Every year on the last week of January, NASA remembers its explorers who died in the line of duty to advance space exploration. They include the crews for the Apollo 1, Challenger's STS-51L and Columbia's STS-107 missions.

The agency honored them in its Day of Remembrance on Thursday.

That day 44 years ago-- Jan. 27, 1967 --the Apollo 1 accident struck. Three astronauts were killed when a fire erupted in their capsule during testing. A review board determined test conditions were hazardous, but NASA didn't prior to the accident. The Apollo 1 disaster became credited for pushing NASA to implement many changes to the Apollo program that enabled later missions to the moon.  

Then 25 years ago on Friday-- Jan. 28, 1986-- seven astronauts died from Challenger's STS-51L mission when the shuttle exploded 73 seconds after liftoff from Kennedy Space Center in Florida. A special commission traced the problem to an “O-ring” seal failure, along with cold weather, that resulted in a leak in one of the two solid rocket boosters. The commission also faulted NASA's decision-making and risk-assessment processes. This first NASA shuttle disaster led the fleet of orbiters to be suspended for roughly two-and-a-half years. 

Next Tuesday marks the eighth anniversary of Columbia's STS-107 disaster--Feb. 1, 2003. During re-entry into Earth's atmosphere, the shuttle disintegrated over Texas approximately 15 minutes before it was scheduled to touch down at Kennedy Space Center.  All seven astronauts on board were killed. Investigators found the key cause to be foam insulation that had shed from the external fuel tank during launch and punched a hole in the shuttle's left wing. The loss brought NASA's safety culture again under scrutiny. Shuttle missions didn't resume until more than two years later.

"The astronauts in [the Challenger] crew were personal friends of mine, as were the astronauts aboard Columbia when it was lost. The Apollo I crew perished while I was studying at the Naval Academy, and I mourned their loss in the line of duty with the nation," said NASA Administrator Charlie Bolden in a statement Thursday. "These brave men and women will always be a part of us, and we are still building on their legacies." 

Some stories remembering the crews:

Memorial Service Marks 25th Anniversary of Challenger Explosion —Orlando Sentinel

25 Years Later: How the Challenger Disaster Brought NASA Down to Earth —USA Today

On Challenger Anniversary: Spare a Thought for Apollo 1 —TIME

NASA's Day of Remembrance —Fox News

25 Years After Challenger: How Grief Inspired Teachers and Students —Space.com

Inside CNN When Challenger Exploded —CNN

Astronaut's Brother Recalls A Man Who Dreamed Big —NPR 

Lt. Colonel Michael Anderson's Legacy Still Felt in Cheney —NorthWest Cable News

In Memoriam—A Rededication to Space —Huffington Post

Remembering The Challenger: 25 Years Later —Neon Tommy

 

The Apollo 1, Challenger's STS-51L and Columbia's STS-107 astronauts:

Apollo 1 crew. From left to right are Edward H. White II, Virgil I. "Gus" Grissom, and Roger B. Chaffee. Photo taken April 1, 1966 by NASA

Challenger's STS-51L crew. In the back row from left to right are mission specialist Ellison S. Onizuka, Teacher in Space participant Sharon Christa McAuliffe, payload specialist Greg Jarvis and mission specialist Judy Resnik. In the front row from left to right are pilot Mike Smith, commander Dick Scobee and mission specialist Ron McNair. Photo taken Nov. 15, 1985 by NASA

Columbia's STS-107 crew. From left to right are mission specialist David Brown, commander Rick Husband, mission specialist Laurel Clark, mission specialist Kalpana Chawla, mission specialist Michael Anderson, pilot William McCool and Israeli payload specialist Ilan Ramon. Photo taken Jan. 1, 2002 by NASA

 

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