Here is an interesting site, a company called Planetary Resources that is planning private mining of asteriods for water and metals, with the ultimate intention of reviving the Earth's economy and making space travel a reality. Ambitious stuff, but exciting and worthwhile nonetheless.
This idea, if it came to fruition, could be the first real step towards minimising our impacts on environmental destruction on Earth, because ultimately asteriod mining will be much cheaper and less damaging to the planet than conventional mining is today. Also, the energy and water resources using in mining will similarly be lessened.
The Planetary Resources solution will not solve what I consider to be Earth's biggest problem, a population of 7 billion people. At a growth rate of approximately 215,000 people a day, space travel will never lessen over-population, we could never get that many people into space. We still need to solve problems at home, and that will ultimately have to come from lessening our need on travel (particularly airline travel), consumption of goods, food wasteage and a reliance on more recycling. Still, the asteriod mining solution, if or when it proves viable, will go a long way to helping.
To really make space travel worth while, we are going to need to build a Space Elevator. With costs ranging from $5000 to $20,000 per kilogram to get material into orbit with conventional rocket technology, a space elevator seems like the only viable option. Since space elevators need to be on the equator, countries like Eucador, Kenya, the Congo or Indoneisa could be very wealthy places in the future.
Perhaps viable space travel is not that far away.