NASA is holding its annual Day of Remembrance today to honor the crew members of Apollo 1 and space shuttles Challenger and Columbia, as well as other NASA employees who have lost their lives while advancing space exploration.

This year marks the 15th anniversary of Columbia’s last space shuttle mission, which suffered a catastrophic and fatal end. The unfortunate event shook the science community and the public, but the lessons taken away from the incident overhauled NASA’s approach to safety concerns and future space travel.

Columbia’s last mission

On January 16, 2003, Columbia embarked on its 28th space shuttle mission, designated STS-107, to carry out a series of microgravity science experiments. However, about 80 seconds into the shuttle’s launch from the Kennedy Space Center, a large piece of foam insulation split off from the “bipod ramp” — a piece used to attach the external tank to the shuttle. Engineers used camera footage to review the incident, and found that the suitcase-sized piece of foam struck the shuttle’s left wing as it fell. However, ground workers weren’t able to determine exactly where the damage occurred or how severe it was. Furthermore, NASA engineers believed that if extensive damage did occur, the crew members wouldn’t be able to remedy the situation while the shuttle was in orbit.

This was the fifth time that foam had been observed breaking off of a bipod ramp, and the problem had become so common that it even had a name: foam shedding. Since the four previous cases of foam shedding occurred without incident, NASA became accustom to the phenomena and didn’t take many preventative measures to stop it from happening in future launches.

On the morning of February 1, 2003, as the Columbia mission was beginning its descent back to Earth, Mission Control starting receiving unusual readings from the tire pressure and temperature sensors on the shuttle’s left wing. They were briefly able to contact the crew, but lost communication before they were able to address the concerns.

Shortly after the shuttle crossed the coast of California, while it was at an altitude of over 230,000 feet (70,100 meters), civilians started seeing debris fall from the sky. Several unsuccessful attempts at communication were made over the next few minutes as Columbia traveled over California to Texas.

Around the time that Columbia should have been landing at the Kennedy Space Center, eyewitnesses near Dallas began reporting that the shuttle had disintegrated overhead. At 9:12 a.m., Entry Flight Director LeRoy Cain instigated NASA's contingency plan and alerted search and rescue teams in Texas and Louisiana.