I'm a big space nerd, so I'd like to make a couple points:
Research into space travel and colonization has direct benefits for life on Earth. The research that goes into building a rocket has applications for the military and aerospace industries. It's a boon for higher education, and thus the towns around them. University towns have many of the benefits of a large city, without the detriments, due to their unique demographics. As for actually colonizing a planet, the benefits for life on Earth are huge. Case in point: if you can survive on Mars, where the atmosphere is lethal, with the incredibly minimal supplies you receive from Earth, you can easily survive on a degraded Earth, where the atmosphere has sublethal effects, resources are diffuse but plentiful, and human capital is massive. The industrial effects are similar. If you can build a biodome and get it to Mars, which would most likely involve some amazing feats of modulation and/or 3D printing, you can absolutely build it on Earth for cheap. More importantly, once factories are up and running for those biodomes, or water filters, or whatever their role is, they can take advantage of economies of scale, so they'll have an incentive to sell those items to more than just a space agency. Space travel is one of the few areas where international collaboration isn't just encouraged, but necessary. The United States and Russia can never fully cut communications, because they both need the other's expertise and infrastructure. Maintaining that relationship obviously isn't either of those countries sole priority, but it still gives them another reason to keep a positive relationship. Research into space also has indirect bonuses. An algorithm to improve images taken by the Hubble Space telescope is also effective at identifying early breast cancer. It's impossible to predict what new discoveries an attempt to colonize Mars may lead to. One final point, people tend to shut down space research without offering any kind of alternatives. The truth is, space research benefits far outweigh the costs, but they're so widespread that it really takes some research to realize it. Saving our own planet is great (and encouraged!), but the best way to do that is by trying somewhere else. It's kind of like going on dates. Some people wait to go on dates until they meet the perfect person, then they find that person and realize they can't get a date with that person because they don't have any practice going on dates.