Launch after repeated delays

STS-51-L was the 25th mission of the Space Shuttle program and the tenth flight of the orbiter Challenger. It carried the Tracking and Data Relay Satellite (TDRS-B) and the SPARTAN-203 instrument to observe Halley's Comet — and teacher Christa McAuliffe as part of the "Teacher in Space" project.

Launch was originally planned for 22 January 1986 but was delayed five times due to weather and technical problems. The night before the final attempt, the temperature at Launch Complex 39B fell below freezing — engineers at Morton Thiokol, maker of the solid rocket boosters, warned in a pre-launch conference that the cold could compromise the rubber O-ring seals, but their objections were ultimately overruled by NASA and Thiokol management.

Challenger finally lifted off on 28 January 1986 at 16:38:00 UTC (11:38 EST) with its seven-member crew. Millions watched the launch live, including schoolchildren across the United States eager to see the first lesson taught from space.