How a habitable mirror planet could accelerate spaceflight

If a habitable Earth-like planet orbited on the far side of the Sun, when might humans have reached it?

The 1960s or 1970s were presented as plausible under intense competition, although the orbital premise was acknowledged as unstable.

An Earth-like planet with a breathable atmosphere would offer a far more compelling destination than the Moon. Such a world could accelerate spaceflight because states would compete for habitable territory without waiting for terraforming. Depending on the effort governments committed, a landing might have been attempted in the 1960s or 1970s, and authoritarian regimes could treat off-world expansion as an ideological prize. The premise itself is acknowledged as dynamically unstable, so this remains a political and technological counterfactual.