Controversy over comets and The Assayer

In 1619, Galileo became embroiled in a controversy with Father Orazio Grassi, professor of mathematics at the Jesuit Collegio Romano. It began as a dispute over the nature of comets, but by the time Galileo had published The Assayer (Il Saggiatore) in 1623, his last salvo in the dispute, it had become a much wider argument over the very nature of science itself. Because The Assayer contains such a wealth of Galileo's ideas on how science should be practised, it has been referred to as his scientific manifesto.

Early in 1619, Father Grassi had anonymously published a pamphlet, An Astronomical Disputation on the Three Comets of the Year 1618,which discussed the nature of a comet that had appeared late in November of the previous year. Grassi concluded that the comet was a fiery body which had moved along a segment of a great circle at a constant distance from the earth,and since it moved in the sky more slowly than the moon, it must be farther away than the moon.

Grassi's arguments and conclusions were criticized in a subsequent article, Discourse on the Comets,published under the name of one of Galileo's disciples, a Florentine lawyer named Mario Guiducci, although it had been largely written by Galileo himself.
Galileo and Guiducci offered no definitive theory of their own on the nature of comets,although they did present some tentative conjectures that are now known to be mistaken.

In its opening passage, Galileo and Guiducci's Discourse gratuitously insulted the Jesuit Christopher Scheiner,and various uncomplimentary remarks about the professors of the Collegio Romano were scattered throughout the work.
The Jesuits were offended,and Grassi soon replied with a polemical tract of his own, The Astronomical and Philosophical Balance,under the pseudonym Lothario Sarsio Sigensano,purporting to be one of his own pupils.

The Assayer was Galileo's devastating reply to the Astronomical Balance.
It has been widely regarded as a masterpiece of polemical literature,in which "Sarsi's" arguments are subjected to withering scorn.It was greeted with wide acclaim, and particularly pleased the new pope, Urban VIII, to whom it had been dedicated.

Galileo's dispute with Grassi permanently alienated many of the Jesuits who had previously been sympathetic to his ideas,and Galileo and his friends were convinced that these Jesuits were responsible for bringing about his later condemnation.
The evidence for this is at best equivocal, however.