recently. In Stirling's The Stone Dogs, as in James Blish's They Shall Have Stars, Earth becomes a dictatorship but a few political refugees escape from the Solar System.
In fact, various other discontented groups also leave the Solar System in:
Robert Heinlein's Methuselah's Children;
the Breakup period of Poul Anderson's Technic History;
Anderson's Rustum History, The Boat Of A Million Years and Harvest Of Stars;
the Great Exodus period of Jerry Pournelle's CoDominium History.
Stirling's The Peshawar Lancers is an alternative history novel set in 2025 but does not feature space travel because, like Anderson's Maurai History, it recounts recovery from a global disaster.
The Ransom Trilogy, CS Lewis' reply to Wells' and Stapledon's future histories, also addresses the history of the future. Issues concerning the future of mankind on Earth are resolved in Volume III. In Volume II, the future is prophesied. Ten thousand years hence, Maleldil, Malacandra, Tor-Oyarsa-Perelendri and many hnau and eldila will descend, destroy the Moon and liberate Thulcandra (Earth) from its present hidden rulers -
- and you cannot get any more alternative than that.