The cause of asteroid Scheila's outburst

October 7, 2011, Europlanet
(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. The observations on 13 and 14 December were acquired at the IAC-80 telescope on the Observatorio de Izaña, Tenerife, the observation on the 17 at the 1.23-m telescope of Calar Alto Observatory, and the observation on the 29 was kindly provided by Joseph Brimacombe using the 0.1-m telescope of the New Mexico Skies Observatory.

( -- A remarkable discovery was made by astronomers on 12 December 2010: an asteroid named Scheila had changed its appearance and looked more like a comet, complete with bright tail. An international team of scientists have used innovative modeling techniques to support the idea that the cause was another object impacting Scheila, ejecting material from the asteroid. The lead scientist of this study, Fernando Moreno of the Instituto de Astrofísica de Andalucía in Granada, Spain, will present the group’s theory — complete with up-to-date and refined estimates of impact date and size — on Friday 7th October in Nantes, France at the joint meeting of the European Planetary Science Congress and the American Astronomical Society’s Division for Planetary Sciences.

Asteroid are rocks that primarily circle the Sun between the orbits of Mars and Jupiter. In contrast, comets tend to have highly elliptical orbits taking them from the frozen extreme edges of our Solar System to within distances so close to the Sun that solar radiation causes material in the to vaporize and stream out, forming the characteristic tails. That an asteroid was observed with a comet-like tail puzzled and required some inventive modeling to explain.

Moreno and his team charted the brightness of Scheila’s tail, noting how it dipped over the course of several weeks. They reached the conclusion that this unconventional tail was caused by another object smashing into Scheila and causing pieces to be blasted off.

Explains Moreno, “The model we used involves a very large number of particles ejected from Scheila. 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.”

It was initially thought that the impact event occurred between 11 November and 3 December 2010; further analysis to be presented at the EPSC-DPS Joint Meeting has now resulted in the date being refined to on or within three days of 27 November 2010. In addition, the latest calculations have dramatically increased the estimated size of the impactor, from 36m in diameter to 60 – 180m.

Moreno elaborates on how his team came to this conclusion: “We applied a scaling law that uses impact velocity to indicate the mass of the impactor and ejected material. 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 metres per second) and the size of the impactor.”

The model created correlates closely with observations taken over four days in December using   the IAC-80 telescope on the Observatorio de Izaña, Tenerife, the 1.23 metre telescope of Calar Alto Observatory, and the 0.1-m telescope of the New Mexico Skies Observatory.

Scheila itself is fairly large as asteroids go at around 110 kilometres across, but it evidently felt the smaller object’s punch.

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