Cape Canaveral, Fla ? After 20 years, the images remain seared into the nation’s consciousness like the Twin Towers on 9-11 and President Kennedy’s motorcade through Dallas.

A smiling, waving schoolteacher and her six astronaut crewmates leave their quarters on a frigid morning at Cape Canaveral.

Two booster rockets arc uncontrollably away from a giant fireball.

Family members stare skyward, not fully comprehending the awful truth of what they are witnessing.

The Challenger accident shattered forever the myth of invincibility that surrounded NASA after landing men on the moon. Now, two decades after the Jan. 28, 1986, catastrophe, the space shuttle faces a bleaker future than it did even in the wake of that stunning disaster. A second shuttle accident, the loss of Columbia three years ago this Wednesday, prompted a fundamental rethinking of the program and a presidential decision to end it by 2010.

“People tend to get complacent and never recognized the shuttle as the high-risk flight test program it is,” said Eugene Covert, professor emeritus of engineering at the Massachusetts Institute of Technology and a member of the Rogers Commission that investigated the Challenger accident. “If you’re going to undertake high-risk activities, you have to expect tragedies.”

Jarvis Junior-Senior High School students Devin Fischer, left, and Miranda Rathbun watch a presentation commemorating the space shuttle Challenger's explosion 20 years ago. The students watched the program Friday in Mohawk, N.Y.

As NASA approaches the program’s retirement date, there are many questions about how the agency will safely close out the shuttle era.

Some of the answers could be forthcoming on Feb. 6, when the White House releases its 2007 budget request for NASA.

Before moving ahead, agency managers still must eliminate an ongoing safety hazard: the shedding of large, potentially damaging chunks of foam insulation from the shuttle’s external fuel tank during launch.