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The Art of Breaking Up: Fragmentation vs. Disintegration
Originally written 2013 Nov 28:
The enigma of C/2012 S1 (ISON), in my opinion, stems from the fact that it has not behaved as predicted nor like any other comet that has been observed recently. From the (now believed) initial outburst when discovered in September 2012, this comet has decided to march to its own drummer. The many faces of C/ISON are rather intriguing and only add to its mystique. Its quiescent phase from May through September resembled a possible Kohutek-like scenario, to its recent outburst, followed by considerable brightening, seem to have put it back on the predicted path of becoming a bright object with magnitude reaching 3.7 before perihelion and hoped for post-perihelion, and comparisons to other comets along the way. However, the continued nagging doubt about its possible fragmentation cannot be completely shrugged off either. Why? Please read on.
The phrase cometary fragmentation immediately brings to mind the beautiful “string of pearls” image of the D/Shoemaker-Levy 9 (SL9) and D for Dead comet, that suffered in multiple ways: the parent comet fragmented in its previous pass by Jupiter and the fragments, all on the same original orbit, impacted Jupiter on its next pass by the planet. Of the total 21 fragments, only few survived to fall to their deaths in the Jovian atmosphere over a period of week (16 – 22 July, 1994). The individual fragments were no larger than 2km or 1.2 mi in diameter, with the scars left on Jupiter persisted for many months.
Other forms of fragmentations were exhibited with comets 73P/Sshwassmann-Wachmann3 (SW3) and 17P/Holmes. Comet 73P/SW3, discovered in 1930, is a periodic comet that returns once every 11 years, started to fragment in 1995 as it went by the sun and finally broke into 66 pieces, of which fragments C is considered to be the largest. 73P/SW2-C is expected to be recovered in June 2017. All fragments follow the original orbit of the parent body, including dust from disintegrating. Comet 17P/Holmes was a late bloomer, discovered in 1862, went through an enormous outburst in 2007, its coma becoming the largest solar system object and a naked eye object, with recovery slated for March 2014. All these comets illustrate that by fragmentation, we implicitly refer to macroscopic particles or chunks of material that remember the parent body’s orbit and properties.
Disintegration is not very different from fragmentation, but occurs on different spatial and temporal scales. It can be a continuous process triggered by internal and external causes. Comet C/1973 E1 (Kohutek), dubbed as the Comet of the Century, was a long-period Oort cloud comet and possibly on its initial passage through the inner solar system. However, it did not live up to its hype, even though it became a naked eye object. C/2010 X1 (Elenin) is an example of a comet that showed promise yet disintegrated after being hit by a massive solar coronal mass ejection (CME). Finally, but not the least, is the example of the famous sungrazer to date, C/2011 W3 (Lovejoy), discovered by Terry Lovejoy (Queensland, Australia), a long- period, Kreutz family member. Although it was affected by its passage around the sun at its perihelion, C/2011 W3 emerged post-perihelion, and eventually its nucleus disintegrated/fragmented a few days later (“headless comet”), yet displayed a beautiful and extremely long curved tail, observable only in the Southern Hemisphere.
Given all these examples, what is the state of C/ISON? Is it fragmenting or disintegrating? There is suggestion of possible nuclear disintegration, given the low amounts of parent volatiles observed. One can imagine a dry dusty nucleus that is releasing dust by erosion or other processes and being leaked along the orbit, causing a tail of a different color, compared to its gaseous coma. Based on my discussions with Dr. W. Reach (Associate Director for Science, SOFIA and comet expert), speculations focus on the brighter "line segment" or spike could represent small fragments or debris leaking out of the nucleus, seen in images of the comet taken around 21 November 2013. “The shape of the tail also gives me a qualitative impression of there being two "comets". One is the nucleus of ISON, at the front of the pack, with its conical and straight-line tails. The other is the "line segment" which I suspect to be a swarm of small fragments, with a leading arc (almost certainly gas lines) and its OWN tail that is brighter than that of the main nucleus at that location. …possibly originated from the same event that produced a new feature in the comet tail on Nov 12-13. The dust seen then most likely follows a synchrone from an outburst that accompanied the brightening by ~2 magnitudes.”
So, C/ISON, “the lollipop” or 2-in-1 comet, with a bright spike-like main (possibly dusty) tail and a large symmetric gaseous coma, may have a dessicated nucleus that is slowly eroding and leaving crumbs of material as evidence. C/ISON will reveal her secrets, hopefully, on perihelion day. Even if does not, C/ISON has fascinated us as it has demonstrated behaviour reminiscent of various famous comets and yet promises its own legacy.
UPDATE: 2013 Nov 29: A fan-shaped feature eventually emerged from behind the sun - and so starts C/ISON's post-perihelion phase.