People who believe in aliens are often thought of as kooky conspiracy theorists.

But a small group of Nasa astronauts thought extraterrestrials exist and that they have been visiting Earth for some time.

At least four of the veteran spacemen became famous for their outspoken view on the existence of aliens.

Some said UFOs have been spotted whizzing over nuclear weapon silos while others claimed to have seen them in the flesh.

Here are four real Nasa astronauts who believed that aliens exist.

Scroll down for video

Edgar Mitchell (pictured) was the sixth person to walk on the moon in 1971 after travelling on the Nasa craft Apollo 14

CONTACT WITH ALIENS 'COULD END LIFE ON EARTH' Lucianne Walkowicz, an astrophysicist at the Adler Planetarium in Chicago, warned in February that making contact with aliens could be catastrophic for the human race. 'There's a possibility that if we actively message, with the intention of getting the attention of an intelligent civilization, that the civilization we contact would not necessarily have our best interests in mind,' she said.. 'On the other hand, there might be great benefits. 'It could be something that ends life on Earth, and it might be something that accelerates the ability to live quality lives on Earth. 'We have no way of knowing.' Advertisement

Edgar Mitchell

Edgar Mitchell was the sixth person to walk on the moon in 1971 after travelling on the Nasa craft Apollo 14.

He experienced an intense spiritual epiphany on his way back from the moon and his since dedicated his life to proving the existence of alien life.

He has claimed that aliens have previously saved humanity from a devastating nuclear war and suggested that the Vatican is covering up their knowledge of an alien race that is trying to share its secrets for a new source of energy.

Mitchell has said that aliens are often spotted above nuclear warhead silos and have stopped nukes from firing off during Cold War tensions.

The astronaut firmly believes that the US government is covering up the Roswell scandal in which a disk-like craft allegedly crashed near a small town in New Mexico.

'The reason for the denial is they didn't know if they [the aliens] were hostile and they didn't want the Soviets to know so they devised to lie about it and cover it up,' he reportedly said.

Mitchell died in hospital on February 4, 2016, at the age of 85 - the eve of the 45th anniversary of his lunar landing.

Mitchell, pictured left in 1970, has claimed that aliens have previously saved humanity from a devastating nuclear war and suggested that the Vatican is covering up their knowledge of an alien race that is trying to share its secrets for a new source of energy

Gordon Cooper

Gordon Cooper was one of seven astronauts selected for Nasa's first manned spaceflight mission.

Code-named Project Mercury, the mission ran from 1958 until 1963 and aimed to get a human into orbit.

Gordon Cooper (pictured) was one of seven astronauts selected for Nasa's first manned spaceflight mission. Code-named Project Mercury, the mission ran from 1958 until 1963 and aimed to get a human into orbit

The craft he flew in was named 'spam in a can' because it was automatically controlled rather than piloted by astronauts on board.

Cooper has claimed that he saw a UFO flying over Germany in 1951.

He also claims to have seen aliens at an experimental US airbase during his time with Nasa.

'I believe that these extraterrestrial vehicles and their crews are visiting this planet from other planets, which are a little more technically advanced than we are on Earth,' he told the UN in 1984.

'We may first have to show them that we have learned how to resolve our problems by peaceful means rather than warfare, before we are accepted as fully qualified universal team members.

'Their acceptance will have tremendous possibilities of advancing our world in all areas.'

Cooper developed Parkinson's disease aged 77 and died from heart failure on October 4 2004.

Cooper (pictured) has claimed that he saw a UFO flying over Germany in 1951. He also claimed to have seen aliens at an experimental US airbase during his time with Nasa

Astronaut Deke Slayton reportedly saw a UFO in 1951. Slayton is pictured here drinking a tube of Vodka given to him by Russian cosmonauts during the historic link up of the Apollo and Soyuz spacecrafts in 1975

Deke Slayton

Deke Slayton was also part of Nasa's Project Mercury, and climbed through the Space Agency's ranks before becoming Nasa's Director of Flight Crew Operations.

But he also reportedly saw a UFO in 1951.

'It looked like a saucer sitting at a 45-degree angle,' he said, according to the Daily Star.

'I didn't have any cameras otherwise I would have shot some pictures.

'At that time – for whatever reason – it just took off climbing and just accelerated and disappeared.'

Slayton was diagnosed with a malignant brain tumour in 1992 and died from the illness, at the age of 69, on June 13, 1993.

Slayton, pictured here in 1973, was also part of Nasa's Project Mercury, and climbed through the Space Agency's ranks before becoming Nasa's Director of Flight Crew Operations

Brian O'Leary

Brian O'Leary was shortlisted for a Nasa mission to Mars in 1967, though the programme was cancelled a year later.

A near-death experience later in his life changed O'Leary's perspective on extraterrestrials.

Dr O'Leary, who became a physics professor at Princeton University after leaving Nasa, said: 'There is abundant evidence that we are being contacted.

'Civilisations have been monitoring us for a very long time and that their appearance is bizarre from any type of traditional materialistic western point of view.'

O'Leary died of intestinal cancer on July 28, 2011, soon after diagnosis, at his home in Vilcabamba, Ecuador.