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The cause of asteroid Scheila's outburst

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 modelling 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.
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Title: Interpretation of (596) Scheila's Triple Dust Tails
Authors: Masateru Ishiguro, Hidekazu Hanayama, Sunao Hasegawa, Yuki Sarugaku, Jun-ichi Watanabe, Hideaki Fujiwara, Hiroshi Terada, Henry H. Hsieh, Jeremie J. Vaubaillon, Nobuyuki Kawai, Kenshi Yanagisawa, Daisuke Kuroda, Takeshi Miyaji, Hideo Fukushima, Kouji Ohta, Hiromi Hamanowa, Junhan Kim, Jeonghyun Pyo, Akiko M. Nakamura

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high velocity impacts and the dust dynamics. As the result, we succeeded in the reproduction of peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 deg, our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of (alpha, delta) = (60deg, -40deg) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m/s. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.

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Title: Optical and Dynamical Characterisation of Comet-Like Main-Belt Asteroid (596) Scheila
Authors: Henry H. Hsieh, Bin Yang, Nader Haghighipour

We present observations and a dynamical analysis of the comet-like main-belt object, (596) Scheila. V-band photometry obtained on UT 2010 December 12 indicates that Scheila's dust cloud has a scattering cross-section ~1.4 times larger than that of the nucleus, corresponding to a dust mass of M_d~3x10^7 kg. V-R colour measurements indicate that both the nucleus and dust are redder than the Sun, with no significant colour differences between the dust cloud's northern and southern plumes. We also undertake an ultimately unsuccessful search for CN emission, where we find CN and H2O production rates of Q(CN) < 9x10^23 s^-1 and Q(H2O) < 10^27 s^{-1}. Numerical simulations indicate that Scheila is dynamically stable for >100 Myr, suggesting that it is likely native to its current location. We also find that it does not belong to a dynamical asteroid family of any significance. We consider sublimation-driven scenarios that could produce the appearance of multiple plumes of dust emission, but reject them as being physically implausible. Instead, we concur with previous studies that the unusual morphology of Scheila's dust cloud is most simply explained by a single oblique impact, meaning this object is likely not a main-belt comet, but is instead the second disrupted asteroid after P/2010 A2 (LINEAR) to be discovered.

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Title: Near Infrared Observations of Comet-Like Asteroid (596) Scheila
Authors: Bin Yang, Henry Hsieh

Asteroid (596) Scheila was reported to exhibit a cometary appearance and an increase in brightness on UT 2010 December 10.4. We used the IRCS spectrograph on the 8-m Subaru telescope to obtain medium-resolution spectra of Scheila in the HK-band (1.4 - 2.5 \mu m) and low-resolution spectra in the KL-band (2.0 - 4.0 \mu m) on UT 2010 December 13 and 14. In addition, we obtained low-resolution spectroscopy using the SpeX spectrograph on the 3-m NASA Infrared Telescope Facility (IRTF) telescope on UT 2011 January 04 and 05. The spectrum of Scheila shows a consistent red slope from 0.8 to 4.0 \mu m with no apparent absorption features, resembling spectra of D-type asteroids. An intimate mixing model suggests that the amount of water ice that might be present on the surface of Scheila is no more than a few percent. The spectrum of the Tagish Lake chondrite matches the asteroid's spectrum at shorter wavelengths (\lambda < 2.5 \mu m), but no hydration features are observed at longer wavelengths on Scheila. Our analysis corroborates other studies suggesting that the comet-like activity of Scheila is likely not caused by the sublimation of water ice. The dust coma and tail may be results of a recent impact event.

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Late last year, astronomers noticed that an asteroid named Scheila had unexpectedly brightened and it was sporting short-lived plumes. Data from NASA's Swift satellite and Hubble Space Telescope show that these changes likely occurred after Scheila was struck by a much smaller asteroid.

Swift and Hubble Probe an Asteroid Crash 

On Dec. 11, 2010, images from the University of Arizona's Catalina Sky Survey, a project of NASA's Near Earth Object Observations Program, revealed the Scheila to be twice as bright as expected and immersed in a faint comet-like glow. Looking through the survey's archived images, astronomers inferred the outburst began between Nov. 11 and Dec. 3.

Three days after the outburst was announced, Swift's Ultraviolet/Optical Telescope (UVOT) captured multiple images and a spectrum of the asteroid. Ultraviolet sunlight breaks up the gas molecules surrounding comets; water, for example, is transformed into hydroxyl (OH) and hydrogen (H). But none of the emissions most commonly identified in comets -- such as hydroxyl or cyanogen (CN) -- show up in the UVOT spectrum. The absence of gas around Scheila led the Swift team to reject scenarios where exposed ice accounted for the activity.



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NASA's SWIFT and Hubble Probe Asteroid Collision Debris

Late last year, astronomers noticed an asteroid named Scheila had unexpectedly brightened, and it was sporting short-lived plumes. Data from NASA's Swift satellite and Hubble Space Telescope showed these changes likely occurred after Scheila was struck by a much smaller asteroid.
This visible-light image of asteroid (596) Scheila was taken with Hubble's Wide Field Camera 3 on Dec. 27, 2010, when the asteroid was approximately 217 million miles away. The asteroid is surrounded by a C-shaped cloud, or coma, of dust particles. The asteroid body, which is approximately 70 miles across, appears star-like because it is overexposed so that the faint dust can be imaged.

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Title: Collisional Excavation of Asteroid (596) Scheila
Authors: D. Bodewits, M.S. Kelley, J.-Y. Li, W. B. Landsman, S. Besse, M. F. A'Hearn

We observed asteroid (596) Scheila and its ejecta cloud using the Swift UV-optical telescope. We obtained photometry of the nucleus and the ejecta, and for the first time measured the asteroid's reflection spectrum between 290 - 500 nm. Our measurements indicate significant reddening at UV wavelengths (13% per 1000 {\AA}) and a possible broad, unidentified absorption feature around 380 nm. Our measurements indicate that the outburst has not permanently increased the asteroid's brightness. We did not detect any of the gases that are typically associated with either hypervolatile activity thought responsible for cometary outbursts (CO+, CO2+), or for any volatiles excavated with the dust (OH, NH, CN, C2, C3). We estimate that 6 x 10^8 kg of dust was released with a high ejection velocity of 57 m/s (assuming 1 {\mu}m sized particles). While the asteroid is red in colour and the ejecta have the same colour as the Sun, we suggest that the dust does not contain any ice. Based on our observations, we conclude that (596) Scheila was most likely impacted by another main belt asteroid less than 100 meters in diameter.

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Title: Hubble Space Telescope Observations of Main Belt Comet (596) Scheila
Authors: David Jewitt, Harold Weaver, Max Mutchler, Stephen Larson, Jessica Agarwal

We present Hubble Space Telescope Observations of (596) Scheila during its recent dust outburst. The nucleus remained point-like with absolute magnitude V(1,1,0) = 8.85±0.02 in our data, equal to the pre-outburst value, with no secondary fragments of diameter =>100 m (for assumed albedos 0.04). We find a coma having a peak scattering cross-section ~2.2x10^4 km^2 corresponding to a mass in micron-sized particles of ~4x10^7 kg. The particles are deflected by solar radiation pressure on projected spatial scales ~2x10^4 km, in the sunward direction, and swept from the vicinity of the nucleus on timescales of weeks. The coma fades by ~30% between observations on UT 2010 December 27 and 2011 January 04. The observed mass loss is inconsistent with an origin either by rotational instability of the nucleus or by electrostatic ejection of regolith charged by sunlight. Dust ejection could be caused by the sudden but unexplained exposure of buried ice. However, the data are most simply explained by the impact, at ~5 km/s of a previously unknown asteroid ~35 m in diameter.

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Weird Asteroid Really a Crusty Old Comet?

A large asteroid known for more than a century appears to actually be a comet in disguise, astronomers say.
Most asteroids are chunks of metallic rock that have virtually no atmospheres. Tens of thousands of asteroids circle the sun inside what's known as the main asteroid belt, a doughnut-like ring that lies between the orbits of Mars and Jupiter.
By contrast, most comets are loose clumps of dirt and ice thought to originate in the Kuiper belt, far beyond the orbit of Neptune. When a comet's oval-shaped orbit brings it close to the sun, its ices vaporise and the comet develops its signature halo of gases and dust.

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Using a small telescope atop Mount Bigelow, University of Arizona astronomer Steve Larson found a rare phenomenon in the night sky - an asteroid that suddenly developed a tail.
This recently recognised class of objects - cometoid, anyone? - is interesting enough to the astronomy community that it snared "discretionary" time Monday night on the Hubble Space Telescope.

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