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Roger Boisjoly

FILE--File picture from January 28, 1986, shows the explosion of space shuttle Challenger over Cap Canaveral/Florida. The space shuttle with seven astronauts onboard explodet 74 seconds after launch in 16 km height. All occupants were killed. (AP Photo/handout)

Television's most powerful moments

According to a new study released by Nielsen, the Sept. 11 terrorist attacks, Hurricane Katrina and the O.J. Simpson verdict were among the most "impactful" moments in television history. Here are some of them.