upgrade to a flight article. Because STA-099's qualification testing prevented damage, NASA found that rebuilding STA-099 as a flight worthy orbiter would be less expensive than refitting Enterprise. Work on converting STA-099 to operational status began in January 1979, starting with the crew module (the pressurized portion of the vehicle), as the rest of the vehicle was still being used for testing by Lockheed. STA-099 returned to the Rockwell plant in November 1979, and the original, unfinished crew module was replaced with the newly constructed model. Major parts of STA-099, including the payload bay doors, body flap, wings, and vertical stabilizer, also had to be returned to their individual subcontractors for rework. By early 1981, most of these components had returned to Palmdale to be reinstalled. Work continued on the conversion until July 1982, when the new orbiter was rolled out as Challenger. Challenger, as did the orbiters built after it, had fewer tiles in its Thermal Protection System than Columbia, though it still made heavier use of the white LRSI tiles on the cabin and main fuselage than did the later orbiters. Most of the tiles on the payload bay doors, upper wing surfaces, and rear fuselage surfaces were replaced with DuPont white Nomex felt insulation. These modifications and an overall lighter structure allowed Challenger to carry 2,500 lb (1,100 kg) more payload than Columbia. Challengers fuselage and wings were stronger and lighter than Columbias. The hatch and vertical-stabilizer tile patterns were different from those of the other orbiters. Challenger was the first orbiter to have a head-up display system for use in the descent phase of a mission, and the first to feature Phase I main engines rated for 104% maximum thrust. Construction milestones (as STA-099) Construction milestones (as OV-099) Flights and modifications After its first flight in April 1983, Challenger quickly became the workhorse of NASA's Space Shuttle fleet, flying six of nine Space Shuttle missions in 1983 and 1984. Even when the orbiters Discovery and Atlantis joined the fleet, Challenger flew three missions a year from 1983 to 1985. Challenger, along with Atlantis, was modified at Kennedy Space Center to be able to carry the Centaur-G upper stage in its payload bay. If flight STS-51-L had been successful, Challengers next mission would have been the deployment of the Ulysses probe with the Centaur to study the polar regions of the Sun. Challenger flew the first American