Read this this morning and thought I would append it to the initial question:

"NASA cut the crew size to just four instead of the typical six or seven to accommodate the smaller Russian Soyuz capsules that would be used as rescue vehicles if Atlantis were too damaged to safely return to Earth. Since the 2003 Columbia accident, shuttle crews have had a second shuttle on standby to mount a rescue mission if needed. But there is no backup shuttle for Atlantis, which is closing out the 30-year-old program with NASA’s 135th and final flight."