it to the vertical orientation and mate it to the external tank. Once the entire stack was assembled, the MLP was carried for to Launch Complex 39 by one of the crawler-transporters. After the Space Shuttle arrived at one of the two launchpads, it would connect to the Fixed and Rotation Service Structures, which provided servicing capabilities, payload insertion, and crew transportation. The crew was transported to the launch pad at T−3 hours and entered the orbiter vehicle, which was closed at T−2 hours. Liquid oxygen and hydrogen were loaded into the external tank via umbilicals that attached to the orbiter vehicle, which began at T−5 hours 35 minutes. At T−3 hours 45 minutes, the hydrogen fast-fill was complete, followed 15 minutes later by the oxygen tank fill. Both tanks were slowly filled up until the launch as the oxygen and hydrogen evaporated. The launch commit criteria considered precipitation, temperatures, cloud cover, lightning forecast, wind, and humidity. The Space Shuttle was not launched under conditions where it could have been struck by lightning, as its exhaust plume could have triggered lightning by providing a current path to ground after launch, which occurred on Apollo 12. The NASA Anvil Rule for a Shuttle launch stated that an anvil cloud could not appear within a distance of . The Shuttle Launch Weather Officer monitored conditions until the final decision to scrub a launch was announced. In addition to the weather at the launch site, conditions had to be acceptable at one of the Transatlantic Abort Landing sites and the SRB recovery area. Launch The mission crew and the Launch Control Center (LCC) personnel completed systems checks throughout the countdown. Two built-in holds at T−20 minutes and T−9 minutes provided scheduled breaks to address any issues and additional preparation. After the built-in hold at T−9 minutes, the countdown was automatically controlled by the Ground Launch Sequencer (GLS) at the LCC, which stopped the countdown if it sensed a critical problem with any of the Space Shuttle's onboard systems. At T−3 minutes 45 seconds, the engines began conducting gimbal tests, which were concluded at T−2 minutes 15 seconds. The ground launch processing system handed off the control to the orbiter vehicle's GPCs at T−31 seconds. At T−16 seconds, the GPCs armed the SRBs, the sound suppression system (SPS) began to drench the MLP and SRB trenches with of water to protect the orbiter vehicle