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The Friday morning observance began with a ceremony honoring the seven crew members lost when Challenger exploded 73 seconds after liftoff from Kennedy Space Center Jan. 28, 1986, Florida Today reported. The crew included teacher Christa McAuliffe.

The disaster reduced the shuttle fleet from four to three and led to the delay or cancellation of many proposed missions.

Chroniclers of the U.S. space program say the explosion taught NASA much about the vulnerabilities of the shuttle and how to make space travel safer, USA Today reported.

The Challenger explosion "was significant, because it set in train a whole set of changes at NASA", says Roger Launius, senior curator in space history at the Smithsonian's National Air and Space Museum.

"We had let our guard down", says Bryan O'Connor, an astronaut at the time of the Challenger accident and now NASA's top safety official. "It's so human to become complacent."

After Challenger, Americans understood "that there is risk to human spaceflight", Marcia Smith, who runs SpacePolicyOnline.com, says. "But that was not enough to deter us from continuing our quest for space exploration."

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