NASA employees throughout the country paused Thursday to rededicate themselves to space exploration and remember their 17 astronaut colleagues who died pursuing it.

"They and their families sacrificed much in the pursuit of their dreams and our dreams. We have not, will not, ever forget what their sacrifice has meant to each of us," Johnson Space Center Director Mike Coats told hundreds of NASA workers.

NASA Administrator Michael Griffin said he would lay a wreath at Arlington National Cemetery in memory of the astronauts lost in the Apollo 1, Challenger and Columbia tragedies.

Saturday marks 20 years since Challenger blew apart as it lifted into space.

Three astronauts died inside the Apollo 1 spacecraft in a fire during a countdown test at the launch pad on Jan. 27, 1967; seven died aboard Challenger when it exploded in 1986, and seven more died as Columbia broke to pieces upon re-entering the Earth's atmosphere Feb. 1, 2003.

"Spaceflight remains the pinnacle of human challenge, an endeavor just barely possible with today's technology," Griffin said in a statement Thursday. "It is an enormously difficult enterprise, made more so by the fact that we are human beings and flawed. The losses we commemorate today are a mute and terrible reminder of the sternness of the challenge, and of awful consequences of our flaws."

A Challenger memorial is planned at the Kennedy Space Center in Florida this weekend.