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.
Flags flew at half-mast at NASA centres on Thursday.
A wreath was laid at the memorial at Cape Canaveral 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, a civilian teacher.
Challenger exploded 73 seconds after launch at an altitude of 14,000 metres on 28 January, 1986, as millions of people watched on television.
Special guests complex on Friday morning included the widow of Commander Scobee.
June Scobee Rodgers said the "entire world knew how the Challenger crew died".
She told the audience to "boldly look into the future" in space and science education.
The BBC reports Challenger was on a mission to deploy two satellites.
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 NASA personnel have been killed since 1964. Among the deaths:
Seven astronauts died aboard the Columbia shuttle in 2003 when it disintegrated upon re-entry to Earth due to a damaged heat shield.
Three people also died aboard Apollo 1 in 1967 after a fire during a launch pad test.