Giant ball of water / human aquarium. It will be ice on the outside like Europa, and a nuclear core heating the whole thing (also possibly like Europa). There’s

Giant ball of water / human aquarium. It will be ice on the outside like Europa, and a nuclear core heating the whole thing (also possibly like Europa). There’s no other practical way. This will be obvious once the majority of humans are permanently living beneath the surface of the ocean. Signed, former NASA guy, unpopular with the spacecraft engineering crowd P.S. Yes, I know; Simpsons did it.