On October 14 in the journal Nature, there was a scientific paper published on new studies about the nature of comet P/2010 A2 (LINEAR). What exactly is it – a unique main-belt comet, or the result of the first documented collision of two small asteroids?

A little history: the comet was discovered January 6, 2010 by the LINEAR automatic sky survey. After being placed on the confirmation page, it became clear that this was no ordinary object. It was not star-like. Furthermore, it did not have any apparent condensation resembling a cometary nucleus. In images, only a tail was visible, without the comet itself. This made astrometric measurement difficult. Immediately after its discovery, the discussion of the nature of the new comet began. They continue to this day, although everyone is beginning to lean toward the version of a collision of two asteroids, where the tail becomes nothing but a dust tail thrown off as a consequence of the collision.

The collisional nature of the formation of the “comet’s” tail was a result of follow-up studies. The time interval for the collision was determined – January to August of 2009. Additionally, the presence of large particles (diameter greater than 1 mm) was noted in the dust tail. That is rather large for ordinary comet dust tails.

New studies were based on the analysis of new images obtained with the help of the Rosetta spacecraft’s OSIRIS camera. Ground-based observational support was carried out at the 3.6-m NTT telescope (New Technology Telescope) at the ESO La Silla Observatory, and also at the 5-m Hale telescope at Mt. Palomar Observatory.

On the basis of numerical modeling of the ejection of dust particles, with consideration of numerous parameters and comparison of the results with the images, the scientists came to the conclusion that the tail formation happened before August of 2009. The greatest agreement between the calculated data and the images was achieved for a collision date of February 10, 2009 (+/- 5 days). From this model the particle sizes were also refined. The result supported the previous conclusion – the dust tail particles have a large size – from millimeters to centimeters and even larger. On the basis of these data the mass of the ejected material was also calculated – 3.7 x 108 kg, which comprises about 16% of the mass of the 120-m asteroid discovered earlier in the images taken with large telescopes. It is thought that this is one of the colliding asteroids. Most likely, the second, smaller object (probable diameter 6-9 meters), was completely destroyed as a result of the collision.

Based on data about the populations of main-belt asteroids, as well as the sizes of the colliding bodies, some conclusions were drawn about the frequency of collisions for a 120-meter sized asteroid – 1.2 billion years; that is, about one collision in 12 years. Since this collision is the first one documented and studied, with the coming on line of new generation surveys this figure will steadily grow.


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