Today marks the 40th anniversary of the Challenger explosion, when the world watched in horror as the shuttle exploded just 73 seconds after liftoff. The other crew members killed were Commander Francis "Dick" Scobee, Michael Smith, Judith Resnik, Ronald McNair, Ellison Onizuka, and Gregory Jarvis. Investigations into the tragedy discovered it was caused by a rubber O-ring seal that failed due to extreme cold. The men and women of the Challenger crew represented the very best of us, brave pioneers who wanted to expand the reach of human knowledge. The night of the Challenger disaster, Ronald Reagan addressed the nation, saying, "The crew of the space shuttle Challenger honored us by the manner in which they lived their lives."