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On 13th December last year we ran a
news item reporting the sudden increase in brightness of 596 Scheila, supposedly an asteroid, but whose behaviour led to suspicions that it might actually be cometary in nature, despite its stable orbit in the asteroid belt.
In an attempt to resolve this strange case, researchers have studied the object using spectra and images obtained with the Swift satellite (left) and the Hubble Space Telescope (right), and have concluded that Scheila is indeed an asteroid, and that the temporary burst in luminosity was caused by an impact with a large meteorite.
As described in the article to be published on 20th May in The Astrophysical Journal Letters, the object that collided with the asteroid probably had a diameter of about 30 metres, and with an impact velocity of around 17,000 km/h, would have made a crater about 300 metres in diameter. This released over 660 thousand tons of material (dust and rocks of various sizes) that then formed an arc-shaped plume around Scheila.
The asymmetrical shape of this plume implies that the impact was at an angle of about 30 degrees, rather than direct.
The mass of material released by the collision was 10 thousand times more than released by the Deep Impact probe from comet 9P/Tempel 1.
The spectroscopic analysis of the plume, that accompanied the asteroid for about a month before dispersing, showed no trace of the molecules typically produced in comets when ultraviolet solar radiation breaks volatile molecules that have sublimed from the nucleus.
This means that the event on Scheila could not have been caused by exposure to solar radiation of surface ices, and the only other explanation is an impact with another asteroid.
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