NASA on Friday marked the 50^th anniversary of Apollo 1 fire accident that killed all the three American astronauts on board - Roger B. Chaffee, Virgil Gus Grissom, and Edward White. NASA marked the historic event by displaying the Apollo 1 capsule's "three-part hatch" at the famous Kennedy Space Center. 

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The three astronauts were trapped in this "three-part hatch" on the fateful day of Jan. 27, 1967. The astronauts made several unsuccessful attempts to come out of the hatch as soon as it was overwhelmed by thick smog, which later grew into a major fire that killed them.

The Apollo 1 fire accident is considered to be the first major accident in NASA's long, illustrious history.  It later became a major case study for the American space agency to avoid future space accidents.

Speaking about the accident, former Gemini and Apollo astronaut Tom Stafford said: "Had that accident occurred in space, we'd have never known exactly what had happened."    

The deaths of these "three great heroes ... helped save at least one other in flight, maybe two," he added.  

NASA's investigation into the accident found that there were several issues with the Apollo 1, mainly with its design including an "electrical wiring issue" and use of "flammable materials throughout the crew cabin."

Following the investigation, NASA successfully continued with its ambitious Apollo space mission that culminated in the "first human landing on the moon" with Apollo 11 mission. However, the Apollo 1 lessons could not stop some major "space tragedies" in the ensuing years.

On Jan 28, 1986, the Challenger space shuttle exploded immediately after take-off, killing all seven crew members. Almost two decades later, the space shuttle Columbia exploded while returning to Earth, killing seven astronauts.

While space analysts and astronomers say that space accidents have become rare, these accidents cannot be ruled out due to the complicated nature of our universe.