The challenge, of course, is figuring out what’s going on.
At first, astronomers assumed Elst-Pizarro was indeed an asteroid that had merely suffered a collision with another one. By this reasoning, we were simply seeing a trail of debris. They even located the most likely candidate to have smashed into it: asteroid 427 Galene.
But that long dust trail displayed changes in both structure and brightness over time, which made no sense. When gravel falls off the back of a truck, it hardly keeps altering its appearance for 5 months.
Meanwhile, light-curve studies of Elst-Pizarro show that it spins rapidly, once every 3½ hours, and that its shed dust particles are the same size as those in cigarette smoke — extremely fine. This made less and less sense by the minute.
Without question, Elst-Pizarro is a mass-losing body and therefore deserves its comet name. But what’s the material, and why is it coming loose?
Nowadays, there are two competing hypotheses. The first says that 133P/Elst-Pizarro is indeed a comet, but one that is barely active because Jupiter captured it and flung it into its present location among the asteroids like a double agent. Its asteroid-like position and orbit are just coincidences.
The second explanation insists that 7968 Elst-Pizarro is a true asteroid of the main asteroid belt family, and that a recent collision unburied ice that had been lurking deep within it. Now it will generate a cometary tail at each perihelion passage until all the newly excavated ice is gone.
Bottom line: No one knows what it is. Take your pick.