To honor all of its fallen astronauts, NASA has conducted a day of remembrance on the 29th anniversary of the disaster that destroyed the space shuttle Challenger and killed the seven astronauts on board.
Ceremonies were set for a number of locations around the U.S., including Arlington National Cemetery in Virginia, the Kennedy Space Center in Florida and the Marshall Space Flight Center in Alabama.
"NASA's Day of Remembrance honors members of the NASA family who lost their lives while furthering the cause of exploration and discovery," the space agency said in a release.
The ceremonies will pay tribute to the three astronauts killed in the Apollo 1 fire in 1967, the seven crew members who died when Challenger exploded on Jan. 28,1986, and the seven astronauts who died when the shuttle Columbia broke apart during re-entry to Earth's atmosphere in 2003.
In the Apollo disaster, a fire broke out in the three-man capsule as it was undergoing a ground test at Cape Canaveral, killing astronauts Gus Grissom, Roger Chaffee and Ed White.
Not quite 20 years later the shuttle Challenger exploded shortly after launch when an O-ring in one of its solid boosters failed, releasing burning gases that caused the shuttle fuel tank to explode.
Killed in the disintegrating shuttle were Francis "Dick" Scobee, Greg Jarvis, Ronald McNair, Judy Resnik, Ellison Onizuka, Mike Smith and teacher Christa McAuliffe.
McAuliffe, from Connecticut, had been selected to join the mission as the first schoolteacher in space.
Space shuttle missions were halted for almost 3 years following that tragedy, then resumed in 1988 with a flight of shuttle Discover and subsequent successful missions for 15 years.
Tragedy stuck again on Feb. 1, 2003, when the shuttle Columbia, suffering unrecognized damage during its launch, broke up during its re-entry into Earth's atmosphere at the end of its mission.
The seven crew members -- Rick Husband, David Brown, Willie McCool, Laurel Clark, Michael Anderson, Kalpana Chawla and Ilan Ramon, Israel's first astronaut -- were all killed.
An investigating board determined that a large chunk of insulating foam from the shuttle's external fuel tank broke off and punched a hole in the leading edge of Columbia's left wing, a hole that allowed superheated atmospheric gases into the wing structure during re-entry, leading to the breakup.
Space shuttle missions ended for good in 2001 and the fleet of remaining shuttles was retired.
NASA Administrator Charles Bolden issued a statement of tribute to "these men and women who were our friends, family and colleagues.
"Let us join together as one NASA Family, along with the entire world, in paying our respects, and honoring the memories of our dear friends," he said. "They will never be forgotten. Godspeed to every one of them."