NASA has marked the 50th anniversary of its moon programme's fatal Apollo launchpad fire with the first public display of the scorched hatch that trapped three astronauts inside their spaceship during a routine pre-launch test.

NASA astronauts Virgil "Gus" Grissom, Edward White and Roger Chaffee died when thick smoke filled the crew module of the Apollo 1 capsule on January 27, 1967, in what was the first deadly accident in the space agency's early days.

The men were unable to open the capsule's three-part hatch before being overcome by smoke.

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The Big Read: Fifty years ago, three astronauts died in the Apollo 1 fire

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The hatch has now been taken out of storage and incorporated into a new display at the Kennedy Space Centre Visitor Complex to honour the fallen astronauts and serve as a reminder of the risks of spaceflight.

"Had that accident occurred in space, we'd have never known exactly what had happened," former Gemini and Apollo astronaut Tom Stafford said at a ceremony to mark the exhibit's opening.

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

Investigators discovered several problems with the Apollo capsule design that led to the fire, including an electrical wiring issue, a pure-oxygen environment and flammable materials throughout the crew cabin.

NASA made dozens of changes and resumed flying in October 1968, setting the stage for the historic Apollo 11 moon landing in July 1969.

Friday's ceremony was one of several events this week in which NASA will pay tribute to the Space Shuttle Challenger crew, killed during launch on January 28, 1986, and the Shuttle Columbia astronauts, who died when that spaceship broke apart as it re-entered the Earth's atmosphere on February 1, 2003.

At twilight, the families of the Apollo 1 astronauts gathered at the base of the seaside launch complex where Grissom, White and Chaffee had been testing their capsule when the fire broke out.

"It's really important that we come together and we don't forget who they were and what they sacrificed. Even more important we remember that we don't ever want to have it happen again," Kennedy Space Centre director and former shuttle astronaut Bob Cabana told the families.

"The lessons learned from Apollo 1 are critical to our future success and I don't ever want them forgotten," Cabana said.

"We got to the moon not in spite of Apollo 1, but because of Apollo 1."