Pollution and Fuel Consumption:
http://www.madsci.org/posts/archives/2000-10/973014746.Es.r.html
Deuterium's Potential As An Alternative Energy:
http://www.springerlink.com/content/w01m77336t108151/
You can find an abundant supply of this on the moon. Space travel will get us to the moon.
Also, use of He-3 from the moon is another, less dangerous energy source. (Deuterium-tritium fusion releases high energy neutrons which will easily damage a containment vessel).
Currently, space travel uses quite a bit of fuel, polluting the environment, but it can bring us better sources of energy back from the moon.