If there’s anything we’ve learned about space travel over the last 56 years, it’s that it’s dangerous.

So space agencies like NASA do all they can to minimize the risks — most importantly, the risks to astronauts’ lives. But learning how to deal with those risks has been an ongoing process.

And in some cases, NASA has had to learn from profound tragedy. One of those tragedies was the Apollo 1 fire on January 27, 1967, which claimed the lives of the three astronauts involved: Gus Grissom, Ed White, and Roger Chaffee. Both Congress and NASA immediately launched investigations, and what they learned changed a lot about how we’ve approached spaceflight ever since.

The plan for the Apollo 1 mission was to test the command module, the capsule on a rocket where the astronauts sit, in low Earth orbit. Since eventually, NASA wanted to send that command module to the Moon. The fire happened during a sort of rehearsal for the launch.

It was what’s known as a “plugs-out” test, meaning that the rocket was unfueled, and it didn’t have the explosive bolts that would separate the different stages of the rocket in-flight. Since there weren’t as many explosives or any fuel around, NASA mission control thought that the test would be safe. Obviously, they were wrong.

After the astronauts got inside the capsule, it was filled with 100% oxygen gas at 115 kilopascals, which is about 15% higher than standard air pressure at sea level. There was an electrical failure during the test, and the resulting spark caused the fire. The pure oxygen atmosphere was completely consumed within thirty seconds, and the astronauts couldn’t open the hatch to escape.

Meanwhile, the smoke billowing out of the command module kept the launchpad personnel from being able to rescue the astronauts, because there were no smoke masks at the launchpad. The investigations into the fire found that several factors, both on an engineering level and on an organizational level, contributed to the tragedy. NASA implemented every suggestion made by the investigation committees, leading to major changes that have made spaceflight much safer, even though it is still dangerous.

One of the biggest engineering-related changes they made was to the gases they used to fill the capsule. The first problem was the fact that they used pure oxygen, which was meant to make the capsule lighter, so it would take less fuel to launch. Plus, breathing pure oxygen would get rid of the nitrogen in the astronauts’ bloodstreams, which meant that they wouldn’t have to worry about nitrogen bubbles forming in their blood during the launch and causing the bends.

But oxygen is really flammable — it’s basically what fire runs on. You don’t want pure oxygen near any kind of spark, and you definitely don’t want it anywhere that a fire could be fatal. The other issue was that the pressure inside the capsule was higher than the pressure outside, and the fire just made that worse.

The hatch opened inward, and the extra pressure inside meant that the astronauts couldn’t open it. That’s why, ever since, instead of using a high-pressure, entirely oxygen atmosphere, NASA has used a mix of gases at standard atmospheric pressure. For the rest of the Apollo missions, the atmosphere inside the capsule during the launch was made up of 60% oxygen and 40% nitrogen, while the astronauts breathed pure oxygen inside their spacesuits.

It was still a higher oxygen concentration than Earth’s atmosphere, but it was low enough that a fire wouldn’t spread too rapidly for astronauts to escape. NASA made another major engineering change, too: they started constructing the command module, and astronauts’ suits, entirely out of non-flammable materials. This way, any electrical spark would have nothing to catch on.

And to make sure the materials they used were up to scratch, the command module was tested to make sure that it would be safe in a fire. But NASA changed more than just the gases and materials they used. They also overhauled their safety procedures and attitude toward spaceflight as a whole.

The tragedy was a huge wake up call for NASA. They realized that, as an organization, they were so focused on beating the Soviet Union in the space race that they’d become somewhat … cavalier … about safety. Before the fire, for example, there were no fire safety procedures in place, and they only had minimal firefighting equipment at the launchpad.

The command module hadn’t been tested in simulations to see if it met any safety standards, and when the test was underway, there were no emergency staff present, like EMTs or firefighters. So NASA implemented a whole series of changes to fix every one of those problems, and resolved to be a lot more cautious in general. In the words of Gene Kranz, who led Mission Control for the Apollo program: “We will never again compromise our responsibilities.

Mission Control will be perfect.”