I believe space colonization is necessary as we continue to contaminate this planet and deplete it's resources. I also believe that space colonization will be strategically necessary in the future. If the United States does not succeed at it, another country will. I am sure there will be volunteers in every branch of science to make the venture successful and able to flourish.
It is definitely ethical; just like any land on the earth could be claimed (even up to today, as seen by the claims of the Arctic and Antarctic) the same can be said for planets elsewhere. Still, the technology isn't there yet, and the effects on humans would be harsh.
Yes, I think that if the technology is ever found so that we can start moving into outer space and live sustainably, then I see no ethical problems with taking advantages of new resources that can be found on another planet, so that we can maybe start saving ours here.
Colonizing space is unethical simply because there are homeless and hungry people in the United States that should be taken care of first. Humans can't live on Mars nor can we grow food on the Red Planet. Let's take care of humanity first before trying to see if Martians need our help.
Space colonization by the United States, or any country, is not ethical. The age of colonization is over and it should not be resumed. Space is a place reserved for scientific exploration and discovery. It should not be a new battleground for countries to fight over resources and political power.