Challenger disaster: Nasa marks 25th anniversary
Hundreds of people have attended an event at Kennedy Space Center in Florida to mark the 25th anniversary of the space shuttle Challenger explosion, in which seven astronauts died.
Special guests at the Cape Canaveral visitor's complex included the widow of Challenger's commander.
The event comes one day after a national day of remembrance for those killed in the 1986 incident.
Flags flew at half-mast at Nasa centres across the country on Thursday.
June Scobee Rodgers, the widow of the doomed vessel's commander, Dick Scobee, spoke at the event on Friday morning, saying the "entire world knew how the Challenger crew died".
"We wanted the world to know how they lived and for what they were risking their lives," Ms Rodgers said.
Ms Rodgers played an important role in establishing the Challenger Center for Space Science Education, which has 48 learning centres.
Challenger exploded 73 seconds after launch at an altitude of 14,000 metres (46,000ft), killing all on board.
The mission was notable because it carried for the first time a teacher, Christa McAuliffe.
The memorial event at Kennedy Space Center drew space agency officials, former astronauts, family and friends of those who died aboard Challenger, as well as school children who were not yet born when the disaster occurred.
Ms Rodgers told the crowd to "boldly look into the future" in space and science education.
The widow, along with Nasa's operations chief, placed a wreath at the bottom of the Space Mirror Memorial, a granite block that lists the names of all those who have died in the Nasa programme since its inception.
Peggy Shecket, whose friend Judith Resnik was aboard Challenger, travelled from the state of Ohio for the memorial.
Ms Shecket was at the launch of the shuttle on 28 January, 1986, when it broke up.
"I miss her terribly," Ms Shecket told the Associated Press news agency of her friend.
"At this age, in our 60s, we could have gone to ladies' weekends together. She probably would have had time that she didn't have 25 years ago because she was so busy," she added.
"The legacy of those who have perished is present every day... and inspires generations of new space explorers," Nasa Administrator Charles Bolden said at a memorial service on Thursday at Arlington National Cemetery in Virginia.
Mr Bolden laid a wreath at the memorial to the seven astronauts, Commander Francis Scobee, Pilot Michael Smith, Mission Specialists Judith Resnik, Ellison Onizuka, Ronald McNair and Payload Specialists Gregory Jarvis and Christa McAuliffe.
"Every day, with each new challenge we overcome and every discovery we make, we honour these remarkable men and women," Mr Bolden said.
The Challenger was on a mission - STS 51L in the official record - to deploy two satellites. It broke up after launch over the Kennedy Space Center in Florida.
Investigators later determined a sealing ring had failed in cold weather, allowing pressurised hot gas to leak from one of the rocket boosters and eventually to breach the external fuel tank.
A total of 24 people have been killed while supporting the space agency's mission since 1964, Nasa said.
In addition to the seven aboard the Challenger, seven other astronauts died aboard the Columbia shuttle in 2003 when it disintegrated upon re-entry to Earth due to a damaged heat shield.
Three men died aboard the Apollo 1 in 1967 in a fire during a launch pad test.
Seven more astronauts died in other incidents while working for the programme.