Orazio Grassi  was born in Savona in 1583 and died in Rome in 1654. He was a Jesuit, a mathematician and an architect well known in history for his strong ideologic battle against Galileo Galilei about the comets origin. The book in which he disputed his thesis was Libra Astronomica et Philosophica published in 1619. The occasion was the discovery of three comets in 1618, one of which was particularly bright: these visions brought up in all astronomers the will to formulate original theories on the comets origin. Galileo had a bright as much as fanciful idea: according to him a vanishing substance had to evaporate from the Earth and to be lightened by the Sun to give birth to a comet! Orazio Grassi was the only one that at first objected harshly to his theory ...and history gave him reason. Grassi had in fact focused the argument with a theory that unconsciously took again all the points that will be demonstrated by Newton and Halley two centuries later.A large and lively correspondence between the two protagonists, where offences and imprecations are at large, testifies the struggle of the disputation: our Grassi used the pen-name of Lothario Sarsi, while Galileo published Il Saggiatore, his most important essay on the subject.