Hi-Seas is not closed loop. You're thinking of something more like Biosphere 2[1] Which had it's flaws.

We've never done this on any significant scale with success.

[1]https://en.wikipedia.org/wiki/Biosphere_2

A Mars colony doesn't have to be closed loop either. If you're bringing a large power source, you can make oxygen and fix carbon from the martian atmosphere. This is already part of the plan for the purposes of making fuel and oxidizer for the return trip.

Think of it more like a nuclear submarine with a greenhouse full of potatoes attached -- we can most likely do that.

Nobody's going to try to launch an entire mangrove forest on the first go. Biosphere2 is a failure of experimental design, not a failed experiment.

>Think of it more like a nuclear submarine with a greenhouse full of potatoes attached -

Maybe, but if we want to make Mars economically self sustaining the correct analogy is nuclear submarine that never surfaces and can totally self repair, including building new nuclear reactors and eventually replace all parts in itself.

They are not experimenting with sustainable biosphere. They eat prepackaged foods and are not completely sealed from their environment. They just simulate trip to Mars psychologically and test foods.

There was Biosphere 2 experiment few decades ago, but they could not keep the biosphere stable.

I'm skeptical of how effective a simulation of a trip to Mars would be.

There's just a huge psychological difference between knowing that you're in space, millions of miles from Earth or any kind of help and knowing you're in a simulation on Earth.

I suppose it's possible to fool people that they're in the former while actually being in the latter. But the ethical issues surrounding such an experiment would make it out of bounds for NASA. It might make for a fun scifi story, though.

Hi-seas isn't self-sustaining in food, water, or atmosphere.