Welcome to the forum. As I see it, there is no "sustainable" no-impact trip to Antarctica. No one needs
to go there, and any travel produces pollution from burning fuel to get there.
However cruises in Antarctica have the most stringent controls of any in the world and low-impact is an important factor for the directors of our tour partners Expedition Trips
It ends up being a compromise, personally I think people should go - as a proportion at least come back as ambassadors for Antarctica and so they make it a place that people care about and a place that people see what is going on all the time.
As for eco-tourism - well maybe I'm just an ascetic, but the only way to not damage the environment even just a tiny bit is to not go anywhere at all - and certainly don't set foot on a jet.
Camping in the garden anyone?