As part of his research into the development of the Extremely LargeTelescope, Professor Gerry Gilmore has extrpolated air traffic control figures and deduced that ground based telescopes will be worthless and astronomy will be impossible by the year 2050 due to aircraft condensation (contrail) polution.
http://news.bbc.co.uk/1/hi/sci/tech/4755996.stm
http://www.makezine.com/blog/archive...s_by_2050.html
Maybe by then the discount air fares that are causing this will apply to space travel and it will be cheap enough to get above the atmosphere for far better views than we currently get from earth.