Wednesday marks a somber anniversary in American history. 40 years ago, seven astronauts were killed when Space Shuttle Challenger exploded during liftoff.

Former astronaut and professor Terry Hart is opening up 40 years later about the friends and colleagues he lost on board the space shuttle Challenger explosion.

To demonstrate this tragic oversight, Feynman dropped a sample of the O-ring rubber in a glass of ice water. The rubber grew rigid in the glass, proving Feynman’s point. “I believe that has some significance for our problem,” he said at a February 1986 hearing of the Challenger commission.

NASA's space shuttle Challenger completed 10 missions before it broke apart during a launch in 1986, killing seven astronauts.

Because a teacher was going to space, millions of schoolchildren watched the Challenger lift off on Jan. 28, 1986. It exploded 73 seconds later.

Marking three decades since the explosion of the Challenger claimed the lives of its seven-member crew, we recap the incident, the investigation, and the lessons learned on the path to space and beyond.