On Thursday NASA will remember the astronauts and astronaut candidates who died in service to U.S. space exploration.

Just 81 seconds into launch, a chunk of foam flew off the shuttle’s external fuel tank and struck Columbia’s heat-resistant panels on its left wing. The damage to the heat shield proved catastrophic when Columbia tried to return to Earth.

NASA is marking the 20th anniversary of the space shuttle Columbia tragedy with somber ceremonies during its annual tribute to fallen astronauts.