The Comet anomaly functions much like a Burner anomaly in that it deals fire damage on close contact, but with a key difference in that the Comet is mobile, hovering around within a certain area at a rapid pace.
The Comet anomaly, unlike the Burner, is extremely powerful: it can kill even a well-armoured stalker if he gets hit directly.
This anomaly does not spawn any kind of artifact.
Shadow of Chernobyl
Call of Pripyat
Comets are found in Zaton and Pripyat, with four at the former and one at the latter. The first two are orbiting around the Circus anomaly field and the last two are in the Burnt farmstead underground. The fifth comet is located in the upper level of the Department Store Basement.