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Hank: Late January and early February are a tough time for the spaceflight industry — really, for anyone who’s passionate about the exploration of space. Over the course of one week, we mark the anniversaries of the only two fatal NASA accidents to happen during flight: the destruction of Space Shuttles Challenger and Columbia. Traveling to space continues to be one of the most dangerous things humanity’s ever done, and space agencies want to do everything they can to minimize that risk. So, after each of the Shuttle accidents, NASA grounded flights for years, while they figured out exactly what went wrong, and how to stop it from happening again. Here’s what we learned from those disasters.

At 11:38 AM Eastern Time on January 28, 1986, Challenger lifted off from the launch pad in Cape Canaveral, Florida, with seven crew on board -- including one civilian, a teacher named Christa McAuliffe. 73 seconds later, the Shuttle broke apart. The cabin landed in the Atlantic Ocean about two minutes later with enough force to crush everything inside.

We know that the accident was caused by a leak in one of the solid rocket boosters. We also know that if it had been up to the engineers, Challenger would never have launched that day. Before this, the coldest it had ever been when the Shuttle launched was about 10.5 degrees Celsius. But on January 28th, it was more like 2 degrees. And some of the engineers working on the Shuttle knew that colder weather was a problem for the O-rings, the rubber seals that hold together the four sections of each solid rocket booster. When it got too cold, the rubber wasn’t as flexible, which meant that the fuel could leak.

They warned management of the danger, with at least one engineer refusing to sign off on the paperwork for the launch. NASA went ahead with it anyway, and seven people died. After that accident, all shuttles were grounded until 1988, and in the meantime, NASA made two important changes to the program. First, they fixed the problem with the O-rings, adding an additional ring to each link between sections, as well as heating pads that made sure they stayed above 24 degrees no matter how cold it was outside. But they also spent those years changing policies and management structure, in the hope that a warning against a launch would never be ignored again.

Those changes seem to have helped, at least for the next 17 years. Then the Space Shuttle Columbia exploded. This time, it was a completely different sort of accident. Seven astronauts launched at 10:39 AM on January 16, 2003, and they made it to space just fine. There was some concern, because it looked like a piece of foam insulation had broken off the external fuel tank, and no one knew if Columbia had been damaged. But on every previous flight, Shuttles had been damaged by debris, and they’d come back safely. So while at least a few engineers were worried, the mission went ahead as usual, with the astronauts working on more than 80 experiments.

Then came February 1st, the day Columbia was supposed to land. As it re-entered the atmosphere, the crew lost sensor readings from the left wing. Then NASA lost contact with the crew. It turns out, the foam had punctured a hole in the left wing. During re-entry, extremely hot atmospheric gas pushed through, into the damaged wing, first disabling the sensors and then destroying the ship. Columbia exploded somewhere over Texas and Louisiana, leaving a field of debris about 400 kilometers long and 8 kilometers wide.

Again, shuttles were grounded for about two years -- there were no more flights until 2005. And in the meantime, the fuel tank was redesigned so it no longer had that piece of foam. But the committee that reviewed the accident made another recommendation: NASA still needed the Shuttles for a few more years, until the International Space Station was completed. But after that, it was time to ground the fleet. They pointed out that the shuttle program had always been kind of experimental. They were now more than 30 years old, and it wasn’t safe to keep using spaceships that were essentially still in development.

So the very last Space Shuttle flight touched down on July 21, 2011, and NASA astronauts have been flying to space on the Russian Soyuz ever since. In the end, it’s difficult to calculate everything we learned from the shuttle program. It enabled us to conduct hundreds of scientific experiments that we could never have done on Earth. It launched the Hubble Space Telescope ...and then repaired it, five times. And it gave us our first real lesson in how to build and operate a re-useable spacecraft -- the world’s first. But maybe the hardest lesson we learned was how much risk we were willing to take, in order to keep the shuttle flying.

In the end, we decided that we didn’t want to learn any more about that. So, today, spaceflight continues -- all around the world! -- and new astronauts are training to fly new spacecraft, from the Dragon capsule by SpaceX, to NASA’s Orion. And every time we send astronauts into space, they travel with the lessons that we learned from the Shuttle, and the fourteen people who died flying it. Everyone who goes to space, goes thanks to them.

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