The first papers about Comet ISON are beginning to appear

We quietly rolled out a new page on the CIOC website recently: publications. You can navigate to it from anywhere on the site from the “Comet ISON” tab then selecting “Publications” from the drop down menu. Why am I bringing your attention to this page? Well, for one, I have a paper there that I want people to read ☺ But also, because there are already some interesting ideas about what has been going on with ISON and what will happen soon.

Going forward, I expect the pace of publications to outstrip my ability to write up layman’s terms summaries of them. But since there are currently only three refereed papers1, it’s still a manageable number so I’ll give it a stab.

The first paper, “Will Comet ISON (C/2012 S1) Survive Perihelion?” by Knight and Walsh looks at Comet ISON’s chances of surviving its dash by the Sun. We considered two ways it might perish: by loss of mass due to the sublimation (rapid vaporization of ice into gas due to the Sun’s heating) of its ices and by loss of mass due to the gravitational pull of the Sun. Based on observations of previous Kreutz sungrazing comets over the last two centuries (see my earlier post for an explanation of what Kreutz comets are) and the current best size estimates for Comet ISON’s nucleus, we concluded that it is likely large enough to survive mass loss due to sublimation.

We then conducted computer simulations of ISON’s passage by the Sun to see how it is likely to be affected by tidal forces. Tidal forces are caused by the Sun’s gravity pulling more strongly on the side of the comet closer to the Sun than the side further from the Sun. In extreme cases this can cause a comet to be elongated into a cigar shape and even to be pulled completely apart! There is evidence of tidal forces destroying comets and asteroids throughout the solar system, most famously Comet Shoemaker-Levy 9 by Jupiter.

Tidal forces are stronger the closer the smaller object gets to the bigger object, so we were pleasantly surprised to find that, despite its sungrazing orbit, there is a good chance that Comet ISON will not be destroyed by tidal forces. Note that I say a “good chance,” not “definitely,” because there are plenty of plausible scenarios where tidal forces could be severe enough to disrupt it. In particular, if Comet ISON is less dense than other comets or if it is spinning very fast in the “prograde” direction (think topspin in tennis), then it may disrupt. On the other hand, we found that if it is spinning in the “retrograde” direction (think backspin in tennis), spinning slowly in either direction, and/or has a typical density, then it is very likely to survive. Given the likely range of the parameters that might affect ISON’s ability to withstand tidal forces, we think it is more likely to survive than be disrupted (but again, this is NOT definite).

This brings up the second paper, “Relating Changes in Cometary Rotation to Activity: Current Status and Application to Comet C/2012 S1 (ISON)” by Samarasinha and Mueller. In this paper, Samarasinha and Mueller looked at the four comets whose rotation periods (how long it takes to spin around one time, equivalent to a day on Earth) have been conclusively measured to change. They found that despite these comets having very different shapes, sizes, levels of outgassing, and orbits, the change in the rotation period could be approximately predicted.

They then applied this methodology to Comet ISON and found that, because it will get very close to the Sun and will therefore experience a tremendous amount of outgassing, Comet ISON may experience a very large change in its rotation period. The change in rotation period could be larger than that measured for any other comet and could potentially cause it to spin so fast that it falls apart! This would obviously be bad for Comet ISON’s chances of surviving its journey by the Sun, and also means that, if ISON breaks up when it is near the Sun, it will be difficult to determine if it was due to tidal forces (as studied by Knight and Walsh) or due to “spin up” from the increased rate of outgassing (or for that matter something else entirely, like spontaneous disruption a la Comet Elenin in 2011).

The third paper is “Outgassing Behavior of C/2012 S1 (ISON) From September 2011 to June 2013” by Meech et al. This will be familiar to those who participated in the Comet ISON Observer’s Workshop August 1-2 or watched the presentations online, as Karen Meech first presented it there. In this paper, Meech et al. presented very nice observations of Comet ISON dating back to September 2011. They then attempted to explain ISON’s brightening behavior (e.g., what I’ve been plotting) by using computer models of the sublimation rates of the three ices thought to control a comet’s activity at large distances from the Sun: CO (carbon monoxide), CO2 (carbon dioxide), and H2O (water).

Meech et al. found that Comet ISON’s activity beyond the “frost line” where water remains frozen (I previously discussed the frost line in a bit more detail) was likely controlled by CO2 (this was the tentative conclusion from Spitzer observations made in June, as Casey Lisse discussed). The really interesting conclusion from their model was that the data can be well fit by a long, slow outburst of CO lasting from late 2011 until early 2013. A CO outburst at this distance is not unheard of -- it has been suggested to explain comet 29P/Schwassmann-Wachmann 1’s periodic outbursts -- but the long duration of this event would be surprising. More data on the brightness over the next few weeks will likely clarify if this model is correct, but at present it is an intriguing explanation for the peculiar flattening of Comet ISON’s brightness throughout the first half of 2013. It may also help us better understand the brightening behavior of other comets reaching the inner solar system for the first time.

1I point this out because “refereed” is the stamp of approval for scholarly papers. It means that the paper has been peer reviewed by other scientists who have checked that the techniques and conclusions are sound. This process weeds out dubious or inaccurate work and ensures that only papers of high scientific quality are published. We are making a point to only include links to papers that have been accepted for publication on the CIOC website. There are other papers that have been posted to arXiv that have not yet been accepted; once these papers are accepted we will include them here. Note however that we are doing this manually and may occasionally miss a paper, so if you find a paper that has been accepted for publication and isn’t on our list, please let us know.