KENNEDY SPACE CENTER, Fla. — It was a solemn day across Florida's space community as NASA memorialized the heroes who gave their lives in the name of space exploration.

Crowds gathered at Kennedy Space Center Visitor Complex's Space Mirror Memorial to honor the 25 people whose names are engraved there.

A bell was rung for each of the heroes who have perished during the six-decade-old U.S. space program. They include test pilots and the crews of Apollo 1, space shuttle Challenger, and space shuttle Columbia.

The annual Day of Remembrance is observed during the most tragic week in NASA history.

The Apollo 1 crew was killed during a launch pad fire on January 27, 1967. On January 28, 1986, seventy-three seconds into flight, Challenger exploded, killing the seven-member crew. The seven-member shuttle Columbia crew was killed when the orbiter broke apart during landing on February 1, 2003.

"Space is an unforgiving environment. It doesn't tolerate human error," Kennedy Space Center Associate Director Kelvin Manning said. "But we are going to have to keep pressing forward to advance exploration, science, and innovation that benefits humanity. How do we do that? With these brave men and women leading the way."

The 25th name was just added to the Space Mirror Memorial: Commercial astronaut Michael Alsbury was killed during a Virgin Galactic test flight in 2014. A wreath was placed the foot of the memorial.

"It's sobering," says Jim Wunderlich of Chicago, who was humbled by the ceremony. "This was a poignant reminder of the sacrifices this astronauts had."

Ceremony guests marked the occasion by putting flowers on the fence, honoring the astronauts who gave the ultimate sacrifice in the name of America's quest for space.

The Space Mirror Memorial is run by the Astronauts Memorial Foundation.