Even after more than 60 years of investigation and debate, the fundamental question remains unresolved: what exactly are UFOs? Each of the many theories seems to explain one or more aspects of the phenomenon. Setting aside deliberate hoaxing, these, and the main points they address, can be summarised as follows:

* Natural or meteorological: explains the absence of widely-acceptable physical evidence

*Inter-dimensional travel / time travel / worm-holes: all take into account the vast size of interstellar space and the velocity limits imposed by the General Theory of Relativity

*Advanced human technology: the secrecy maintained by many governments and their apparent lack of interest in researching UFOs is understandable if they are their operators!

* Hallucination & psychological pathology: all of the above points, together with the abduction phenomenon and very close encounters, are explicable if we assume that the events occurred in the witnesses own minds.

There is also, of course, the explanation that was widely accepted at the start of the 'Flying Saucer' era: that UFOs are spacecraft operated by extra-terrestrial biological entities (EBEs) To many people - particularly those who grew up during the heady years of the 'Space Age' between 1957 and 1972 - this seems by far the most logical and credible explanation: we were making our first stumbling steps into Space: surely the ultimate expression of this 'outward urge' would be to visit other star-systems and contact other intelligent life forms. If our species, then why not others?

But, ironically, it is the exploration of near- Earth space that has caused many people to doubt the extra-terrestrial hypothesis: " If UFOs are spacecraft operated by alien astronauts, (the argument runs) then why don't our astronauts encounter them in Space? Or on the Moon?" So, in the final analysis, the 'ET Hypothesis' largely stands or falls upon this question: do US Astronauts and Soviet Cosmonauts believe in an extra-terrestrial origin for UFOs and have they encountered them during their missions? If the answer is 'No!' then it becomes almost impossible to support the 'alien spacecraft' identification.

Having been born in 1951, I have watched the beginning and the sad decline of human space exploration: from the age of six I was fascinated by rockets. I constructed my own increasingly complex devices, culminating in a liquid-fuelled missile when I was seventeen. Starting in 1961, I kept a diary of every manned mission and, so determined was I to become an astronaut, that ultimately I joined the Royal Navy as a pilot. Now, four decades later, as the owner of a space collectibles company, I have been privileged to meet many of my childhood heroes both professionally and socially.

The space travellers I have met, particularly those from the United States, are very patriotic. Although occasionally critical of the way some recent administrations have marginalised the space program, they generally support the official NASA line when discussing UFOs and other contentious issues in public. However, a few late-night sessions with a pitcher of margaritas or Tanqueray martinis have revealed some fascinating insights. The conclusion I have reached is that, during their missions many astronauts and cosmonauts have witnessed objects that defy immediate explanation.

Furthermore, since most of the pre-Shuttle spacemen have thousands of hours experience as test pilots or military aviators, it is perhaps unsurprising that a number have reported encountering UFOs in the atmosphere. Some intriguing (non-verbatim!) quotes:

James McDivitt, Gemini IV
In 1965, McDivitt witnessed an apparently metallic cylinder with antenna-like structures, which he attempted to photograph. When I asked why he didn't wake his fellow crew-member, Edward White, he replied that "...it wasn't in the mission profile. I was on watch, Ed was scheduled for a sleep period. You have to understand the military way of things: you do what's in the profile!" Later, however, both astronauts reported two egg-shaped object leaving glowing trails. McDivitt told me that the photographs of these on many websites are either fakes or merely show reflections in Gemini IV's window: he has never seen prints of the exposures he made, and feels it is likely they were over exposed due to the UFO's proximity to the Sun.

Scott Carpenter, Mercury 'Aurora 7'
Asked if it was true that he had reportedly stated: "At no time, when the astronauts were in space were they alone: there was a constant surveillance by UFOs." Carpenter replied that he had said '...something like that'. I asked if he was referring to the 'fire-flies' he and John Glenn had observed during their Mercury missions: these he dismissed as ice crystals or frozen urine. I have spent time with Carpenter on three separate occasions and asked for clarification: other than to confirm the quote above, he has not commented further. (Except to remark, in apparent contradiction, that he has never personally seen a UFO!)

James Lovell, Gemini V11, Gemini XII, Apollo 8, Apollo 13
I asked Lovell about the 'bogies' that were reported on his Gemini VII mission. He stands by the original statement that he and Frank Borman were not looking at the top stage of their Titan II launch vehicle, which they could distantly see at the same time. He also confirmed that he and Buzz Aldrin observed two 'bright unknowns' during their Gemini XII mission. Lovell laughs at the interpretation of his Apollo 8 communication about Santa Claus, remarking that the tongue-in-cheek comment was, after all, made on Christmas Day!

Gene Cernan: Gemini IX, Apollo 10, Apollo 17
When asked to confirm the reported sightings of UFOs during his three space missions, Cernan (paraphrasing his widely-published quote) said: "Look: I've publicly said that I believe they (UFOs) are real and that they come from someplace else: from some other technological civilization. There is too much evidence not to (believe that)"

Richard Gordon: Gemini XI, Apollo 12
I asked Gordon about the reported encounter with a UFO during the Gemini XI mission. He confirmed that both he and Pete Conrad had seen a bright object, which Conrad had photographed. Gordon stated that this had been identified by NASA as the Soviet Proton 3 satellite: he felt this was unlikely, since the object had visible length, which would not have been the case with a 3 metre long spacecraft 450 km away.

Dr Edgar Mitchell: Apollo 14 moonwalker
I have met Dr Mitchell a number of times and discussed the UFO phenomenon with him in a general way: he famously grew up in Roswell, NM. in October 2011, he amazed me by casually mentioning that he had been briefed that US intelligence agencies are actively working with four alien species, which he went on to describe! I asked if he would include this information in the next day's lecture: he agreed and did so!

My conclusions: Having broached the subject of extraterrestrial life and UFOs with a number of Astronauts, I personally believe that they have more to say on the subject than they feel comfortable about disclosing. Their natural reluctance to say or do anything controversial, or that would expose the Astronaut Corps to ridicule, prevents them from making speculative statements. This, in my view, makes the disclosures of space travellers such as Popovich, Mitchell, Cooper and Slayton that much more credible.

Astronauts / Cosmonauts who have confirmed a belief in the reality of the UFO phenomenon: The internet is crowded with alleged accounts of UFO encounters by space travellers from the US and former Soviet Union: the great majority of these are complete fabrications. The reticence shown by many of the Astronauts to discuss the subject is generally either because they have nothing to discuss or because they have grown weary of being misquoted. Occasionally, however, someone will confirm a belief in the possibility of extraterrestrial life or even admit to having personally witnessed a UFO. Here is a partial list: