© Fernando Moreno
(Left to right): images of (596) Scheila corresponding to 2010 December 13, 14, 17, and 29. The upper row corresponds to the observations, while the lower row to the models. The tails clearly show a bifid pattern with a central spike in the sunward direction, although it is not detectable in the December 29 image. Except for this latter case, the modeled images are rendered using the same color code for the intensities as the corresponding observed images in the top row.

On December 12, 2010, something very unusual happened to asteroid Scheila. For a short period of time, its appearance changed dramatically and it even developed a comet-like tail. Now a group of international scientists headed by Fernando Moreno of the Instituto de Astrofísica de Andalucía in Granada, Spain have created a computer model which may explain this weird activity... an impact.

In results revealed October 7th in Nantes, France at the joint meeting of the European Planetary Science Congress and the American Astronomical Society's Division for Planetary Sciences, the team explained their theory of how this innocent asteroid may have been crashed into by a smaller object. Moreno and his team plotted the brightness curve of Scheila's newly developed "tail" - watching how it declined over a period of weeks. Their conclusion was that Scheila was either responsible for bumping into an uncatalogued object - or the object bumped into it causing a debris trail.

"The model we used involves a very large number of particles ejected from Scheila." explains Moreno. "We took into account gravity from the Sun, pressure radiation on the ejected particles, and Scheila´s gravity, which has a strong effect on the particles in its vicinity owing to its large mass."

Just when did this crash occur? The first indications placed the "asteroid accident" at a period of somewhere between November 11 and December 3, 2001. But, thanks to refined studies the team has placed the smash-up to on - or within - three days of November 27, 2010. With a size of about 110 kilometers across, Scheila isn't very large and the impactor was estimated to be anywhere from 60 to 180 meters in diameter. That's quite enough to send visible pieces flying into space!

"We applied a scaling law that uses impact velocity to indicate the mass of the impactor and ejected material." concludes Moreno. "We know the impact should be about 5 kilometres per second because that's the average velocity of asteroids in the Main Belt. Using this number we predicted both the ejection velocity of the particles (50 to 80 meters per second) and the size of the impactor."

As for asteroid Scheila, she's also a step off the beaten path, too. It belongs to a class known Main-Belt Comets - objects which have orbital characteristics of Main-Belt Asteroids - but sometimes behave like a comet. The reason why they have outbursts still isn't clear. While these new modeling techniques may lend credence to the impact theory, there's also a strong possibility of gaseous emissions. However, astronomers from the University of Maryland and Institute for Astronomy, University of Hawaii have ruled out venting in Scheila's case.

Source: EuroPlanet News.