Flight Lieutenant Stan Hubbard's Farnborough discs

14 August 1950, although some accounts give 15 August. 11:27 in the morning. Hubbard and two other officers were on the airfield when they heard a subdued humming noise, "like a model Diesel motor." Hubbard, the only one wearing sunglasses, looked up. Almost directly overhead was a flat disc, light pearl grey, which he put at about 50 feet in diameter and 5,000 feet up.

It flew a series of S-turns. As it oscillated, light glinted off different parts of it. It crossed the sky in about 30 seconds on a heading of 100 degrees, at an estimated 800 to 1,000 mph. The two others heard the noise. Only Hubbard saw the object.

Decades later he described it to the historian David Clarke. "It looked like an edge-on view of a sports discus." It had a mother-of-pearl quality, no windows or portholes, and "a mass of tiny crackling, sparkling lights" around the edge. He remembered a heavy humming, a crackling hiss and a strong smell of ozone. In that later account the disc was about 100 feet across and only 700 to 1,000 feet up.

5 September 1950. 16:09. Hubbard stood on the RAE watch-tower with five other RAF officers, waiting for a display by the Hawker P.1081 prototype. Then a disc appeared low in the sky, in the direction of Guildford and Farnham.

In Hubbard's words, it was "fluttering, as though bordering on instability, in a hovering mode." Then it would "swoop off in a slight dive at incredibly high speed and in quite stable flight, then stop abruptly and go into another fluttering hover mode." It performed a strange falling leaf motion before diving towards the horizon. The sighting lasted about 10 minutes. By the end, according to Clarke, the little crowd had swelled to more than a dozen RAF personnel.

Both events went to the Ministry of Defence's Flying Saucer Working Party, a secret committee of scientists and intelligence officers set up under Sir Henry Tizard. Asked what he thought it was, he said it "was not something that had been designed and built on this Earth." He later added: "clearly, from the effect it had on the team, it was the wrong answer." The witnesses were told not to discuss the matter, even privately. Hubbard said the subject was described to him as "highly classified."

The committee's final report, DSI/JTIC Report No. 7, came in June 1951. It found it "impossible to believe" that an unconventional aircraft could cross a populous and air-minded district like Farnborough without attracting the attention of more than one observer. Hubbard, it concluded, was the victim of an optical illusion or had misjudged a normal aircraft. The tower group, it said, saw some quite normal aircraft at extreme range and were led by the previous report to believe it abnormal.

Hubbard's answer was blunt. "Absolute rubbish. My engineering experience convinced me it was not of this earth." Sceptics note that his own size and height estimates changed between 1950 and 2002. Others point to the two officers who heard the hum, and the trained aircrew who watched the second object for ten minutes.

The report stayed secret for about 50 years. In 2001 the MoD released it to David Clarke and Andy Roberts, who met Hubbard in 2002. Hubbard died in 2014, aged 93. Late in life he said the events made him "question everything I know about the rules of physics."