Jan. 28 (UPI) -- The National Aeronautics and Space Administration on Thursday held its annual Day of Remembrance for astronauts who have died in the exploration of space -- which this year falls on the 35th anniversary of the Space Shuttle Challenger disaster.
The memorial is held each January and honors those who died "furthering the cause of exploration and discovery."
Included in Thursday's observance were the crews of Challenger, Apollo 1 and the Space Shuttle Columbia.
Virgil "Gus" Grissom, Roger Chaffee and Ed White were killed on Jan. 27, 1967, during a test launch at the Kennedy Space Center in Florida. All three were in the crew capsule when a fire started and engulfed the compartment.
Brian Odom, acting NASA historian, called the Apollo 1 disaster a "terrible tragedy" Thursday. "We remember the loss of those lives. We don't stop our quest, we don't stop our mission, but we continue to move forward in that mission of scientific discovery," Odom said.
"The astronauts were the nation's first three-man space crew in the Apollo program designed to put men on the moon in 1969," UPI reported at the time. "An electrical spark which ignited the pure oxygen of the pressurized cabin was the apparent cause of the fiery explosion aboard Apollo 1."
Brother Lowell Grissom and daughter Sheryl Chaffee attended an event Thursday at the Kennedy Space Center. Sheryl Chaffee heads the non-profit Astronauts Memorial Foundation, which built and maintains the Space Mirror Memorial at the Kennedy Space Center, where family members laid a wreath.
"We honor these heroes and the lessons from these heroes that history continues to teach us,"' said Janet Petro, deputy director of NASA's Kennedy Space Center. "We continue to dedicate ourselves to the safety of those who are coming the future."
The loss of life in each accident "changed NASA," she said.
"These ordinary people doing extraordinary things left their families so empty," said Mike Leinbach, who was the last launch director for the Space Shuttle program in 2011. "We owe these families so much."
At Arlington National Cemetery, in Washington, D.C., acting NASA Administrator Steve Jurczyk led another event, which included a wreath-laying ceremony at the Tomb of the Unknown Soldier and monuments that honor all three crews.
"NASA has a unique culture that is fueled by possibility, set on a path to the next giant leap for humanity, and guided by its history," Jurczyk said in a statement.
"The lessons of our past are the enduring legacy of the brave women and men who did not put limits on what could be achieved, and we all recognize the honor of being counted among them as part of the NASA family."
Events were also held at the Johnson Space Center in Houston, the Marshall Space Flight Center in Huntsville, Ala., and Glenn Research Center in Cleveland.
"We will never forget the courage it takes or the lives given in the name of space exploration," said Marshall Center Director Jody Singer in a video.
"They bravely went forward into the unknown," astronaut Butch Wilmore said in the Marshall Flight Center video. "We always want to recall back to where we are now from where we've been. They're part of our family. ... They paid with their lives and we never forget because remembering is vitally important."
This year's Day of Remembrance comes 35 years to the day after Challenger exploded 73 seconds after liftoff at the Kennedy Space Center.
All seven astronauts, including schoolteacher Christa McAuliffe, were killed in the disaster, which grounded the Space Shuttle program for more than two years. McAuliffe, McNair, Dick Scobee, Michael Smith, Judith Resnik, Ellison Onizuka and Greg Jarvis died aboard Challenger.
In 2003, NASA lost seven more astronauts when Columbia broke apart as it re-entered the Earth's atmosphere. Rick Husband, William McCool, Michael Anderson, David Brown, Laurel Clark, Ilan Ramon and Kalpana Chawla were killed aboard the spacecraft.