Around the world, there are few people as beloved in their home country as Yuri Gagarin is in Russia.

But more than 50 years on, the circumstances behind his untimely death remain deliberately shrouded in mystery, designated classified by Vladimir Putin's government.

The Soviet Union caught the world off-guard when they announced in April 1961 that a man had been to outer space and back.

Yuri Gagarin just before his successful space flight. (AAP)

That man was Yuri Gagarin, a short and charming Air Force officer with a smile that, according to one rocket scientist, "lit up the darkness of the Cold War".

In the words of Purdue University's Michael Smith, a historian specialising in Soviet space exploration, Gagarin was "idolised by Russians".

"People wanted to know him, see him, photograph him, and be seen with him," Dr Smith told 9News.

"By the count of reliable public opinion polls in Russia today, he remains one of the greatest and admired Russians of recent history, right up there with Joseph Stalin and Vladimir Putin."

Yuri Gagarin became a beloved international celebrity after his successful trip to space. (AP)

Gagarin was charismatic for sure, but his celebrity in the USSR was rooted in something more substantial.

He was a "national hero" and more, according to Australian National University history fellow Leonid Petrov.

"He was considered to be the symbol of Soviet superiority against the West," Dr Petrov told 9News.

"He was considered a beacon of progress and the Soviet face of the space program."

Which is why his sudden death at the age of 34 rattled the USSR to its core – if the symbol of Russian superiority was dead, what did that mean for Russia?

The hero

The United States was well-and-truly unprepared for Russia's advances in the space race in the 1950s, with the launch and orbit of the first satellite Sputnik.

America was desperate to get a man into outer space with the Mercury program, but they were beaten again.

The Soviets rushed to launch, meaning there was an extraordinary amount of risk involved.

The Vostok 1 used to rocket Yuri Gagarin into space. (AAP)

Gagarin was chosen for his physical and psychological endurance, his mathematical nuance, his fantastic memory, and his height.

At 157cm (5ft 2), Gagarin was short enough to fit into the cramped Vostok 1 spacecraft cockpit.

"With Yuri Gagarin, people learned of it only on his return," Dr Petrov said.

Yuri Gagarin (second from left) and other cosmonauts with Soviet Premier Nikita Khrushchev in 1962. (AAP)

"There could have been some unsuccessful space programs, but that information is closed so we don't know what went wrong."

The Soviets were nervous enough about the impression of failure that they kept most of their launches a secret until they had been successful.

So when Gagarin landed back on Earth only a handful of people knew he had left.

The space program was so secretive that Gagarin's uniform bore Soviet insignia so Russian authorities would not mistake him for a US spy.

The crash

That secrecy about the space program lasts to this day, which is why historians know so little about Russian failure in space.

And while a 29-volume report on Gagarin's death at the age of 34 remains classified to this day.

"Everything that surrounded the final flight of Yuri Gagarin continues to be closed," Dr Petrov told 9News.

"What we know about his death continues to be covered in secrecy."

Yuri Gagarin was the first man in space. (AAP)

Seven years after returning to Earth, Gagarin was on a routine training flight when his Mig-15UTI crashed.

Gagarin and his flight instructor Vladimir Seryogin were killed.

But 51 years on, the world hasn't received a straight answer on why.

Dr Smith said the Russians kept the cause of his death a secret so as not to put the spotlight on the Soviet system.

A Mig-15 similar to the one Yuri Gagarin was flying when he died. (AAP)

"Telling the truth would have meant breaking the power of the official propaganda that had draped Gagarin for years," he said.

"Hence the secrecy and conspiracy against the truth. The state had to lie to protect its investment in the hero."

The official theories over his death are banal: unexpected weather conditions or birdstrike.

But neither Gagarin or his instructor deployed the ejector seats, which suggests something else.

The theories

As the nation mourned, conspiracy theories of Gagarin's death began to fill the void.

Russians speculated that Gagarin was flying drunk, or his plane was sabotaged.

Others suggested he survived and was committed to a psychiatric institution, or that he staged his own death to escape his celebrity status.

Of course, some have blamed aliens.

In 2010 independent investigators claimed Gagarin panicked when he realised an air vent was open in the cockpit, and the jet plummeted into a deep dive.

Yuri Gagarin's remains are carried by Soviet leaders as part of his funeral procession in Moscow. (AP)

"None of the weird conspiracy theories about his death ring true," Dr Smith said.

"Be they about Gagarin's reckless flying or some kind of sabotage."

The witness

A train on the Moscow Metro is painted to pay tribute to Yuri Gagarin. (AAP)

In 2011 the Kremlin released a declassified document saying Gagarin probably crashed after swerving to dodge a weather balloon.

But a few years later, perhaps the second-most famous Russian cosmonaut came forward with a witness account he had been forced to keep secret.

Aleksey Leonov was the first person to do a spacewalk, spending 12 minutes on the outside of the Voskhod 2 spacecraft in 1965.

And Leonov happened to be nearby on the day Gagarin died, taking part in the recovery of the wreckage and the search for his body.

Leonov is convinced there was another fighter jet flying dangerously close to Gagarin.

"We knew that a Su-15 was scheduled to be tested that day, but it was supposed to be flying at the altitude of 10,000m or higher, not 450-500m," he told RT in 2013.

Soviet cosmonaut Alexei Leonov making the first spacewalk in 1965. (AAP)

"It was a violation of the flight procedure."

Leonov played a part in writing the incident report, but he discovered decades later that his writing had been replaced with someone else's.

The end and beginning of a Cold War

The collapse of the Soviet Union initiated a new era of openness under President Boris Yeltsin.

Classified documents from the Soviet era began to be published, and there was hope answers would finally be provided into Gagarin's death.

Russian cosmonaut Alexei Leonov with Vladimir Putin in 2012. (AAP)

But under Yeltsin's replacement, Vladimir Putin , the historic archives snapped shut.

Communism may be over, but the Russian nationalism under Putin relies on reliving past glories.

"To rule over Russia today, the Putin system relies on the heroic memory and nostalgia of past Soviet successes, be it Stalin's victory in the Second World War or Gagarin's flight," Dr Smith said.

"All Putin's Russia seems to have these days, along with its militarisation and foreign wars, is misremembered history."

It is why the Kremlin vetoed a fresh investigation into Gagarin's death in 2007.

And why, 51 years after Gagarin was killed, the 29 volumes about the plane accident remain a tightly-held secret.

Vladimir Putin's Kremlin vetoed an investigation into Yuri Gagarin's death. (AAP)

Yuri Gagarin remains a national hero in Russia, and for the sake of the country's glory, his legacy will remain incomplete but untarnished.

Read more: The 1983 false alarm that nearly ended the world

Yuri Gagarin remains a beloved figure in Russia. (AAP)