The suspicious nature of Tycho Brahe's death in 1601, and Johannes Kepler's possible role in his end, constitutes one of history's greatest unsolved murder mysteries.  The question has produced many claims from the plausible to the extraordinary, but it appears that while Kepler may in fact have had some motive for killing his superior, the evidence does not definitively point to him.  Kepler's involvement is just one of three reasonable theories.  The second suggests that his death was accidental; the final theory proposing that the perpetrator was in fact his cousin, Erik Brahe, working as a part of a broader conspiracy.

Tycho Brahe (1546-1601) was the famous and gifted Danish astronomer of the sixteenth century who contributed to the beginning of modern astronomy.  He made key observations of Mars and many stars, using his own personal observatory but without the use of a telescope (which had yet to be invented), and argued for his own version of the Copernican model.  Towards the end of his life, Brahe gained a notable assistant for his work on Mars, Johannes Kepler (1571-1630).  However, the two did not enjoy a particularly friendly relationship due to Brahe's condescending attitude as well as his withholding of the mass of his data about Mars from Kepler.  While it may be unlikely that Kepler desired to kill Brahe for any insults suffered, some wonder whether if Kepler may have killed him to obtain his data, which Kepler needed to validate his own theories.  Indeed, Kepler's seizure of Brahe's work after his death confirms this suspicion for many.  But at the same time, Brahe's efforts to have Kepler made an imperial mathematician under Holy Roman Emperor Rudolf II, in part to advance his own agenda, complicates these calculations as Kepler likely would not have eliminated his crucial, reputable supporter for the position if he truly wanted the job.

The common explanation for Brahe's death at the time—the one that Kepler himself put forth—was that at the banquet at the Holy Roman Emperor's court in Prague that Brahe attended several days before his death, Brahe desperately had to urinate but did not do so, not wanting to be leave and be impolite.  This caused some sort of urinary infection which killed him eleven days later.  This tale has been overshadowed by the 1991 discovery of high levels of mercury present in Brahe's body in the last few days of his life.  (As mercury is well known to be quite dangerous to humans, the discovery has in turn led to the belief that Brahe’s cause of death can be traced back to the kidneys rather than the bladder.)  For those who believe that Kepler killed Brahe, given the questionable story he offered, his physical proximity in Brahe's final days, and his actions after Brahe's death, the discovery of the mercury levels appeared to provide confirmation of a murder and even an indication of the murder weapon.

Joshua and Anne-Lee Gilder, authors of Heavenly Intrigue: Johannes Kepler, Tycho Brahe and the Murder Behind One of History's Greatest Scientific Discoveries and promoters of the Kepler-as-murderer theory have suggested that there was a second, heavier and ultimately lethal dose of mercury given by Kepler, who would have had access to Brahe's laboratory and chemicals.  However, the fact of mercury poisoning is not enough to convict Kepler.  Kepler could have been accidentally poisoned—it does happen, even today.  Brahe did deal with mercury and other chemicals in his experiments, and at the end he may have ingested mercury in the hope that it would cure him of the ailment that befallen him at the banquet.

Finally, there’s an even more thrilling plot proposed by Peter Andersen, who claimed that Brahe's Swedish cousin, Erik Brahe, at the behest of the Danish king Christian IV.  While Tycho had enjoyed the blessing and support of the previous king of Denmark, who had provided Tycho with the island on which he had built his research institute, his fortunes reversed after the ascension of Christian IV, who had Tycho's castle seized and destroyed.  This, in turn, prompted Tycho to seek the support of the Holy Roman Emperor at whose banquet he fell ill.  The exact cause behind Christian IV's blatant hostility is unknown, but a popular rumor of the time was that Brahe had had an affair with Christian's mother, posing a serious threat to Christian's legitimacy and security as king. Indeed, Erik Brahe, a most dissolute and disreputable figure, met with Christian IV and other enemies of Tycho shortly before Tycho fell ill, and his presence in Tycho's home in the final days of his life (especially after only having become familiar with Tycho earlier that year) and some very questionable entries in Erik's diary around the time point to him as a potential suspect.

While Kepler certainly could have killed Tycho Brahe, it’s also possible that Brahe’s own cousin did the terrible deed. And, of course, it really could have been an accident, prompted by Brahe’s own overly acute sense of decorum.  In the absence of more convincing and definitive evidence, it may never be known if Kepler had a role in his superior's downfall, or indeed if there was a role to be had at all.