KENNEDY SPACE CENTER, Fla. — Today is a solemn day of reflection and honor for NASA.

The agency is observing its annual Day of Remembrance, when it honors the astronauts killed in the Apollo 1, Challenger and Columbia tragedies.

The anniversaries of all three disasters were last week. The three crew members of Apollo 1 were killed in a preflight test in 1967. The crew of Challenger — including the first teacher picked for the astronaut program, Christa McAuliffe — died in 1986. Columbia broke up returning to Earth in 2003.

In addition to an observance this morning at the Kennedy Space Center Visitor Complex, NASA Administrator Jim Bridenstine is scheduled to take part in a wreath-laying ceremony at Arlington National Cemetery with family members of the lost astronauts.

"As our nation continues to lead the world in forging a pathway to space, we honor the legacy of these brave men and women. We vow to continue building the bold future they envisioned for all of humanity on this Earth and beyond. Though our friends, colleagues, and family members have left this Earth, they will always be with us as we journey among the stars," a statement by President Donald Trump said.

Flags at all NASA facilities are also being flown at half-staff.

"NASA has learned hard lessons from each of our tragedies, and we will keep that knowledge at the forefront of our work as we continuously strive for a culture of safety that also enables us to still reach for the stars," Bridenstine said in a statement. "What President Reagan said of the Challenger crew applies to each of our fallen colleagues, who 'honored us by the manner in which they lived their lives.' "

NASA postponed its Day of Remembrance to today, because most of its workers were furloughed during the partial government shutdown.