>>63812088
>>63812421
Pre-positioned orbital batteries may be the best bet, as they are at least "possible" with tech available today, albeit with some caveats such as creating SSTO spacecraft or further perfecting reusable tech. We may need something that is purely hypothetical, a space elevator, to truly take on an alien threat within a ~100 year warning window. Without warning, defending Earth with anything approaching 'conventional' (*nukes included*) would be practically impossible.

Think gaylo 2's Cairo station, but with less 'comfortable' living space so it would be more like a pencil than a big sword (crew would inhabit spaces similar to ISS), realistically most/all of the systems directly firing the gun would, could, and should be automated.

We would very likely lose, specially with alien overmatch (they could just glass us from orbit, or terraform the planet to be uninhabitable for us, but pleasant for them), especially if we lack any orbital defensive fleet (not just orbital batteries).