that time in history. Had plans to launch United States Department of Defense payloads from Vandenberg Air Force Base gone ahead, Discovery would have become the dedicated US Air Force shuttle. Its first West Coast mission, STS-62-A, was scheduled for 1986, but canceled in the aftermath of the Challenger disaster. Discovery was retired after completing its final mission, STS-133 on March 9, 2011. The spacecraft is now on display in Virginia at the Steven F. Udvar-Hazy Center, an annex of the Smithsonian Institution's National Air and Space Museum. Construction milestones Features and upgrades During its construction, Discovery was fitted with several black tiles near the middle starboard window where there should have been white tiles. It is unknown if this was the result of a harmless manufacturing mishap or done intentionally to give a distinctive look to the shuttle. This feature has been called 'teardrop' and allowed Discovery to be told apart from the rest of the fleet without looking at its name, although often unnoticed by the uninitiated. The spacecraft weighed roughly less than Columbia when it was brought into service due to optimalizations determined during the construction and testing of Enterprise, Columbia and Challenger. Discovery weighs heavier than Atlantis and heavier than Endeavour after further weight-saving adjustments were made. Part of the Discovery weight optimizations included the greater use of quilted AFRSI blankets rather than the white LRSI tiles on the fuselage, and the use of graphite epoxy instead of aluminum for the payload bay doors and some of the wing spars and beams. Upon its delivery to the Kennedy Space Center in 1983, Discovery was modified alongside Challenger to accommodate the liquid-fueled Centaur-G booster, which had been planned for use beginning in 1986 but was cancelled in the wake of the Challenger disaster. Beginning in late 1995, the orbiter underwent a nine-month Orbiter Maintenance Down Period (OMDP) in Palmdale, California. This included outfitting the vehicle with a 5th set of cryogenic tanks and an external airlock to support missions to the International Space Station. As with all the orbiters, it could be attached to the top of specialized aircraft and did so in June 1996 when it returned to the Kennedy Space Center, and later in April 2012 when sent to the Udvar-Hazy Center, riding piggy-back on a modified Boeing 747. After STS-105, Discovery became the first of the orbiter fleet to undergo Orbiter Major Modification (OMM)