Which U.S. space shuttle exploded on January 28, 1986, killing all seven astronauts on board?

The worst accident in the history of the U.S. space program, the Challenger disaster was caused by the failure of an O-ring seal due to cold weather. The seven astronauts who perished included the first Asian American, the second Black American, and the second and third women ever to go to space. Perhaps the most devastating loss, however, was Christa Macauliffe, the first teacher in space. Selected by Ronald Reagan's 'Teacher in Space Project', Macauliffe's students — as well as many thousands of their peers in classrooms across the country, tuned in to see the thrilling takeoff and later, Macauliffe's planned delivery of the first school lesson taught from space. Seventy-three seconds after lift-off, the shuttle exploded before the nation's eyes.

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