The STS-135 patch represents the space shuttle Atlantis embarking on its mission to resupply the International Space Station. Atlantis is centered over elements of the NASA emblem depicting how the space shuttle has been at the heart of NASA for the last 30 years. It also pays tribute to the entire NASA and contractor team that made possible all the incredible accomplishments of the space shuttle. Omega, the last letter in the Greek alphabet, recognizes this mission as the last flight of the Space Shuttle Program.
http://spaceflight.nasa.gov/gallery/images/shuttle/sts-135/html/sts135-s-001.html
http://commons.wikimedia.org/w/index.php?title=File:ISS_Utilization_Guide_2015.pdf&amp;page=99
NASA