Millions watched the fireball in the sky when, 10 years ago this Sunday, the Challenger shuttle exploded 8.9 miles above the Atlantic Ocean, killing the seven Americans aboard.

Just 73 seconds into liftoff, the shuttle was traveling at 1,460 mph - nearly twice the speed of sound - when its pressure seals gave way in the cold and failed to contain the combustible rocket gases.

Many UA students still remember where they were when they saw the launching's live broadcast.

Richard Keller, mechanical engineering junior, was in his eighth-grade science class when the shuttle lifted off on Jan. 28, 1986.

"All the teachers were so excited about having a fellow educator getting the chance to go into space, but I knew something had gone terribly wrong when I saw the frantic look on my science teacher's face when smoke started coming out of the Challenger," he said.

Despite engineers' warnings about the cold weather's effect on the shuttle's pressure seals, or O-rings, NASA and booster-maker Morton Thiokol Inc. chose to launch the Challenger.

"There is always a risk when sending anything or anyone into space, and the astronauts knew that," said Gary Pennington, agricultural and resource economics junior.

The O-rings were made to perform at temperatures of 50 degrees and above but could be expected to perform within the 40 degree ranges.

The temperature at launch time was 36 degrees.

"Five or six degrees can make a big difference in the accepted functioning of an instrument," said John Brownlee of Students for the Exploration and Development of Space.

William Earley, finance sophomore, said he was stunned as he watched the explosion on his television at home.

"This mission was the beginning of exploratory space travel for non-professional astronauts. It had a nice charisma of having a teacher on board," he said.

Christa McAuliffe, a schoolteacher from Concord, N.H., was to use the shuttle as a classroom.

The other NASA crew members were shuttle commander Francis "Dick" Scobee; pilot Michael Smith; Greg Jarvis, a Hughes Aircraft Co. engineer; Ronald McNair, the second African-American in space; Ellison Onizuka, the first Asian-American in space; and Judith Resnik, the second American woman in space.

The Associated Press contributed to this story.