The email said: 'Do not think for one moment that a space agency would suddenly decide to spend billions of dollars to build and send a spacecraft on a 12-year journey to simply take some close-up images of a randomly-picked-out comet floating in space.
'Comet 67P is not a comet.'
WHY ARE COMETS IMPORTANT?
Comets are of great interest to scientists because they are the oldest and most primitive bodies in the solar system that we know of.
They preserve the earliest record of material from the nebula from which our sun and planets formed.
While the planets in the solar system have gone through chemical transformations, comets have remained almost unchanged in the 4.5 billion year history of the solar system.
It is also thought that they brought certain elements to planets, including water.
They also carry complex organic molecules that may triggered life on Earth.
The whistle blower, who insisted on remaining anonymous for fear of reprisals, insisted the comet 'is only being disguised as such' and appears able to change trajectory at will.
The letter added: 'As for who is disguising it and why I cannot answer; all I can tell you is that whatever this thing is, it does show signs on its outside of machine-like parts and unnatural terrain.
'Whatever this object is it did not ask to be found or scrutinised, and it appears for good reason.'
Other theorists spoke of 'alien structures' in images of the comet, in particular a bright white 'metallic' disc on or hovering above the 2.5 mile (4km) wide lump of ice and dust, and a sculpted non-human face.
Esa unwittingly sent the speculation into overdrive when it disclosed that the comet was 'singing'. Clicking oscillations in the object's magnetic field, below the range of human hearing, sounded strangely tuneful.
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