A UFO travelling at 3000mph in Glenavy was reported to the MoD in December 1989, newly released documents from the Ministry have revealed.
And while the MoD suggested the object was a 'helo' (helicopter), the person who spotted it dismissed such the suggestion.
Letters between the two sides have been released to the National Archives by the MoD as part of a series of files on UFO sightings across the UK.
A total of 19 files covering sightings between 1986 and 1992 have been made available online.
Reporting the sighting in Glenavy, the MoD said in a letter it would be investigated for security reasons.
They stated: "We have been advised that there were no reports of any terrorist activity.
"RAF Aldergrove has informed us that there were no fixed wing aircraft in the area at the time stated.
"Two rotary wing movements occurred within five minutes either side of the reported sighting.
"A Gazelle departed Aldergrove enroute to south east at 16.40 and a Wessex enroute from Lisburn landed at Aldergrove at 16.55.
"I suggest..probably wtinessed a the arrival or the departure of a helo at Aldergrove."
Such a suggestion was quickly dismissed and the correspondent said: "Thank you for your letter concerning the sighting of a UFO.
"I did not realise that a helicopter travels in excess of 3000mph at Aldergrove Airport."
In another case, details were revealed about UFO activity near Belfast International Airport in 1990 when it was reported that a muffled sound was heard.
In the letter the MoD suggested that the Civil Aviation Authority make a report and the RUC should also investigate the matter.
"If a muffled bang was heard there may be recoverable evidence...I cannot comment on there being some form of rational explanation of the sighting" says the file.
"We do not think we should draw on any conclusions or assumptions for the sighting until the RUC have carried out an investigation."
It went on: "HQ NI may be able to help solve the matter which may relate to the discharge of a weapon."