Workers at NASA's Kennedy Space Center halted all activity and fell silent for 73 seconds Monday in memory of the seven astronauts who died aboard the space shuttle Challenger five years ago.

The observance began at 11:38 a.m., the time of Challenger's liftoff on Jan. 28, 1986. The shuttle exploded 73 seconds later.Among the dead was Christa McAuliffe, a teacher from Concord, N.H., who had planned live lessons from orbit for the nation's students. She was the first ordinary American to head into space.

At Concord High School, where McAuliffe taught, students and faculty paused for a moment of silence at the same time as NASA workers. Five years ago, cheering students jammed the school's auditorium watching the launch on television.

In a brief message televised to employees throughout the launch site, Kennedy Space Center director Forrest McCartney noted that 13 shuttle flights have been conducted since the disaster.

While NASA and contractor employees stopped work, security guards lowered flags to half-staff.

No official observances were held at Johnson Space Center in Houston or at NASA headquarters in Washington. NASA Administrator Richard Truly, however, issued this message to employees:

"Today marks the day that we in the NASA family remember the seven astronauts who flew aboard Challenger. We will never forget them. It has been through your tireless efforts in the interim that we have profited from the important lessons we learned."

Shuttle solid fuel booster rockets have been redesigned since the accident, caused by a leak in a joint on Challenger's right booster.

As one reminder of the crew's mission, the Challenger Center for Space Science Education held a live teleconference at noon for more than 1 million students and 38,942 teachers at 936 sites.