[elurg]

A lot of stuff is known about it. Both the US and USSR first operated long-duration space stations nearly half a century ago.

Gravity and radiation are well understood and we can definitely build machinery that can tolerate them well for many years.

> we have never entirely isolated a group of humans from the biosphere for even a year, never mind indefinitely. The times it was tried, it failed.

Why doesn't the ISS count? It gets resupplied every few months but a Mars base would be resupplied every two years so systems only have to scale to last 10x as long.