Monday marked the 27th anniversary of the explosion of the space shuttle Challenger, which killed all seven crew members, including New Hampshire schoolteacher Christa McAuliffe. The shuttle exploded 73 seconds after liftoff from Cape Canaveral on Jan. 28, 1986. McAuliffe was a social studies teacher at Concord High School. The Concord School District and the city have observed the Challenger anniversary quietly through the years, partly to respect the privacy of the McAuliffe family. No formal events were scheduled Monday. For the 25th anniversary in 2011, the McAuliffe-Shepard Discovery Center held a tribute to teachers, followed by a program about McAuliffe.