This is a pun on the phrase 'no comment' which is usually said by politicians when they wish to not comment on a cetain topic, but 'comet' is used in it's place because it sounds similar and because comets are a celestial object consisting of a nucleus of ice and dust and, when near the sun, a tail of gas and dust particles pointing away from the sun, which is something that astronomers study
A politician who had been an astronomer was always saying ' no comet ' .