There are so many other modes of transport than air travel. The amount of CO2 released into the atmosphere is just ridiculous. The development of trains is coming on really well, and they use not even nearly as much fuel as air travel.
Also, the amount of crucial fuel that a rocket need to take off and for the whole journey is atrocious.
The planet may be fragile in a sense, but it is certainly not as fragile as air travel. Manned air travel actually allows us to reduce the burden on the land which is as more a part of the planet than the air.Sure we have the ozone and the atmosphere and air quality to look at, but in the end, water pollution and emissions from vehicles and trains are just the same. Not to mention the vast difference in infrastructure necessary for land travel as opposed to air. The world would probably be better if we all traveled by manned air craft, in place of land transportation.