(596) Scheila – the asteroid that became a comet

A week ago, Steve Larson of the Catalina Sky Survey came across a bright but peculiar looking comet. At 13th magnitude it was strange to find a new comet that bright in a part of the sky that had been picked over numerous times in the past month or two. Even weirder was the appearance of the comet, a bright stellar core with 2 tails, one straight and one curving almost 180°. But the oddest thing about this new find is that the object isn’t new at all. In fact, it had been discovered over 100 years ago back in 1906. Only from the time of discovery till two night ago it was known as asteroid (596) Scheila.

Up until ~15 years ago, the line between comets and asteroids was sharp and easy to see. Asteroids were rocks baked dry by billions of years of orbiting relatively close to the Sun. Comets were dirty snowballs born in the outer reaches of the Solar System. Sure there were a few objects that straddled the line such as Comets Encke and Wilson-Harrington but those objects could be hand-waved into one camp or the other.

But in 1996 the discovery of Comet Elst-Pizzaro changed all that. This comet was on a boring Main Belt asteroid orbit and probably had been for most of the history of the Solar System. Here was an example of an asteroid that was not bone dry but had, somehow, retained some volatiles over the past 4.5 billion years. It wasn’t long before more ‘Main Belt Comets’ were found: 176P/LINEAR, 238P/Read, P/2008 R1 (Garradd), P/2010 R2 (La Sagra). And now we get to add the large, 100+ km asteroid (596) Scheila to the list. [Note, that though Comet P/2010 A2 (LINEAR) is also on a Main Belt orbit, recent studies suggest its cometary appearance was the result of an asteroid-to-asteroid collision rather than cometary outgassing.]

I was finally able to get an image of this object a few nights ago. The ghostly ‘wings’ extending out from Scheila is dust ejected during its outburst. Based on images from the Catalina Sky Survey, the outburst occurred a day or two before December 3. So the image below was taken 11-12 days after the outburst.


3 thoughts on “(596) Scheila – the asteroid that became a comet

  1. Michael Watson

    *Can* an object at as great a distance from the Sun as 3.1 AU, like 596 Scheila, have enough sublimating ice to create a substantial tail like this?

    1. Carl Hergenrother


      Sure can. The question really isn’t could an object at a distance of 3.1 AU from the Sun have enough ice to create Scheila’s dust tail, but rather, should an object that has spent so much time this close to the Sun still have ice.

      There are plenty of comets that have been observed to possess large comae and tails at much further distances. Hale-Bopp was observed to be active at a distance of 25.7 AU (at this distance the activity appears to be driven by CO ice). Also there a large number of active comets that never get closer to the Sun than 3.1 AU.

      Scheila joins a rapidly growing list of objects in the Main Belt that appear to have substantial reservoirs of ice. In addition to the other cases of activity from Main Belt comets, the spectral signature of ice has recently been detected on the surfaces of 2 large carbonaceous outer Main Belt asteroids, (24) Themis and (65) Cybele. It’s starting to look like all carbonaceous asteroids may be ice-rich bodies.

      – Carl

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