@MrSmith… I get that too… But they don’t have to be mutually exclusive, frankly the place to start there is stopping spending money on things that trash the world.
Thing is, space travel helps us to look up, which is exactly the sort of thing that encourages people to think bigger- whether that means mars, or this world, it’s easy to ignore global issues when you don’t think outside of your own local world. There’s also likely to be applicable tech, both predictable and unpredictable- the obvious stuff is in biosphere management, and really efficient resource use and reuse. None of that’s really stuff we couldn’t possibly do here, but there we couldn’t possibly not do it, motivation’s important.
I can’t remember who said it… Humans work best at the 11th hour, which is just as well because even when we see a crisis coming we tend not to deal with it til we absolutely have to. In space travel, you start on the 11th hour- you never have the choice of putting it off for generations like you can do in a big blue world, that’s always going to force results. Ironically living on a benevolent planet is what gives us the option of wrecking it.
The technical challenges are enormous – how do you shield the crew from radiation for example?
Why would you want to do that? Cosmic rays are awesome!