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A City on Mars: Reality kills space settlement dreams
(arstechnica.com)
This is a most excellent place for technology news and articles.
If we're talking about space colonization then "at least for now" doesn't apply any more.
There are vast quantities of oxygen available everywhere in the solar system. Extracting it is really not hard. There's a technology demonstrator generating oxygen on Mars right now. If you're arguing against space colonization because you're assuming that every bit of resources the space colony uses will have to be sent there from Earth, you're completely missing the basic concept of space colonization.