Although larger ships would cause more environmental damage and create an enormous rescue challenge in case of an accident, they also offer greater comfort and more amenities.
Space Shuttle Endeavour was staged for a possible rescue mission in case Space Shuttle Atlantis gets into trouble while the crew is fixing the Hubble Telescope.
Because Atlantis will not be able to reach the space station if it gets into trouble, or is damaged, on its Hubble flight, the Endeavour orbiter will be made ready on the pad for a rescue mission in case it is needed.
