Most known comets come from either the Kuiper belt (beyond Neptune) or the Oort cloud (out towards the stars).  So-called Halley type comets (HTCs) are a very under-studied group intermediate between these populations. It is not clear if they are more similar to the Kuiper belt or the Oort cloud comets, and their origins remain unknown. In a new paper (in press at The Astronomical Journal) David Jewitt and colleague Jane Luu used several telescopes to compare HTC 12P/Pons-Brooks with the class prototype 1P/Halley.  Surprisingly, although their orbits are very similar, 1P and 12P behave completely differently.  Starting at 4 au from the Sun, 12P exhibited a series of massive explosions caused by supervolatile ice (probably CO or CO2), each of which gave the comet a peculiar horned appearance. By comparison, activity in comet Halley varied smoothly on approach to the Sun, with no similar outbursts or morphology.  The trapping and long-term preservation of supervolatiles are only possible at very low temperatures.  One possibility is that the two comets were formed at different places and temperatures in the Sun's protoplanetary disk, such that 1P either never had or has already lost most of its supervolatile material.  Future measurements of other HTCs are needed to reveal the true diversity of this enigmatic population. - Dave