After the "Challenger" disaster, NASA returned to using a sequential numbering system, with the number counting from the beginning of the STS program. Unlike the initial system, however, the numbers were assigned based on the initial mission schedule, and did not always reflect actual launch order. This numbering scheme started at 26, with the first flight as STS-26R—the R suffix stood for "reflight" to disambiguate from prior missions. The suffix was used for two years through STS-33R, then the R was dropped. As a result of the changes in systems, flights under different numbering systems could have the same number with one having a letter appended, e.g. flight STS-51 (a mission carried out by "Discovery" in 1993) was many years after STS-51-A ("Discovery's" second flight in 1984). It wasn't until STS-127 in 2009 where the flight numbering system returned to a standard and consistent order.