The United States on Thursday marked a day of remembrance for astronauts who have died in the line of duty, particularly the victims in the explosion of the space shuttle Challenger 25 years ago.
Schoolchildren and space enthusiasts around the world watched live January 28, 1986 as the Challenger lifted off carrying seven astronauts including the first teacher, Christa McAuliffe, to embark on a mission to space.
The shuttle exploded 73 seconds after launch at an altitude of 14,000 meters (46,000 feet), killing everyone on board. Engineers determined that the blast was caused by the failure of a joint seal due to cold weather.
"We have seen that achieving great things sometimes comes at great cost and we mourn the brave astronauts who made the ultimate sacrifice in support of NASA missions throughout the agency's storied history," President Barack Obama said in a statement.
"We pause to reflect on the tragic loss of the Apollo 1 crew, those who boarded the space shuttle Challenger in search of a brighter future, and the brave souls who perished on the space shuttle Columbia."
A total of 24 people have been killed while supporting the space agency's mission since 1964, NASA said.
Among them were seven astronauts killed aboard the Columbia in 2003 when the space shuttle disintegrated upon re-entry to Earth due to a damaged heat shield that was compromised by a broken off piece of insulation.
Three astronauts died aboard the Apollo 1 in 1967 when a fire broke out during a launch pad test.
"This year marks the 25th anniversary of the loss of Challenger -- a tragedy that caused us to completely re-think our systems and processes as we worked to make the shuttle safer," said NASA chief Charles Bolden.
"The nation will never forget January 28, 1986, nor its indelible images."
Bolden attended a wreath-laying ceremony at Arlington National Cemetery outside Washington on Thursday, and flags at NASA stations were flown at half-mast.