Sergei Korolev[0], chief architect of the N1 rocket[1] which never really made it off the ground. I believe the show mentions him surviving heart surgery, and the show's creator points to this as the 'divergence point'. The rockets the Soviets use to land on the Moon in the show were N1.

[0]: https://en.wikipedia.org/wiki/Sergei_Korolev

[1]: https://en.wikipedia.org/wiki/N1_(rocket)