CAPE CANAVERAL, Fla. - When Endeavour lifts off, former schoolteacher Barbara Morgan will be seated on the lower deck in the middle, exactly where Christa McAuliffe sat 21 years ago. And she'll be flying aboard Endeavour, the space shuttle that was built to replace the Challenger. Morgan, who was McAuliffe's backup, is used to all the comparisons. In fact, she welcomes them. "I think the great thing about it is that people will be thinking about Challenger and thinking about all the hard work lots of folks over many years have done to continue their mission," Morgan said last month....