From the extreme temperatures to the lack of oxygen, gravity, and pressure, outer space is incredibly hostile to human life. It takes years of preparation and safety tests to get humans into space, and every step of the way is another chance for fatal errors to occur. Although hundreds of people have successfully visited outer space over the last 70 years, space travel has also seen its fair share of tragedy. (This is the number of space travelers every year since space travel began.)

To determine the worst disasters in space flight history, 24/7 Wall St. utilized a compilation of significant incidents and close calls in human space flight published by the National Aeronautics and Space Administration. The incidents on our list include those that led to injuries or deaths or the loss of missions or vehicles. Medical evacuations were excluded.

During the early days of space travel, the United States and the Soviet Union led the Space Race; hence, the majority of disasters in space flight have taken place in U.S.- and Soviet- or Russian-led projects. Many of the disasters occurred during training incidents in high-oxygen environments where a single spark could ignite a raging fire. Others occurred during launches or during the dangerous re-entry back into the earth’s atmosphere.

Click here to see the worst disasters in space flight history

Not all of the spacecraft involved were designed for entry into outer space. Some were participating in sub-orbital or atmospheric flights, which nevertheless require the use of high Mach speeds and can subject crew members to high g-forces. While some astronauts survived these disasters, as the crew of Apollo 13 famously did in 1970, others experienced in-flight breakup of their vehicles, or devastating crash landings. Here are the largest spacecraft to crash back to earth.