For many wannabe astronauts, the idea of venturing into the great unknown would be a dream come true. But over the past 50 years, there's been a slew of spaceflight-related tragedies that are more akin to an astronaut's worst nightmare.

In the last half-century, about 30 astronauts and cosmonauts have died while training for or attempting dangerous space missions. But the vast majority of these deaths occurred either on the ground or in Earth's atmosphere — below the accepted boundary of space called the Kármán line, which begins at an altitude of about 62 miles (100 kilometers).

However, of the roughly 550 people who have so far ventured into space, only three have actually died there.

The fatal frontier

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Early in the space race, both NASA and the USSR experienced a surge in deadly jet crashes that killed a number of pilots testing advanced rocket-propelled planes. Then, of course, there was the Apollo 1 fire in January 1967, which killed astronauts Gus Grissom, Ed White, and Roger Chaffee in a horrific manner. During a launch simulation, a stray spark within the cabin of the grounded spacecraft, which was filled with pure oxygen, ignited. This led to an uncontrollable fire that quickly overwhelmed the doomed crew, leading to their tragic deaths as they struggled in vain to open the pressurized hatch door.