If the Strategic Defense Initiative was problematical for the Reagan administration, other efforts in space were more promising. In 1981 the U.S. launched the space shuttle Columbia -- the first reusable manned spacecraft. Between 1981 and 1985, the shuttle demonstrated extraordinary versatility, with astronauts conducting experiments, taking photographs, and launching, retrieving and repairing satellites while in orbit. But in January 1986, tragedy struck: the space shuttle Challenger exploded 73 seconds after takeoff, instantly killing six astronauts and a schoolteacher who was to have been the first ordinary citizen in space. Space shuttle missions were postponed indefinitely while NASA set out to redesign the shuttle for safety. By the time the United States successfully launched the shuttle Discovery in late 1988, there had been over 300 changes in the shuttle's launch systems and computer software.