It is no where near the limits of our reach. We already have people living in space year round without any local resources. What we do seem to lack however is any faith in ourselves.
If you want to re-supply people on Mars you would have to escape Earth's gravity well completely, which is crazy expensive.
>Estimates of the return on investment in the spaaace program range from $7 for every $1 spent on the Apollo Program to $40 for every $1 spent on spaaace development today. [1]
That money is generally spent in the US, on good, high-paying jobs.[2] The products NASA has created are EVERYWHERE[3] — The general rule of thumb is if it's wireless, fireproof, or small, it's using technology based off NASA work.
1: http://www.nss.org/settlement/nasa/spaceresvol4/newspace3.ht... 2: http://www.huffingtonpost.com/lauren-lyons/misconceptions-na... 3: http://www.investopedia.com/financial-edge/0812/the-roi-of-s...
And not have any dividends at all in terms of sustainable environmental cycles....
But those don't have much to do with creating sustainable ecosystems (farming, direct atmospheric management, etc). They help, to be sure! But most of the innovations in robotics, batteries and solar panels aren't coming out of the space program these days, AFAIK.
The technology involved in going to mars will significantly advance (and cheapen) several areas that would otherwise be ignored. Currently we see SpaceX developing cost-saving technology such as landing rocket boosters and ramping up mass production.
In order to live on mars we would need specialized farming equipment, chemistry equipment, robots, interstellar transportation and communication, knowledge on how to terraform at a massive scale and so on. It would be a boon to numerous commercial interests and likely pay for itself and then some.
You're joking, right? The ISS gets everything from Earth, even its water, and they're so bad at self-sustainability that they are not able to wash their own dirty clothes (I shit you not). Oh, and they throw away their feces and only recently were able to grow some lettuce on board. That's the state of the art right now.