In just a few days, NASA will begin remembrances for a tragically somber week for the agency.

Kennedy Space Center Director Bob Cabana was just a few months into his NASA astronaut career on January 28, 1986.

He was working in a shuttle system simulation trainer when he heard what had just happened at 11:39 a.m. ET — the moment the shuttle Challenger was destroyed.

"When they came and interrupted the simulation and told us, we just couldn't believe it," he said.

"I didn't believe it. It was nonacceptance," he said. "It was, 'The crew got out, the crew's OK, they did an abort or something.' It was total nonacceptance that we lost the crew."

The seven-member Challenger crew was killed when the orbiter broke apart 73 seconds after liftoff from Kennedy Space Center in Florida. A lengthy investigation determined a joint in the right solid-rocket booster failed, allowing burning fuel to leak out and make contact with the other booster and external tank.

The shuttle broke apart, and with no escape system for the crew, they were killed.

It was a milestone 25th space shuttle mission and the first launch from Pad 39B.

Cabana's duties in mission control were often alongside the Challenger crew at Johnson Space Center in Houston.

On the job for just a few months, Cabana remembers a conversation he had with Mission Specialist Ellison Onizuka, one of the Challenger 7.

"He said, 'I know it's really tough when you move to a new place, getting settled and everything. If you need to work on your car or anything, I got all the tools. Just come on over, and we'll do it my garage,' " Cabana said. "I'll never forget that."

Among those killed was teacher Christa McAuliffe, heralded nationally beforehand as the first teacher in space.

Challenger became NASA's first orbital mission during which astronauts were lost.

Cabana says, even 35 years after the disaster, remembering those lost is the way to make space flight safer for the future.

"(It's important) that we do not forget, that we bring this forward, that we look at the lessons learned and continue to remember," he said.​

Next week, NASA will hold its annual Day of Remembrance ceremony, honoring the astronauts killed in the Apollo 1 fire in 1967, the Challenger disaster, and the seven-member crew of the shuttle Columbia, which broke apart in 2003. All happened in different years within the span of a week.