of TAL sites changed over time because of geopolitical factors. The exact sites were determined from launch to launch depending on orbital inclination. Preparations of TAL sites took four to five days and began one week before launch, with the majority of personnel from NASA, the Department of Defense and contractors arriving 48 hours before launch. Additionally, two C-130 aircraft from the space flight support office from the adjacent Patrick Space Force Base (then known as Patrick Air Force Base) would deliver eight crew members, nine pararescuers, two flight surgeons, a nurse and medical technician, and of medical equipment to Zaragoza, Istres, or both. One or more C-21S or C-12S aircraft would also be deployed to provide weather reconnaissance in the event of an abort with a TALCOM, or astronaut flight controller aboard for communications with the shuttle pilot and commander. This abort mode was never needed during the entire history of the Space Shuttle program. Abort once around An abort once around (AOA) was available if the shuttle was unable to reach a stable orbit but had sufficient velocity to circle Earth once and land at around 90 minutes after liftoff. Around five minutes after liftoff, the shuttle reaches a velocity and altitude sufficient for a single orbit around Earth. The orbiter would then proceed into re-entry; NASA could choose to have the orbiter land at Edwards Air Force Base, White Sands Space Harbor, or Kennedy Space Center. The time window for using the AOA abort was very short, just a few seconds between the TAL and ATO abort opportunities. Therefore, taking this option because of a technical malfunction (such as an engine failure) was very unlikely, although a medical emergency on board could have necessitated an AOA abort. This abort mode was never needed during the entire history of the Space Shuttle program. Abort to orbit An abort to orbit (ATO) was available when the intended orbit could not be reached but a lower stable orbit above above Earth's surface was possible. This occurred during mission STS-51-F, when Challenger's center engine failed five minutes and 46 seconds after liftoff. An orbit near the craft's planned orbit was established, and the mission continued despite the abort to a lower orbit. The Mission Control Center at Johnson Space Center observed an SSME failure and called "Challenger-Houston, abort ATO." The engine failure was later determined to be an inadvertent engine shutdown