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

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.

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

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.