When I think about alternate history, I follow some self-imposed rules. First, the Point of Divergence (POD) has to be plausible. How one defines “plausible” is a matter for debate; the two examples I have given above seem plausible to me.
Second, the POD has to be “of this universe.” By that I mean, one should not invoke features of the physical universe that did not/could not exist at the time of the POD, such as a dense martian atmosphere, a subterranean lunar civilization, or a space-enthused President Richard M. Nixon.
Finally, small changes are preferred over large ones. The examples I give in my second paragraph above are, I contend, representative of small changes; the proverbial flap of a butterfly’s wings. Large changes would include a Soviet victory in the moon race or a 1969 decision to land Americans on Mars by 1986. These both constitute detours from real history too large to have occurred without many preceding PODs.