of Soviet territory, perhaps near a small town, or maybe somewhere near Mongolia. They and their spacecraft were soon rescued.
The reason why all the details of this aborted mission aren’t clear is that the Soviet Union were not used to telling or admitting failures to other countries. The only reason that the United States learned about the aborted flight was because they requested information they felt was vital in terms of keeping their Apollo ASTP crew safe.
The mission that came to be known later as Soyuz 18-1 or 18a was the first in-flight abort of any manned spacecraft. Years later, another Soyuz would barely escape from a burning rocket while on the pad–but that is another tale.
Andrew LaPage has more on the thankfully-short history of launch aborts in this 2014 article.