 KSC's Shuttle Landing Facility (SLF) was the orbiters' primary end-of-mission landing site, although the first KSC landing did not take place until the tenth flight, when "Challenger" completed STS-41-B on February 11, 1984; the primary landing site until then was Edwards Air Force Base in California, subsequently used as a backup landing site.The SLF also provided a return-to-launch-site (RTLS) abort option, which was not utilized.The SLF is among the longest runways in the world.