After a six-year hiatus, American space flights with astronauts resumed with the launch of the space shuttle Columbia in April 1981. The shuttle program progressed smoothly from there, with three more orbiters entering service in 1983–1985. But that all came to an end with the tragic loss of the "Challenger" (STS-51-L) on January 28, 1986, taking with it seven astronauts, including Christa McAuliffe, who was to have been the first teacher in space. In full view of the world, a faulty O-ring on the right solid rocket booster allowed hot gases to burn through the external fuel tank and cause it to explode, destroying the shuttle in the process. Extensive efforts were made to improve NASA's increasingly careless management practices, and to make the shuttle safer. Flights resumed with the launch of "Discovery" in September 1988.