During aerial reconnaissance missions, the SR-71 operated at high speeds and altitudes (Mach 3.2 and ) to allow it to outrace threats. If a surface-to-air missile launch was detected, the standard evasive action was simply to accelerate and outfly the missile. On average, each SR-71 could fly once per week due to the extended turnaround required after mission recovery. A total of 32 aircraft were built; 12 were lost in accidents with none lost to enemy action. During 1988, the USAF retired the SR-71 largely due to political reasons; several were briefly reactivated during the 1990s before their second retirement in 1998. NASA was the final operator of the type, retiring their examples in 1999. Since its retirement, the SR-71's role has been taken up by a combination of reconnaissance satellites and unmanned aerial vehicles (UAVs); a proposed UAV successor, the SR-72 is under development by Lockheed Martin, and scheduled to fly in 2025. The SR-71 has been given several nicknames, including "Blackbird" and "Habu". the SR-71 continues to hold the world record it set in 1976 for the fastest air-breathing manned aircraft, previously held by the related Lockheed YF-12.