According to NASA, the Space Shuttle Main Engine design, three of which powered Discovery, is "the most advanced liquid-fuelled rocket engine ever built". The fuel used by these engines is super-cold liquid hydrogen, kept at a temperature of -253C, which NASA reports is "the second coldest liquid on Earth". Inside the engines, this hydrogen fuel is combusted with liquid oxygen in a reaction that reaches temperatures of up to 3136C, "hotter than the boiling point of iron". This creates a high-speed stream of gas which ultimately generates the thrust necessary for launching the shuttle. Professor George Fraser, director of Leicester University's Space Research Centre says this exhaust gas, made from the combination of hydrogen and oxygen, consists of water vapour and as such does not harm the atmosphere, making the use of Nasa's main shuttle engines fairly environmentally safe. --SOURCE: http://news.bbc.co.uk/2/hi/uk_news/magazine/4130980.stm

Although Shuttle main engines are relatively clean burning, Solid rocket motor (SRM) exhaust contains chlorine, an important stratospheric constituent that plays a crucial role in the chemistry of ozone. Models suggest that a significant fraction of SRM exhaust chlorine might be in an active form as a result of afterburning reactions and be available for ozone destruction as the exhaust mixes with the ambient stratosphere. If afterburning does produce free chlorine, the SRMs used by the Space Shuttle have the potential to cause nearly complete ozone loss in regions up to several tens of kilometers in radius, depending on altitude.

The hydrogen gas and oxygen gas both have to be produced and this process uses more energy than what the resulting gasses themselves possess--basic 2nd Law of Thermodynamics. That means you have to burn coal or natural gas etc to make both of these gases, which means the total energy stream involved in powering the launch is not "clean" but releases literally tons of greenhouse gases. Of course the solid boosters are very dirty as well. Since hydrogen can be made either by striping H's off of other hydrocarbons or by electrolysis which would be powered by mostly coal or natural gas in the US, fueling the shuttle uses tons of fossil fuels at a time when we are suppose to be trying to conserve. We protend that the Shuttle is doing "cutting edge" research, but they're not developing warp drives on the international space station. And the notion that is put forward that we're going to colonize space is just ridiculous when you consider how much fuel is used to put 6 astronauts into space with maybe a few hundred pounds of extra equipment. Now imagine taking basic building supplies and equipment to build a house. The only way you could imagine it being possible would be dropping off a few dozen people who would have to revert to stone-age living and rebuild to current technology from existing natural resources. If you think about the colonization of the US, how many years ship loads of supplies had to be brought from Europe to keep us going during the early years. It sounds very sexy and romantic on movies, but we will destroy life on this planet and still have nothing to show for it except maybe some nice wallpaper for our PC desktops. We need to wise up and start using our vital resources for better purposes.

not as much as is generated by all the cars driving in the US in one day. factor in only about 4 launches a year, and the shuttle is a pretty clean vehicle. it does use a lot of fuel, but the main by-product is water vapor. it does put off some pretty nasty stuff, but NASA is pretty environmentally conscious. the space center is actually located on a wildlife refuge.