Friday 1 June 2012

The origin of Comet P/2006 VW₁₃₉.

Comet P/2006 VW₁₃₉ is a Main-Belt Comet, one of a recently discovered group of objects, which inhabit the main Asteroid Belt, but which produce a tail similar to that of a comet when at their closest to the Sun. Asteroids and comets have traditionally been viewed as very different types of objects; asteroids being rocky objects of the inner Solar System, Comets being icy objects from the outer system that occasionally get thrown into the inner system, melting as they do so (strictly speaking sublimating, the solid ices turning directly to gasses in the vacuum of space) and generating snowy tails that point away from the Sun. 

Main-Belt comets do not fit into either of these neat categories, they inhabit the Main Asteroid Belt, but have eccentric orbits (i.e. orbits that are not perfect circles centered on the Sun) and when they are closest to the Sun produce comet-lie tails, suggesting that they are primarily icy objects, similar to comets.

The orbit of P/2006 VW₁₃₉. Green squares are sightings; (a) 30 August 2011, (b) 5 November 2011, (c) 12-14 November 2011, (d) 22 November-4 December 2011, (e) 16-19 December 2011, (f)  7 January 2012. (A) Represents the aphelion, the point in the orbit where the orbiting object is furthest from the Sun. (P) Represents the perihelion, the point in the orbit closest to the Sun. Also shown are the orbits of Mercury, Venus, Earth, Mars and Jupiter. The scale is in Astronomical Units (AU), one AU being equal to the average distance between the Earth and the Sun. Hsieh et al. (2012).

In a paper published on the arXiv database at Cornell University Library on 22 May 2012, and also due to be published in the Monthly Notices of the Royal Astronomical Society, Bojan Novaković of the Department of Astronomy at the Faculty of Mathematics at the University of BelgradeHenry Hsieh of the Institute for Astronomy at the University of Hawaii, and Alberto Cellino of the INAF–Osservatorio Astronomico di Torino, discuss the origin of P/2006 VW₁₃₉, and its relationship to other objects in similar orbits.

Novaković et al. compared the orbit of P/2006 VW₁₃₉ to that of 24 other objects in similar orbits, calculating their orbits backwards to look for a common origin. 19 of these objects were found to be in stable orbits; the remaining 5 were in unstable orbits, and therefore unlikely to have been there very long. These objects are most likely to have been shifted into their current orbits in the recent past (though this might mean a hundred thousand years or so) by encounters with other objects, and will most probably be shifted out of their current orbits within the next hundred thousand years or so. Of the remaining 19 objects, 11 could be traced backwards to a likely common origin with P/2006 VW₁₃₉, about 7.5 million years ago, in the breakup of a larger body, probably as the result of a collision. 

Furthermore the parent body probably belonged to a group of asteroids known as the Themis Family, which are thought to have originated in the breakup of a larger body about 2.5 billion years ago. This suggests that both the objects of the 'P/2006 VW₁₃₉ Family' and the Themis Family may also be substantially comet-like in composition, i.e. icy rather than rocky in composition.

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