NASA held an emotional memorial service at Cape Canaveral, Florida Thursday for the seven astronauts killed when the Space Shuttle Challenger exploded moments after launch 30 years ago.
Hundreds attended the service, including relatives of all U.S. astronauts and space workers killed in the line of duty since NASA's founding in 1958.
They include three astronauts killed in a launch pad fire in 1967 and the seven who died when the Space Shuttle Columbia broke apart on its return to Earth in 2003.
President Barack Obama sent out a statement Thursday, saying reaching for "unbounded heights" has helped make the United States what it is today.
"We must never forget the courageous Americans who made the ultimate sacrifice to expand the boundaries of understanding. They knew the risks and still chose to put their lives on the line so that future generations could lead lives made better by advances in science, technology and a deeper understanding of our universe and humanity's place therein."
Obama quoted former president Ronald Reagan, who said on the day of the Challenger disaster in 1986, "The future doesn't belong to the fainthearted. It belongs to the brave."
Americans had taken safe space flight for granted by 1986 and were stunned beyond words when the Challenger exploded one minute and 13 seconds after lifting off from Cape Canaveral.
Millions saw the tragedy on television as it happened. It is believed the astronauts died almost instantly.
Among those killed was school teacher Christa McAuliffe, America's first-ever civilian astronaut.
An investigation revealed that a fuel leak in one of the space shuttle's booster rockets led to the accident.